Septic Tank Size Specification FAQs

Index to all Q&A on septic tank size / capacity
Adding bedrooms or people vs septic tank size FAQs
Septic tank size & weight calculation FAQs
Septic tank sizing for large number of users FAQs
Septic tank / system repair or replacement cost FAQs
Shared septic tank / drainfield FAQs
Temporary surges in septic tank loading FAQs

Can my Existing or Old Septic Tank Be Used if I add People, Bedrooms, or a Business? FAQs

Will my existing septic tank permit adding a bedroom and more occupants?

I have a 3 bedroom 1 bath house. I am looking to knock out the wall separating a very small bedroom and a medium sized bedroom - turning 2 bedrooms into 1. Then I am looking to build a bedroom and a second bath in my unused garage.

The inspection report I have when I bought the house says I have a 1000+ gallon septic tank. Will this hold the additional bathroom or will the town make me replace the tank for a larger one? On 2012-08-09 by John

Answer by (mod)

John, your question is a good one because the answer is messy, or has several possible replies:

1. your local building department or health department will either allow continue using the existing tank or not - or in some cases will require a bigger septic tank and more drainfield capacity if more occupants or more bedrooms are being added - it's a local official decision.

You could reasonably argue that the total number of bedrooms at the property is NOT changing so neither should your septic system design requirements. Local decision, I can't speak for your guys.

2. In any event you should look at SEPTIC TANK PUMPING SCHEDULE (in the ARTICLE INDEX found at the end of this article) and pump your tank at a frequency based on size and number of occupants unless you have more sophisticated data like actual sludge and scum measurements, or if you use a "points system" method of determining septic tank pumping interval (also found in that article).

3, The septic tank size requirements that designers use are in the table in the article SEPTIC TANK SIZE and as you saw, thereare based on wastewater flow estimates - detailed

at SEPTIC SYSTEM BASICS

and at WATER QUANTITY USAGE TABLES

Can we add a bedroom and not enlarge the septic?

I have a similar question to one below - we have a purchase option on a summer house that clearly has 4 bedrooms, but the septic plan is only for 3 bedrooms - have no idea why (maybe because of the poor soil/ledge around it, as this is an island home in Maine?).

In fact, most of the time there will only be 2-4 people there, but we would like to rent it so there will be some periods of time with up to 8 people. There is only 1 bathroom with a shower, no dishwasher, but a washing machine.

What are the implications for this - just more frequent pumping? Or should be convert one of the bedrooms to a study? On 2012-06-06 by Karen Rea

Answer by (mod) -

Karen,

Watch out: more frequent pumping can help protect the drainfield from damage, but nevertheless, if your system was designed for just 3 bedrooms, or nominally, 3-6 people, larger numbers of visitors risk overloading the drainfield with septic effluent - and shortening its life as well.

So in the short run the risk is contaminating the environment and in the longer run there is some risk that the high occasional usage, by overloading the system wastewater by volume, won't pemit enough settling time in the septic tank, thus pushing fine floating solids into the drainfled, clogging it.

Can I add a bedroom without putting in a bigger septic tank since the original one was approved years ago?

20 yrs ago a larger than needed septic tank was installed with a future intent of adding a 3rd bedroom.So they installed a 750 gal. Tank back then.

Now 20 yrs later it appears the county Health department may require a 1000 gal tank.

Is it possible that i could be grandfathered in since due dillagence was established 20 yrs ago just in case we add a bedroom? On 2016-07-04 by Will

by (mod) - Watch out: septic tank grandfathering rules won't prevent a costly drainfield failure or high cost of frequent septic tank pumping

Will, the "grandfathering" clause in some buliding codes is a local policy not somehting one an answer about an unknown property at an un-known location, city, state or Province, and country.

However I can speculate that as in concept "grandfathering" pertains to structures that were already-built before current applical building codes, an d as your third bedroom did not then and does not yet exist, I would expect local officials to say no.

