Photograph of  a conventional septic tank during installation.Septic Tank Size Specification FAQs

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Septic tank size requirement FAQs:

This article series provides septic tank size tables to determine the required size or capacity of a septic tank, and it explains how to calculate septic tank volume based on septic tank inside dimensions measured in feet, and we discuss the sizing, installation, and functions of septic tank tees to prevent septic system clogging.

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Questions & Answers about Septic Tank Size Calculations & Requirements

Septic tank cross section showing layers and volumes (C) Daniel Friedman These questions & answers about septic tank sizing or size calculations were posted originally at SEPTIC TANK SIZE - home.

How big does our septic tank need to be? Typically the septic tank volume for a conventional tank and onsite effluent disposal system (such as a drainfield) is estimated at a minimum of 1000 gallons or 1.5 x average total daily wastewater flow.

On 2017-05-25 by (mod) re: how to calcualte the various volumes of septic tanks by shape, freeboard, and effects of tank outlet height, sludge & scum layers


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 STATIC HEAD, WELL 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.

On 2017-05-25 by emile

how to calculate the volume of a septic tank only for black water? how to calculate the free board

On 2017-05-25 by (mod) - Re: options for treatment of urine & water wastewater systems


Regarding your question of treatment of urine and water mixture, I can't answer without knowing the intended application.

Normally urine and other wastewater flows either into a public sewer and on to a sewage treatment plant or into a private septic system where the wastewater in a conventional system is treated by bacteria in the septic tank (45%) and by both aerobic and anaerobic bacteria in the soil of the soakaway bed (55%).

Aerobic septics and other designs treat wastewater to different levels at different states of the process. Still other systems may use a disinfection step combining UV light or chlorine injection.

On 2017-05-25 by emile

what are the best technologies to treat urine and water mixture?
thank you

On 2017-05-09 by (mod) re: what's the size of a 1250 gallon septic tank?


From the table at SEPTIC TANK SIZE

1200 gallon: 111 x 78 x 61inches for the tank plus a couple of feet on sides for placing the tank and for making piping connections

On 2017-05-09 by Deric

What is the size of the hole for a 1250 gallon septic tank

On 2017-04-06 by (mod)


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.

On 2017-04-06 by Tony

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 2017-03-16 by (mod)

Cheryl, as you'll read in the article at SEPTIC TANK SIZE septic tank size is not a function of house square feet, but rather it depends on the number of occupants, bathrooms, volume of wastewater to be produced, and probably, depending on where you live, a minimum septic tank size specified by local codes and ordinances.

On 2017-03-16 by Cheryl parker

Im building a 980 square foot tiny house. What is the requirement size septic tank

On 2017-01-20 by (mod)


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. The question is not just septic tank size but the size, design, layout of the effluent absorption field. I'd consult a local septic 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.

On 2017-01-20 by pavan

We have a working staff of 30 members,how much capacity of septic tank is preferred for us,

We have a industry, working staff - 30 no.s, how much capacity of septic tank is required for our use.

On 2017-01-20 by (mod) - septic tank size for 30 employees


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.

The question is not just septic tank size but the size, design, layout of the effluent absorption field. I'd consult a local septic 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.

On 2017-01-20 by pavan

We have a working staff of 30 members,how much capacity of septic tank is preferred for us,
We have a industry, working staff - 30 no.s, how much capacity of septic tank is required for our use.

On 2016-12-20 by Daniel

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 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 2016-11-15 by akbruce

Azmat al i- You don't design a tank, you buy a tank. You design a system, but usually that is required to be done by an engineer. The fact that you are asking how to do it illustrates why that is the case (plus you couldn't build one as cheaply as buying one).

On 2016-11-11 by Anonymous

Drawing for 7ft. Deep ×6 with.×9ft lenth

On 2016-10-13 by Azmat ali

How to design a septic tank

On 2016-10-05 by Rosa

Question?? What size tank and number of gallon capacity is needed for a duplex consisting of 2 units occupied by 1 person each ??

On 2016-07-04 by (mod) - septic tank grandfathering rules

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.

On 2016-07-04 by Will

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 cnty. Health dept 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 2016-06-24 by (mod) re: how many openings are there in the top of a septi ctank?

Usually there are openings at both ends of all septic tanks - as one needs to be able to inspect or repair both inlet and outlet baffles;

Some septic tanks such as older round steel tanks had a single removable cover over the whole tank;

Other septic tanks, particularly larger ones will have three or more openings including inlet end, outlet end, and center compartment, and a septic tank with more than three compartments will have additional openings for inspection or cleaning.

An aerobic septic tank will have an opening over the inlet, the aerator and aeration chamber, over any settling chambers.

Other septic tank systems will sport openings over an additional pumping chamber if one is used to lift effluent to a raised elevation, raised bed or mound system.

When your tank is pumped your pumper can use a light and mirror to make at least a limited inspection of the tank interior.

On 2016-06-23 by Duke Of Hudson

"CONTINUED":used to lift a square opening. Do you think this is the only opening used for pumping this size septic tank,or do I need to remove" all the soil" from the top of the tank to be sure? I don't know any history for this septic tank!

I'm planning on having my septic tank pumped(emptied). The top of the septic tank is aprox. 9ft. X 4ft.,I've located one opening,towards one end of the tank. I've dug across the center,from one end to the other end,there's a "metal hoop on one end that's bent over" and doesn't appear to be an opening,the other end of the tank has a metal hoop also & is obviously

On 2016-04-12 by (mod) - septic tank size capable of handling an added bedroom?


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.

On 2016-04-12 by Victoria

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 2016-03-23 by (mod) - does number of bathrooms determine septic tank size?

Good question, Kent.

I have not found a table converting "number of bathrooms" to septic tank size. Even "bathroom" is unclear since more water will be used in a full bath than in a half bath or a water closet alone.

Here is the Kansas septic design guide

In that Kansas septic design guide at table 7 you'll see that Kansas uses number of bedrooms, as we discuss below.

This link provides other Kansas guides for septic & graywater systems.

For that matter, one person might live in a one-bedroom, 3 bath home.

That's probably why other common tables for guesstimating septic tank size estimate daily wastewater flow by guessing at number of occupants - in turn some tables guess at number of occupants by specifying the number of bedrooms.

Ask your local building department if they have a reference or citation and please use our page bottom CONTACT link to send me that information - what we learn will help other readers.

On 2016-03-22 by Kent

I've been told that if I buy a house in Saline County, Kansas, the size of the septic tank is determined by the number of bathrooms in the house. All I have been seeing is water flow in gallons per day to determine the size of the tank. Any help or suggestions where to find the correct answer?

On 2016-02-23 by (mod) re: tank dimensions don't come out to the exact volumes quoted for those "sizes"

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.

On 2016-02-23 by QestionGuy

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 2016-01-27 by Wendy

How many bedrooms can a 1,000gal 2 chamber tank accomidate in Washington State???

On 2015-12-15 by (mod) - designing a septic system for 1200 people


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.

On 2015-12-15 by Anonymous

How to calculate the size of a septic tank for 1200 persons


Continue reading at SEPTIC TANK SIZE or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



Or see SEPTIC DRAINFIELD LOCATION for help in finding an existing septic soakaway bed, leach field or drainfield



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Septic Tank Types - Articles

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