Photograph of  a conventional septic tank during installation.Table of Required Septic Tank Sizes
     

  • SEPTIC TANK SIZE - CONTENTS: Septic tank sizing tables provide a guide to choosing a septic tank size based on water usage. Minimum recommended septic tank sizes based on building occupancy or wastewater volume. How does weather affect the necessary septic tank size. Design, Code Requirements & Maintenance for Two-Compartment Septic Tanks
  • POST a QUESTION or READ FAQs about how to calculate the necessary size of a septic tank
  • REFERENCES

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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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SEPTIC TANK SIZE TABLE - Table of Required Septic Tank Size Based on Daily Water Usage

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.

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:

  • (1) Source: Florida ASHI Seminar, Kissimmee, October 1993, included sheet from Environmental Health Services, March 17, 1992.
  • Some intermediate table entries were eliminated but can be recalculated by interpolation
  • In most U.S. jurisdictions we surveyed, the minimum permitted tank size for new septic installations is 1000 gallons. Special site considerations may require design by a septic-engineer before local health departments can approve an installation.
  • WATER USAGE TABLE provides companion data if you don't already know your daily wastewater volume
  • This table is for sizing residential septic tanks.

    Separately, COMMERCIAL SEPTIC DESIGN discusses commercial septic tank and drainfield sizing based on wastewater flow estimates. There we also include excerpts from the EPA Design Manual's discussion of how a septic tank size should be determined, including safety factors but balancing design capacity safety with avoiding unnecessarily high costs.

SEPTIC TANK SIZE 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, 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 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 .

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

  • Number of bedrooms (as in our table above). For example the Alaska state regulation (18 AAC 72) specifies that the minimum septic tank size shall be 1000 gallons for a home with three or fewer bedrooms. (Most septic design guides presume an occupancy of two people per bedroom, so if your home is more crowded your septic tank size should be increased).
  • Septic pumping chambers: use of a septic system pumping chamber: the same example regulations as above state that

    If a septic tank incorporates an integral lift station pumping chamber, the required tank size must be increased by 250 gallons [4]
  • Local weather temperatures: the same example septic regulation just cited also points out that septic tank sizes need to be considered in view of the climate where they are installed. In very cold climates such as Alaska and northern Canada, because cold temperatures reduce the anaerobic rate of digestion of sewage solids, the septic tank sizes need to be increased - more size, more net free area, longer septic tank retention times are all needed. Conversely in warm or hot climates where the level of bacterial action is comparatively greater, septic tank sizes may be a bit smaller.

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

Septic tank dual compartment - Alaska DECTwo 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 entire tank, both compartments, will need to be filled with wastewater before any effluent will begin to flow out of the septic tank and into the drainfield or soakaway bed.

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 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 SEWAGE LEVELS in SEPTIC TANKS 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 Filters

COMPUTING SEPTIC TANK CAPACITY - 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: Inspecting, Testing, & Maintaining Residential Septic Systems. Also see The Septic 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.

Septic Tank Dimensions by Tank Size

Septic system failure days after tank pumpout (C) Daniel FriedmanWhile 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 Example 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 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
    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

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

 

Continue reading at SEPTIC DRAINFIELD SIZE or select a topic from the More Reading links shown below.

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

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

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