Besides, building standards describe a minimum not a maximum performance level. In fact you most-likley want your septic system to work properly, have a long life and to avoid discharging sewage into the groundwater or environment. That means to me that your tank is too small in absolute terms.

That in turn means that it won't treat the volumeof seweage adequately and that you'd be asking for trouble in the form of more rapid failure of the drainfield or sewage in your well water

Can an engineer say my perfect-condition steel septic tank is ok to use for a pizza home delivery business?

by (mod) - Watch out: wishful thinking may be setting you up for a costly septic system failure or violation of local codes

Corey,

We'd need more information to answer your question, including

1. Local building codes and regulations, zoning regulations about operation of a takeout food business from your home - ask your local building or zoning department about permitted uses of your property before spending money on anything else for this idea, as you'll need to operate legally both to avoid getting in trouble and worse, to avoid making someone sick by making a health-mistake.

2. If the use is permitted, you'll need to review any changes in the anticipated wastewater volume at your property as that may exceed the capacity of your septic system.

I apologize that it sounds a bit glib, but as long as a property use is permitted, you can usually do almost whatever is needed - if you are able to spend enough.

An "engineer" isn't the person who approves your septic system for an expanded use, that's your local health, building, or code department.

However you might need a septic engineer to describe what's installed and its capacity to assert that it meets the local requirements, or to give you a plan to make it meet what's required.

Be sure to read COMMERCIAL SEPTIC DESIGN 

Watch out: Search InspectApedia.com for STEEL SEPTIC TANKS to see details about metal septic tanks and their common hazards and problems. I would be careful about making assertions that the septic tank is in "perfect working order" without inspection and testing of both the tank and the drainfield.

Can we get our 3-bedroom home septic tank re-classified for 4 bedrooms?

We purchased a new 4 bedroom home last year. We have recently found out that the septic system was done for a 3 bedroom home and is listed with the county as a 3 bedroom home. Is there a way to have our current septic system re-classified or approved for our 4 bedroom home?

All of our real estate listings, floor plans, etc., that were given to us when we purchased the home say it's 4 bedrooms. The 4th bedroom has a closet, walk-in as a matter of fact and a window for escape in case of a fire.

We know that listing our home for resale as a 3 bedroom home versus a 4 bedroom home will decrease our value and we feel mislead by the builder and the realtor representing the builder. Do you have any suggestions or do we have any recourse? Thank you! On 2016-04-12 by Victoria

Answer by (mod) - septic tank size and drainfield must be capable of handling an added bedroom

Victoria:

You need the septic system design, tank details, field details, site plan, and then you'll need to review that with your local building department and perhaps a septic engineer.

You should discuss with your lawyer the ethics as well as legal exposure of listing as a 4-bedroom home a property approved for only 3 bedrooms as well as the question of what was disclosed to you or represented to you when you purchased the home.

Septic System Specifications for More People / Larger Number of Users FAQs

Can a single conventional septic system support 30 homes?

Are there special standards for shared septic systems in a seasonal community (May to September).

Could a single centrally located conventional septic system support 30 2 bedroom, 1 bath cabins? Alternatives? On 2017-09-07 by Steve

Reply by (mod) - No: Shared septic conventional septic tank and drainfield system for 30 homes is improbable

Steve

Shared septic systems would, in my opinion, fall under community septic system regulations.

There is almost no chance that a conventional septic tank and drainfield would support 30 2 bedroom cabins - potentially 120 people at maximum daily use. You'd need a small onsite wastewater treatment plant.

Only if you could establish a much smaller maximum simultaneous usage might it be reasonable for the local health or building code officials approve something different. Typically a septic system is designed for the maximum use that could be made of it.

So for example a couple building a new four bedroom house would not be successful in arguing to the local code approvers that only two of them would live in the house.

An astute code official would reply "yes but the home is designed with four bedrooms; the septic for that home has to be able to handle the people who could be in the home, including guests or the larger family of a future owner.

