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
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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.
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)
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
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
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.
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 
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
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  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
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.
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
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 Example Septic Tank Outside Dimensions, Capacity, & Other Data
Septic Tank Size
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.
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
P. O. Box 439
4365 Steiner Street
St. Bonifacius, MN
Tel 800-328-3420 (Customer Service)
Fax 800-874-2371 (Customer Service)
"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: email@example.com, retrieved 3/31/14, original source: http://www.greertank.com/septictanks.html
"Septic Tanks, Septic Tank Systems - the superior traditional two chamber design", Crystal Sewage Treatment
Bolton House Farm
Telephone: 01759 369915
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.
4700 Fremont Street (P.O. Box 4583)
Lincoln, Nebraska 68504
Phone: 402-467-5221, Tel (Sales): 402-465-1237, Email: firstname.lastname@example.org
Also see our septic tank examples and septic tank properties or data at
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 Pennsylvania State Fact Sheets relating to domestic wastewater treatment systems include
Pennsylvania State Wastewater Treatment Fact Sheet SW-161, Septic System Failure: Diagnosis and Treatment
Pennsylvania State Wastewater Treatment Fact Sheet SW-162, The Soil Media and the Percolation Test
Pennsylvania State Wastewater Treatment Fact Sheet SW-l64, Mound Systems for Wastewater Treatment
Pennsylvania State Wastewater Treatment Fact Sheet SW-165, Septic Tank-Soil Absorption Systems
Document Sources used for this web page include but are not limited to: Agricultural Fact Sheet #SW-161 "Septic Tank Pumping," by Paul D. Robillard and
Kelli S. Martin. Penn State College of Agriculture - Cooperative Extension, edited and annotated by
Dan Friedman (Thanks: to Bob Mackey for proofreading the original source material.)
 Septic Tank/Soil-Absorption Systems: How to Operate & Maintain [ copy on file as /septic/Septic_Operation_USDA.pdf ] - , Equipment Tips, U.S. Department of Agriculture, 8271 1302, 7100 Engineering, 2300 Recreation, September 1982, web search 08/28/2010, original source: http://www.fs.fed.us/t-d/pubs/pdfimage/82711302.pdf.
 "How to Maintain Your Septic System", Alaska Department of Environmental Conservation, Division of Water, retrieved 8/8/12, original source: http://dec.alaska.gov/water/wwdp/onsite/maintain_septic.htm [copy on file as Alaska_Septic_Care.pdf]
 Installers Manual for Conventional Onsite Domestic Wastewater Treatment and Disposal Systems", Alaska Department of Environmental Conservation, Division of Water, retrieved 1/15/2001, original source: [copy on file as Alaska_Certified_Installer's_Manual.pdf]
Tyler, E. T., R. Laak, E. McCoy, and S. S. Sandhu. 1977. "The Soil as a Treatment System." in Home Sewage Treatment. ASAE publication 5-77
Septic Tank Maintenance - Guide M-113, R. Craig Runyan, Extension Water Quality Specialist, New Mexico septic tank size table , scheduled for update 11/07 (pending), Cooperative Extension Service, College of Agriculture and Home Economics, New Mexico State University (Las Cruces)
US EPA Onsite Wastewater Treatment Systems Manual [online copy, free] Top Reference: US EPA's Design Manual for Onsite Wastewater Treatment and Disposal, 1980, available from the US EPA, the US GPO Superintendent of Documents (Pueblo CO), and from the National Small Flows Clearinghouse. Original source http://www.epa.gov/ORD/NRMRL/Pubs/625R00008/625R00008.htm Onsite wastewater treatment and disposal systems,
Richard J Otis, published by the US EPA. Although it's more than 20 years old, this book remains a useful reference for septic system designers.
U.S. Environmental Protection Agency, Office of Water Program Operations; Office of Research and Development, Municipal Environmental Research Laboratory; (1980)
"International Private Sewage Disposal Code," 1995, BOCA-708-799-2300, ICBO-310-699-0541, SBCCI 205-591-1853, available from those code associations.
"Manual of Policy, Procedures, and Guidelines for Onsite Sewage Systems," Ontario Reg. 374/81, Part VII of the Environmental
Protection Act (Canada), ISBN 0-7743-7303-2, Ministry of the Environment,135 St. Clair Ave. West, Toronto Ontario M4V 1P5 Canada $24. CDN.
Manual of Septic Tank Practice, US Public Health Service's 1959.
