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Photograph of  a conventional septic tank during installation.Septic Tank Size Requirements
Septic tank size calculations, tables, codes

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Septic tank size specifications & requirements:

This article provides a septic tank size table to determine the required size or capacity of a septic tank. The typical residential septic tank size required for a given average daily sewage wastewater flow in gallons is provided in a table of septic tank sizes.

We also include tables of typical septic tank size dimensions and capacities for concrete septic tanks, plastic or fiberglass septic tanks & steel septic tanks.

This chapter also 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. Septic tank size requirements and how to calculate septic tank capacity are explained.

Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow & How to Calculate the Size (in gallons) of a Septic Tank are reflected in the table. This chapter summarizes guidelines on the required septic tank size based on anticipated level of daily gallons of sewage wastewater flow.



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Tables, Codes, Calculations of Required Septic Tank Size

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.

Here we also discuss the smallest recommended septic tank sizes based on building occupancy or wastewater volume. One should also ask, how does weather affect the necessary septic tank size?

Article Contents

Table of Required Septic Tank Size Based on Daily Waste-Water Volume in Gallons

Average Sewage Wastewater
Flow - Gallons Per Day
Minimum Septic Tank Size in Gallons
of Effective Capacity Needed (1)
0-500900
601-7001200
801-9001500
1001-12401900
2001-25003200
4501-50005800

Notes to Table:

Table of Required Septic Tank Size Based on Number of Bedrooms in a Home

How big does our septic tank need to be based on the number of bedrooms in the home? Some jurisdictions use the number of bedrooms rather than number of occupants or estimated daily wastewater flow to guide homeowners and septic installers in choosing a septic tank size.

Septic Tank in Gallons Size Based on Number of Bedrooms

Number of bedrooms Minimum Septic Tank Capacity (Gallons) Minimum Septic Tank Liquid Surface Area (sq.ft.)
0 bedrooms 750 gal. (2) - obsolete in NYS  
1, 2, or 3 bedrooms 1,000 gallons 27 sq.ft.
4 bedrooms 1,200 gallons 34 sq.ft.
5 bedrooms 1,500 gallons 40 sq.ft.
6 bedrooms 1,750 gallons 47 sq.ft.

Notes to the septic tank sizing table:

Original source: New York State NYS75-A. Wastewater Treatment Standards,[image] Update 12 January 2015 to current table that lumps 1-3 bedrooms together and adds 6-bedroom homes.

The current standard document is provided as a PDF and can be seen in its entirety at REFERENCES.

The older version of this table permitted 750 gallon septic tanks in some jurisdictions and permitted 1,500 gallon septic tanks for 6-bedroom homes. Some current standards such as New Mexico still permit these smaller septic tank sizes).

For example, New Mexico [reference citation] uses this standard. Other experts estimate that occupants use between 50 and 100 gallons of water per person per day in a home in the U.S. We can use that guesstimate to compare different septic tank size guidelines.

Also see WATER USAGE TABLE [web page].

The current and somewhat more stringent septic tank size used in the U.S. by New York (Public Health Law 201(1)(1) Section 75-A) standard includes these comments on septic tank sizing:

Tank size requirements for more than six bedrooms shall be calculated by adding \ 250 gallons and seven square feet of surface area for each additional bedroom.

A garbage grinder shall be considered equivalent to an additional bedroom for determining tank size.

2. Watch out: 750 gallon septic tanks are smaller than the minimum 1000g size required for new construction in many jurisdictions world-wide.

 

Septic Tank Size Requirements May Be Set or Adjusted by Local or State or Provincial Ordinances, Lift Stations, Weather

In some jurisdictions government ordinance may simply specify the minimum allowable septic tank size by

Design, Code Requirements & Maintenance for Two-Compartment Septic Tanks

Septic tank dual compartment - Alaska DEC

Two compartment septic tanks do a somewhat better job of removing suspended solids from wastewater than single compartment septic tanks, and some jurisdictions, including Alaska (18 AAC 72), require that two compartment septic tanks be used. But we have not found regulations that translate that design difference into different septic tank size requirements.