See the system specifications and options at SEWAGE TREATMENT PLANTS, RESIDENTIAL 

Also read COMMERCIAL SEPTIC DESIGN 

Design for a Septic System for Primary & Secondary Schools in African Community

I'm an engineer volunteering with an NGO to design, and construct sanitation facilities for African primary, and secondary schools.

There is a current proposed black water only septic system project. How do you design/size a black water only septic tank? The only influent water will be approximately 2gal/flush. On 2019-01-30 by Michael

Suggestions by (mod) -

Michael

I think you'd use a conventional tank and drainfield if your site has space and usable soils. With a 2 gallon flush volume there ought to be sufficient water for the tank and fields to operate. The modest total wastewater volume probably means that if your soils have decent perc you won't need much total area.

Typical design-retention time for septic tanks is 1-3 days in some sources or better, 2-3 days but there is not a single exact answer since tank sizes and shapes vary. The formula below gives a reasonable septic tank effluent retention time in days based on the tank volume and daily wastewater volume.

Details are at our article on EFFLUENT RETENTION TIME

by Michael

Thanks Dan. Then a standard 2day+/- septic tank retention time should provide sufficient process time?

by (mod) - Septic system designs for concentrated wastewater,

Michael

That's how it appears to me, except for cases of very little wastewater volume;

The case you describe, in extreme is called concentrated wastewater, for which there has been research. Examples:

• De Graaff, Marthe S., Hardy Temmink, Grietje Zeeman, and Cees JN Buisman. "Anaerobic treatment of concentrated black water in a UASB reactor at a short HRT." Water 2, no. 1 (2010): 101-119.
• Luostarinen, Sari A., and Jukka A. Rintala. "Anaerobic on-site treatment of black water and dairy parlour wastewater in UASB-septic tanks at low temperatures." Water Research 39, no. 2-3 (2005): 436-448.
  
• Gray, N. F. "The influence of sludge accumulation rate on septic tank design." Environmental Technology 16, no. 8 (1995): 795-800.
  
  The point of articles like the last one is we need sufficient water volume for settlement time and treatment, and to avoid blocking the tank or clogging it to the point that wastewater can't flow through it and effluent out to the drainfield.

by Michael - black water septic tank design is the same as comingled.

Again, thanks Dan. Bottom line, black water septic tank design is the same as comingled. Got it.

Building Rental Houses, need septic design for a septic tank to support 96 people

I would like to build a septic tank to accommodate to 96 people (48) bedroom. I bought a (10000sqm) plot in order to build rented houses.
Kind regards
Thulani
Contact no 0609633554
email mlumax75@gmail.com On 2018-10-20 by Thulani M Ndlovu

Answer by (mod) - a conventional septic tank can't do the job - can't be big enough, nor can the drainfield. You need an onsite wastewater plant.

Depending on occupancy and level of wastewater use you may need to install an onsite mini-wastewater treatment facility since a conventional home-type septic tank and drainfield may not be adequate.

A conventional septic tank and drainfield would have to be larger than feasible as would probably be a chain or set of septic tanks.

The question is not just septic tank size but the size, design, layout of the effluent absorption field as well.

I'd consult a local septic engineer who also can take into consideration the properties of the soil where your facility is located, site shape, layout, slope, available space, and local codes and restrictions.

See details, research, and specifications at SEWAGE TREATMENT PLANTS, RESIDENTIAL 

Septic system Design to support 220 RV Campsites in Maine

I just purchased 75 acres of heavily wood ,predominantly level property in Maine. My plan is design & build 220 r/v-campsites. Phase one of four will break ground may/2017 and calls for the installation of 55 individual campsites with water power & waste hookup.

My question is it feasable or possible to install one large downstream septic & field instead of individual designs for each site..my calculations indicate the measurements will be 250 x 400 x 10.5 deep (inside measurements) plastic/fibreglass tank.

I know its possible but will it pass Title-V regs. I have experience in the construction trades and know individual inspectors can be tough..what ya thoughts On 2016-12-20 by Daniel

Reply by (mod) - a conventional septic system won't have the capacity.