Onsite Wastewater Disposal, R. J. Perkins;
Quoting from Amazon: This practical book, co-published with the National Environmental Health Association,
describes the step-by-step procedures needed to avoid common pitfalls in septic system technology.
Valuable in matching the septic system to the site-specific conditions, this useful book will help you install a reliable system in
both suitable and difficult environments. Septic tank installers, planners, state and local regulators, civil and sanitary engineers,
consulting engineers, architects, homeowners, academics, and land developers will find this publication valuable.
Onsite Wastewater Treatment Systems, Bennette D. Burks, Mary Margaret Minnis, Hogarth House 1994 - one of the best septic system books around, suffering a bit from small fonts and a weak index. (DF volunteers to serve as indexer if Burks/Minnis re-publish this very useful volume.)While it contains some material more technical than needed by homeowners, Burks/Minnis book on onsite wastewater treatment systems a very useful reference
for both property owners and septic system designers. We refer to it often.
While Minnis says the best place to buy this book is at Amazon (our link at left), you can also see this book at Minnis' website at http://web page .pace.edu/MMinnisbook
Septic Tank/Soil-Absorption Systems: How to Operate & Maintain [ copy on file as /septic/Septic_Operation_USDA.pdf ] - , Equipment Tips, U.S. Department of Agriculture, 8271 1302, 7100 Engineering, 2300 Recreation, September 1982, web search 08/28/2010, original source: http://www.fs.fed.us/t-d/pubs/pdfimage/82711302.pdf
Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
John Cranor is an ASHI member and a home inspector (The House Whisperer) is located in Glen Allen, VA 23060. He is also a contributor to InspectApedia.com in several technical areas such as plumbing and appliances (dryer vents). Contact Mr. Cranor at 804-747-7747 or by Email: email@example.com
Home Inspection Education Home Study Courses - ASHI@Home Training 10-course program. Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
The Home Reference Book, a reference & inspection report product for building owners & inspectors. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
The Home Reference eBook, an electronic version for PCs, the iPad, iPhone, & Android smart phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter inspectaehrb in the order payment page "Promo/Redemption" space.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
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Builder's Guide to Wells and Septic Systems, Woodson, R. Dodge: $ 24.95; MCGRAW HILL B; TP;
Quoting from Amazon's description: For the homebuilder, one mistake in estimating or installing wells and septic systems can cost thousands of dollars. This comprehensive guide filled with case studies can prevent that. Master plumber R. Dodge Woodson packs this reader-friendly guide with guidance and information, including details on new techniques and materials that can economize and expedite jobs and advice on how to avoid mistakes in both estimating and construction. Chapters cover virtually every aspect of wells and septic systems, including on-site evaluations; site limitations; bidding; soil studies, septic designs, and code-related issues; drilled and dug wells, gravel and pipe, chamber-type, and gravity septic systems; pump stations; common problems with well installation; and remedies for poor septic situations. Woodson also discusses ways to increase profits by avoiding cost overruns.
Country Plumbing: Living with a Septic System, Hartigan, Gerry: $ 9.95; ALAN C HOOD & TP;
Quoting an Amazon reviewer's comment, with which we agree--DF:This book is informative as far as it goes and might be most useful for someone with an older system. But it was written in the early 1980s. A lot has changed since then. In particular, the book doesn't cover any of the newer systems that are used more and more nowadays in some parts of the country -- sand mounds, aeration systems, lagoons, etc.
US EPA Onsite Wastewater Treatment Systems Manual Top Reference: US EPA's Design Manual for Onsite Wastewater Treatment and Disposal, 1980, available from the US EPA, the US GPO Superintendent of Documents (Pueblo CO), and from the National Small Flows Clearinghouse. Original source http://www.epa.gov/ORD/NRMRL/Pubs/625R00008/625R00008.htm
Wells and Septic Systems, Alth, Max and Charlet, Rev. by S. Blackwell Duncan, $ 18.95; Tab Books 1992. We have found this text very useful for conventional well and septic systems design and maintenance --DF.
Quoting an Amazon description:Here's all the information you need to build a well or septic system yourself - and save a lot of time, money, and frustration. S. Blackwell Duncan has thoroughly revised and updated this second edition of Wells and Septic Systems to conform to current codes and requirements. He also has expanded this national bestseller to include new material on well and septic installation, water storage and distribution, water treatment, ecological considerations, and septic systems for problem building sites.