In some states (Connecticut since January 1991) septic tanks now consist of two compartments in order to do a more effective job, and increasingly other jurisdictions (Alaska, Pennsylvania) require that new and up-graded onsite wastewater disposal systems use two-compartment septic tanks.

Image adapted from Alaska DEC [3] The dashed lines illustrate the liquid level (red) and the difference in elevation (green) between the inlet and outlet septic tank pipe connections.

More about these measurements is at SEPTIC TANK TEES where we discuss repair procedures and backwards septic tanks.

In a two compartment septic tank the wall separating the two compartments will have an opening that allows liquid effluent to flow into the second compartment, keeping floating scum and settled sludge in the first compartment (mostly).

The septic entire tank, both compartments, will need to become filled with wastewater before any effluent will begin to flow out of the septic tank and into the drainfield or soakaway bed. That happens naturally during normal building plumbing system usage over the course of several days for typical septic tank sizes.

So when you observed about two feet of waste in the septic tank, then left the system unused, you'd expect to find exactly the same amount in the tank weeks later. Only a very slight drop in level might occur, less than an inch - caused by evaporation - because you left the tank open (and dangerous).

Watch out: if your tank is a two compartment type the solids, floating scum and settled sludge are accumulating at the inlet portion of the tank. Inspecting at the final septic tank outlet end will not discover sludge and scum early enough to prevent septic system damage.

Such septic tanks may have a center inspection port which admits tank access at the outlet of the sludge/scum containing compartment. That's where to test in two-compartment septic tanks.

Please see SEPTIC TANK LEVELS of SEWAGE for details about how to interpret abnormal levels of sewage found in the septic tank (too high or too low).

Septic media filter systems also require either two septic tanks or a two compartment tank - see SAND FILTER SEPTIC DESIGNS

How to Compute Septic Tank Size and Capacity in Gallons

How do we compute the volume in gallons that a septic tank provides based on simple measurements we can make?

Septic Tanks are usually about 4.5 feet wide x 8.0 feet long x 6 feet tall. Tanks are typically buried 4 inches to 4 feet deep depending on local site conditions, shape, slope, and other factors.

Here is the basic math for computing septic tank capacity (volume) in gallons. Measurements are in feet, taken of inside dimensions of the septic tank.

How to Calculate the Septic Tank Capacity in Gallons

Round Septic Tanks 3.14 x radius squared x depth (all in feet) = cubic capacity. Cubic capacity x 7.5 = gallons capacity.
Rectangular Septic Tanks Length x Width x Depth in feet x 7.5 = gallons
Rectangular Septic Tanks
(alternative method 1)
Length x width in inches / 231 = gallons per inch of septic tank depth. Multiply this number by septic tank depth in inches to get gallons
Rectangular Septic Tanks
(alternative method 2)
Length x Width x Depth in feet / .1337 = gallons

COMPUTING EXAMPLE - An Example of Computing Septic Tank Size, Capacity or Volume in Gallons

One gallon of water has a volume of .1337 cubic feet. For a rectangular septic tank, multiply depth (or inside "height") in feet times width times length. Divide this figure by .1337 to establish the number of gallons in the septic tank.

Example 1: how many gallons is held in a a 4ft. deep x 5ft. wide x 8 ft. long septic tank? If the tank dimensions were 4ft. x 5ft. x 8ft. = 160 cubic feet. Using the conversion factor to convert cubic feet to gallons, 160 / .1337 = 1196 - or about a 1200-gallon tank.

One cubic foot of volume can contain 7.481 gallons of liquid. So a second approach to calculating septic tank actual size or capacity in gallons is to multiply the septic tank volume in cubic feet by this constant, which we round up to 7.5 gallons/cubic foot.