You need deisgn help from local septic engineer who knows what designs are accepted by local authorities and also who also can take into consideration the properties of the soil where your facility is located, site shape, layout, slope, available space, and local codes and restrictions.

See details, research, and specifications at SEWAGE TREATMENT PLANTS, RESIDENTIAL 

How big does the septic tank have to be to support 1,200 people?

How to calculate the size of a septic tank for 1200 persons ? On 2015-12-15 by Anonymous

by (mod) - designing a septic system for 1200 people requires more than a simple tank and drainfield

Anon:

If you are designing an onsite waste disposal system for a facility housing 1,200 people, a conventional residential septic tank such as those shown in the tables at

SEPTIC TANK SIZE would never be adequate. More likely you'll need an on-site wastewater treatment plant designed by a sanitary engineer.

The engineer will consider daily wastewater flow and will also consider the physical size of your building site and where you can locate both treatment facility and effluent or wastewater disposal facility.

See details, research, and specifications at SEWAGE TREATMENT PLANTS, RESIDENTIAL 

Small Septic Tank FAQs

Is a 36-inch small septic tank usable?

Can you use a square tank 84”x84”x39”deep concrete tank as a septank if you have all the hooks for it On 2020-05-20 by John -

by (mod) - Not very useful; shallow septic tanks have very limited capacity

Relatively shallow septic tanks may get local approval, but in my view there is a significant risk of early drainfield failure as shallow depth, further reduced by sludge and scum layers means inadequate settling time in the tank.

Inadequate settling time means pushing solids into and thus damaging the absorption field or drainfield.

Can I use a 550 gallon water tank as a septic tank?

Can a 550 gal water tank be used as septic tank ? On 2017-07-15 by Brandon -

by (mod) - No, it's too small; you'd be pumping it every few months and risk costly drainfield failure

Not in any useful nor effective way

A water tank is the wrong shape, lacks the appropriate access connections for clean-out or connections to the Sewer Inlet or a fluid outlet, and by virtually all contemporary standards 550 gal would be too small. A water tank is also not designed to be buried nor withstand corrosion, earth pressure Etc.

It's a great question but IMO a very bad idea.

Inspector says I need a larger septic tank for the house I'm buying

I'm purchasing a 4 bedroom house in Wisconsin and we are scheduled to close early next week. I will be the only person living in the house. I just found out the septic tank is a 1,000 gallon concrete tank and the septic permit was for a 2 bedroom house.

The inspector recommended installing a 1,500 gallon plastic tank to the tune of $15,000. Is this something I need to worry about?

by (mod)

Rather than pouring $15,000 into the ground before it's necessary, you may find you can simply have the septic tank pumped more-frequently, and when the tank is pumped, have the sludge and scum levels measured as that will tell you if you're pumping often enough.

Watch out: keep in mind that buying an older home with a septic system means you probably don't know the condition of the system and you could be facing a costly failure. Was your inspector a home inspector or did you have an actual septic inspection and test performed? On 2020-08-05 by Douglas -

For expanding septic system capacity can we just add another small septic tank?

I am planning to build a house. 1800-2000 sq. feet. with 2.5 baths room. The vast majority (95%) of the time it will only be my wife and myself (2 people). We also already have a small building. Living space 400 sq. ft.

No one would live there. only for occasional guest. It has 1 bath. so total 3.5 baths. 2 people 95% of time.

I already have a 1,000 gallon septic system system consisting of two 500 gallon tanks. two drain field lines each being 67 feet in length. According to the septic report the soil type is 3. If by chance the county comes back and tells us the system is to small.

Can we just add one additional 500 gallon tank to increase the volume instead of installing a totally new system. If not, would it be cheaper to install a separate system for either the house of the small building? thanks for any help you can provide. On 2017-04-06 by Tony

by (mod) No, the drainfield will be too small

Tony

Typically the sizing of a septic system involves BOTH the tank capacity AND the area and capacity of the drainfield. Just adding a tank won't address the second half: how to dispose of effluent. The system, to avoid a failure and contamination, has to be able to handle the maximum daily wastewater inflow.

But indeed I'd look at extending the existing system before installing a whole new one.

If the septic tank is too small, can I just add a bigger tank?

If you have to have a bigger tank installed is it as simple as just changing the size of the tank? i.e. 1000 gal - to 1200 gal. On 2012-07-24 by gord

by (mod) - yes but the drainfield may be too small, too.

Gord, essentially, yes. But considering the costs involved, one would not rush to replace a 1000g septic tank with a 1200 gallon unit unless the original tank was damaged or otherwise needed replacement.

That added 200 gallons of capacity and thus improvement in settling out of solids and greases is expensively bought - most people would pump more often and defer the new septic tank.

Watch out: if you're adding a larger septic tank to handle an increased wastewater load or number of occupants at a property, the septic soakaway or leachfield (drainfield) size will usually also to be increased to handle the increase in effluent to be disposed-of.

Septic Failure With Undersized 900 gallon cinder block Septic Tank

I rent a home with an very outdated, cinder block constructed 900 gallon septic tank. The tank would need to be frequently drained (2-3 months) with just 2 people using a normal amount of water. The waste would overflow into the yard.

We were told by the company that pumped it out that it was not in compliance. What are our options as far as requesting it be replaced? We were told it isn't even a legitimate septic tank. On 2012-07-10 by Anon

Reply by (mod) - hazards of "outdated" septic tanks; concrete block septic tanks are unsafe and also probably cause drainfield failure

Outdated septic tank: while I too am very pessimistic about the functionality of an old, under-sized, site-built concrete block septic tank (leaks into and out of the tank are likley as is tank collapse a risk), the symptoms you describe seem to mix up a drainfield failure with a septic tank failure.

If you had a 900 gallon capacity septic tank that were intact, that is not collapsed, and if it were pumped out - thus removing sludge and scum, you've got about 900 gallons of wastewater volume - the tank is filled in less than a week of normal use; septic tanks are normally "full" of wastewater all the time.

The overflowing waste into the yard is more likely to be a blocked effluent line or saturated drainfield - it sounds to me as if the entire SYSTEM needs replacement, is unsanitary, and thus in failure.

Your options: inform the owner that there is an unsafe (collapse risk) and unsanitary condition (backing up sewage onto the property surface) that make the building not only not habitable but at risk of issues with local code authorities.

Watch out: I would be SURE to keep everyone away from the septic tank area; a collapsing masonry block septic tank, if someone falls in, is likely to be a fatality.

See an example of a failed concrete block septic tank at SEPTIC FAILURE LAWSUIT

And read CONCRETE BLOCK SEPTIC TANK LIFE FAQs - cinder block septic tanks are unsafe

Septic Tank Size Calculation FAQs

Tell me how to calculate the volumes of septic tanks by shape, freeboard etc

How do you calculate the volume of a septic tank only for black water? Hw to calculate the free board? On 2017-05-25 by emile

Answer by (mod) re: how to calculate the volumes of septic tanks by shape, freeboard, and effects of tank outlet height, sludge & scum layers

Emile,

The volume of a septic tank is calculated based on the shape of the tank - calculating the volume of a rectangular shape

Vol = Length x Width x Height of the rectangular-shaped septic tank.

or for septic tanks that are round or a cylindrical shape and excluding adjustments for ends of a tank that are not flat, you would use

volume of a cylinder V = pi x r² x h 

where pi = 3.1416,

r = cylinder radius (1/2 the diameter) and

h = the cylinder height or length of pipe in our case

Watch out: be sure you use consistent units: feet, inches, meters, centimeters etc. For example if you're calculating the volume in inches and calculate cubic inches of volume you then need to convert to gallons (or liters) where

G = the volume in gallons = 0.004329 gallons per cubic inch

This formula and discussion is

at WELL DYNAMIC HEAD & STATIC HEAD DEFINITION

The actual volume of wastewater in a properly-working rectangular septic tank is easier to calculate than a cylindrical one since we have to consider just the volume of the container up to the bottom of the outlet pipe.