Example 2: how many cubic feet and how many gallons are held in a septic tank of typical dimensions of 4.5 ft. wide x 8.0 ft. long x 6 ft. high. (4.5 x 8 x 6) = 216 cubic feet. Since one cubic foot can contain 7.481 gallons, which we round up to 7.5 gallons per cubic foot: 216 x 7.5 = 1620 gallons of septic tank capacity - this is probably nominally a "1500-gallon septic tank".

Note that if the dimensions given by your septic contractor are the external dimensions of the tank rather than the internal dimensions then the volume given by this calculation will come up with a septic tank size estimate that is higher than the actual tank capacity - the error is due to failure to allow for the thickness of the septic tank walls.

So for fitting a septic tank into a tight spot, the outer dimensions of the septic tank are important. But for accurate calculation of the capacity of a septic tank you need to use the septic tank internal dimensions.

This article is part of our series: SEPTIC SYSTEM INSPECTION & MAINTENANCE COURSE.

Also see our home page: THE SEPTIC SYSTEM INFORMATION WEBSITE

Also see the basic septic system design information links at SEPTIC SYSTEM DESIGN BASICS: Choosing Septic Tank Size, Absorption System Size - basic septic system volume and absorption system design guides.

On 2017-08-23 by Robby - how do I calculate the size of a septic tank based on influent and effluent BOD loadings?

I am working on a project that will utilize septic tanks (several in parallel and series). The WW flows are 20,000 GPD, and influent WW BOD strength is about 450 mg/L and it will be permitted with an effluent strength of 100 mg/L.

The effluent will be disposed of via surface irrigation. I have not been able to find any literature on how to size a septic tank using influent and effluent BOD loadings. All literature I have found is based on number of bedrooms. This OSSF system will serve a Kid's Summer Camp.

Thanks and I look forward to your response. Robby

This question was posted originally at SEPTIC SYSTEM DESIGN BASICS

On 2017-08-23 by (mod) - how to size a septic tank using influent and effluent BOD loadings

Right. Yet there has to be an engineering assumption underneath the "number of bedrooms" that translates to "number of occupants" that in turn translates to average daily wastewater flow. That's what we both should look for and report back here.

See for starters WATER QUANTITY USAGE GUIDE https://inspectapedia.com/septic/Water_Quantity_Usage_Tables.php

and be sure to see SEPTIC TANK SIZE http://inspectapedia.com/septic/Septic_Tank_Size_Tables.php

In my OPINION the rule of thumb of 50 to 100 gallons of water per person per day may not be the right match for a kids' summer camp
but it would be really interesting to calculate:

- toilet flushes
- showers
- use of laundry facilities
- dishwashing and cleaning by camp staff

The volume of wastewater in the net free area between the top of sludge and bottom of scum determines the treatment time in the tank as new wastewater flows into the system. That in turn will be an important determinant of BOD measurement results.

On 2017-08-15 (mod) - how to calculate the required septic tank size

Boieez

Thanks for the septic tank capacity question. Please hop over to SEPTIC TANK SIZE

Also see SEPTIC SYSTEM DESIGN BASICS at http://inspectapedia.com/septic/Septic_System_Design_Basics.php#FAQ

where in reply to a reader question we discuss average daily wastewater flow per person design assumptions and refer to a companion article WATER QUANTITY USAGE TABLES

On 2017-08-15 y boieezcar@gmail.com - computation of septic tank capacity per person

please provide proper computation of septic tank capacity per person

This question and answere were posted originally at SEPTIC TANK PUMPING MISTAKES

Septic Tank Dimensions by Tank Size & Type

Concrete, Plastic / Fiberglass, Steel Septic Tank Dimensions

Septic system failure days after tank pumpout (C) Daniel Friedman

While septic tank sizes are normally given in gallons or liters of wastewater capacity, the actual physical dimensions of a septic tank can be important for some considerations.