Watch out: you have to remember that a septic tank is never normally full to its top.

If you want to calculate just the "liquid" volume of the septic tank you need to measure the current thickness of the floating scum layer and bottom sludge layer and thus to subtract those from the first volume that you calculated.

The "freeboard" in the septic tank has to be the volume between the bottom of the septic tank outlet pipe (assuming again a properly-working septic system with no blockages in the outlet or drainfield) and the top of the septic tank baffle or tee. That's because any depths higher than the tee top mean that the outlet is blocked and the system is not working normally.

This sketch of a SEPTIC TANK CROSS SECTION shown above and used in several of our articles

inculding SEPTIC TANK LEVELS of SEWAGE illustrates my comments.

Starting at SEPTIC TANK INSPECTION PROCEDURE you can see examples of tools used to measure the scum and sludge layers.

Good luck on your test, Emile.

Septic tank dimensions don't come out to the exact volumes quoted for those "sizes"

The tank sizes by dimension do not come out to be the tank size (in gallons) listed. For instance, the tank dimensions for a 2000 gal tank are 162" x 78" x 64", which comes out to be 468 cf, or 3500 gal. On 2016-02-23 by QestionGuy

by (mod) Nominal septic tank capacity is not equal to actual volume

Thanks QG you make an excellent point.

For example septic tank dimensions and capacities I copied actual septic tank dimensions from several manufacturers as well as their rated or nominal tank sizes for those dimensions.

Keep in mind that outside dimensions of a tank are going to be larger than inside dimensions, varying depending on the tank wall thickness - for concrete tanks or site-built masonry tanks. (Metal and plastic or fiberglass tanks have such thin walls that the OD vs ID distinction is not important).

Nominal septic tank actual internal volume or "size" is therefore not the same as exact external volume, with a greater difference between those if a tank is thick-walled concrete than if it is thin-walled plastic or fiberglass.

Nominals septic tank capacity will be different (and less than) the actual internal volume of the tank because a septic tank in proper operation is filled only to the bottom of the tank outlet pipe, never to the tank top.

Examples of Septic Tank Volume or Size or Quantity & Weight Calculations

Calculate the number of gallons in a gallons 48×48×72inch septic tank

How much gallons 48×48×72 tank holds On 2018-02-28 by Kg Gotti

by (mod) - A 48×48×72 septic tank is too small for most applications

Kg

Let's do the math:

48" x 48" x 72" = 168,888 cubic inches

One U.S. gallon = 231 cubic inches

168,888 cubic inches = 730 gallons

Watch out: that s ank is too small to serve as a septic tank in most jurisdictions.

What's the capacity of a 4' x 4' x 42" septic tank?

I have a septic tank with these dimisions 48 inches wide × 48 inches long × 42 inches wide How many gallons are there? On 2017-10-12 by Anonymous

by (mod) - Inadequate: a 450 gallon tank is too small

Sure Anon

48 x 48 x 42" = 103,680 cubic inches of volume in the septic tank, assuming you measured the INSIDE dimensions of the tank (else we'd be off by the wall thickness.

1 U.S. gallon = 231 cubic inches

103,680 cubic inches of tank volume / 231 cubic inches per US Gallon = 448.8 US gallons - nominally a "450 gallon tank" -

Watch out: 450 gallons is, by the way smaller than would be permitted by typical codes in any normal septic system

What's the weight of a 250 gallon concrete septic tank?

How much does 250 gallon concrete septic tank weigh? On 2018-10-09 by Anonymous

by (mod) - 1,700 to 2,250 pounds; but no 250 gallon tank can serve by itself as a septic tank

Anon

The weight of a septic tank varies enormously depending on the materials from which it is constructed and its design details such as dimensions, wall thickness, use of re-bar vs steel mesh reinforcement, and cover design, openings, thickness, etc.

Watch out: in addition and getting to the specifics of your question there are no 250 gallon septic tanks. Typically the minimum septic tank size varies depending on local regulations but is likely to be 1200 to 1500 gallons