For example on a very rocky site where excavation to bury a septic tank runs into trouble the installer may decide to choose a shallow but large "flat" septic tank.

In my limited experience with shallow septic tanks they don't work so well and need to be pumped more often. I suspect that the net free area in total volume may meet spec but effluent settlement time may still be too little.

The septic tank under the pile of straw at the top of the septic system failure photo at left was just such a shallow or "flat" septic tank. Here is a table of some typical septic tank dimensions and other properties.

Table of Concrete Septic Tank Outside Dimensions, Capacity, & Other Data

Rectangular
Concrete
Septic Tank Size
(Gallons Capacity)
Tank Length
(Inches)
Tank Width
(Inches)
Tank Depth (Height)
(Inches)
Comments

SEPTIC TANKS, CONCRETE
600 gallons 78 56 60 Not legal in many jurisdictions
800 96 67 57 Not legal in many jurisdictions
1000 heavy duty 96 78 61  
1000 low profile 120 67 57 low profile septic tank
1000 /600 145 in. 68 in. 61 in. two compartment tank
1200 111 78 61  
1250/750 162 78 64 two compartment tank
1600 145 78 61  
1600/1400 174 90 73 two compartment tank
2000 162 78 64  
2600 147 90 73  
3000 165 92 76  
5000 204 96 93  

...

Plastic or Fiberglass Septic Tank Typical Dimensions

Plastic / Fiberglass
Septic Tank Size
(Gallons Capacity)
Tank Length
(Inches)
Tank Width
(Inches)
Tank Depth (Height)
(Inches)
Comments

SEPTIC TANKS, FIBERGLASS / PLASTIC
 
500 spherical 64 64 64 holding tank or pumping tank, Norwesco
750 linear 96 60 51 sold as single or dual compartment
900       Snyder Industries.
1050 126 60 51  
1250 161 60 51  
1500 191 60 51  

...

Steel Septic Tank Typical Dimensions

Steel
Septic Tank Size
(Gallons Capacity)
Tank Length
(Inches)
Tank Width
(Inches)
Tank Depth (Height)
(Inches)
Comments

/septic/Steel_Septic_Tanks.phpTanks.htm">SEPTIC TANKS, STEEL
 
500 gallons round 46 46 73

Cylindrical, laid horizontally.

In the U.S. older round cylindrical septic tanks were placed end-wise, that is with a flat end down and a flat cover on top. Recangular steel septic tanks are installed in low-profile forms in problem sites in some jursidictions.

750 58 58 73 "
1000 58 58 96 "
1250 58 58 120 "
1500 58 58 144 "
2000 58 58 192 "

Notes:

Manhole risers: for septic tanks buried at depth that prevents ready access-to and removal of cleanout & inspection covers a manhole riser with a concrete or steel cover is recommended to permt proper inspection & cleaning. Covers must be of sufficient strength & security to prevent collapse & to prevent a septic tank fall-in hazard.

Concrete & some steel tanks are usually rectangular; plastic & fiberglass septic tanks may be roughly rectangular, oval, or even round spherical. Spherical tanks are usually used for pumping stations not for primary septic tank applicatins.

Dimensions & properties taken from surveys of septic tank manufacturers (concrete & other materials).

  • "Environmental Health Practitioner Manual: A resource manual for Environmental Health Practitioners working with Aboriginal and Torres Strait Islander Communities", Australia Department of Health, Department of Health GPO Box 9848, Canberra ACT 2601, Australia, retreived 3/31/14, Original source: https://www.health.gov.au/internet/publications/publishing.nsf/Content/ ohp-enhealth-manual-atsi-cnt-l~ohp-enhealth-manual-atsi-cnt-l-ch2~ohp-enhealth-manual-atsi-cnt-l-ch2.9
  • Norwesco, Inc., P. O. Box 439 4365 Steiner Street St. Bonifacius, MN 55375-0439 Tel 800-328-3420 (Customer Service) Fax 800-874-2371 (Customer Service) http://www.NORWESCO.com
  • "Septic Tanks", Greer Tank & Welding, Inc., 3117 107th Street Greer Steel in Lakewood Washington Woodworth Industrial Park Lakewood, Washington 98499 Tel: 800-725-8108, email: info@greertank.com, retrieved 3/31/14, original source: http://www.greertank.com/septictanks.html
  • "Septic Tank Dimensions & Weights", Huffcutt Concrete, Inc., retrieved 3/31/14, source: http://www.huffcutt.com/tank-weights.php
  • "Septic Tanks, Septic Tank Systems - the superior traditional two chamber design", Crystal Sewage Treatment Unit 2 Bolton House Farm York North Yorkshire YO41 5QX United Kingdom Telephone: 01759 369915 Email: info@crystaltanks.com Website: http://www.crystaltanks.com, Tel: 01759-369915, retrieved 3/31/14, original source: http://www.crystaltanks.com/septic_tank_crystal.html


    For sewage tanks In the U.K. also see "Code of Practice - Flows & Loads 4 - Sizing Criteria, Treatment Capacity for Small Wastewater Treatment Systems" & "Code of Practice - Guide to the Installation of Small Wastewater Treatment Systems" and

    "Code of Practice – Guide to the Desludging of Small Wastewater Treatment Systems" provided by britishwater.co.uk at http://www.britishwater.co.uk/publications/Publications_and_Technical_Guides.aspx
  • Snyder Industries, Inc. Corporate Headquarters 4700 Fremont Street (P.O. Box 4583) Lincoln, Nebraska 68504 Phone: 402-467-5221, Tel (Sales): 402-465-1237, Email: sales@snydernet.com

On 2017-07-15 by (mod) - Can a 550 gal water tank be used as septic tank

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 not a great idea.

On 2017-07-15 by Brandon

Can a 550 gal water tank be used as septic tank

On 2017-05-25 by (mod) - why can't I do some separate treatment of urine and sewage that will permit installing a septic system on a site that the authorities say is too small.

Emile:

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.

But ultimately the septic system components that dispose of wastewater effluent have to be able to do that effectively in the available space.

On 2017-05-25 by (mod) - - how is the volume of a septic tank calculated?

Septic tank dimensions and volume (C) InspectApedia.com

Emile,

The volume of a septic tank is calculated based on the shape of the tank - calculating the volume of a rectangular shape Vol = L x W x H or of a cylindrical shape

The formula and discussion are 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.

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 http://inspectapedia.com/septic/Image4e.jpg used in several of our articles inculding SEPTIC TANK LEVELS of SEWAGE

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

Large Capacity Septic System Design Requirements

On 2016-01-23 by Mark - size for a septic tank in a population area of 650 people

What can be the appropriate size for a septic tank in a population area of 650 people?

Posted originally at SEPTIC SYSTEM DESIGN BASICS

On 2016-01-24 by (mod) - for 650 people - use an onsite wastewater treatment plant

Mark,

No conventional septic tank would be reasonable for 650 people; the septic tank would have to be millions of gallons, and even then you'd need a thousand acres or so for a conventional drainfield.

For a population of 650 people, you'd need an onsite wastewater sewage treatment plant.

References & authorities for large capacity onsite wastewater treatment plants

...


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

Or see SEPTIC TANK SIZE FAQs

Or see SEPTIC TANK, HOW TO FIND

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

Or see SEPTIC TANK PUMPING SCHEDULE

Or see SEPTIC TANK LEVELS of SEWAGE

Or see these

Septic Tank Types - Articles

Also see our septic tank examples and septic tank properties or data at

Suggested citation for this web page

SEPTIC TANK SIZE at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

INDEX to RELATED ARTICLES: ARTICLE INDEX to SEPTIC SYSTEMS

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