Questions & answers about how to calculate the necessary size of a septic tank
This article provides a septic tank size table to determine the required size or capacity of a septic tank. Septic tank size requirements and how to calculate septic tank capacity are explained.
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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.
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.
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.
Links to related septic system testing and design information are given.
Use of this information at other websites, in books or pamphlets for sale is reserved
to the author. Technical review by industry experts has been performed and is ongoing - reviewers are listed at "References."
Comments and suggestions for content are welcome.
This article is part of our series:
Inspecting, Testing, & Maintaining Residential Septic Systems.
Also see The Septic Information Website
SEPTIC TANK SIZE TABLE - Table of Required Septic Tank Size Based on Daily Water Usage
How big does our septic tank need to be?
Table of Required Septic Tank Size for Daily Water Usage Volume in Gallons
Average Sewage Wastewater
Flow - Gallons Per Day
Minimum Septic Tank Size in Gallons of Effective Capacity Needed (1)
0-500
900
601-700
1200
801-900
1500
1001-1240
1900
2001-2500
3200
4501-5000
5800
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
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. 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 .
Septic Tank in Gallons Size Based on Number of Bedrooms
0-2 bedrooms
750 gal. (2)
3 bedrooms
1000 gal.
4 bedrooms
1200 gal.
5-6 bedrooms
1500 gal.
Notes to the septic tank table:
2. 750 gallons is smaller than the minimum 1000g size required for new construction in some jurisdictions.
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.
Calculating 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.
Questions & Answers regarding this article
Questions & answers about how to calculate the necessary size of a septic tank
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Additional technical contributors & reference sources for this article are listed below.
Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.
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)
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
Advanced Onsite Wastewater Systems Technologies, Anish R. Jantrania, Mark A. Gross. Anish Jantrania, Ph.D., P.E., M.B.A., is a Consulting Engineer, in Mechanicsville VA, 804-550-0389
(2006), Advanced Onsite Wastewater Systems Technologies.
Outstanding technical reference especially on alternative septic system design alternatives. Written for designers and engineers, this book is not at all easy going
for homeowners but is a text we recommend for professionals--DF.
AEROBIC SEPTIC SYSTEMS
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.
Design Manuals for Septic Systems
US EPA Onsite Wastewater Treatment Systems Manua [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 Books
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
Planting Over Septic System Component", Daniel Friedman (author/editor, InspectAPedia.com), The Innovator, Winter/Spring 2008, BCOSSA, British Columbia OnSite Sewage Association, 201-3542 Blansard St., Victoria BC V8X 1W3 Canada
Septic System Owner's Manual, Lloyd Kahn, Blair Allen, Julie Jones, Shelter Publications, 2000 $14.95 U.S. - easy to understand, well illustrated, one of the best practical references around on septic design basics including some advanced systems; a little short on safety and maintenance. Both new and used (low priced copies are available, and we think the authors are working on an updated edition--DF.
Quoting from one of several Amazon reviews: The basics of septic systems, from underground systems and failures to what the owner can do to promote and maintain a healthy system, is revealed in an excellent guide essential for any who reside on a septic system. Rural residents receive a primer on not only the basics; but how to conduct period inspections and what to do when things go wrong. History also figures into the fine coverage.
Test Pit Preparation for Onsite Sewage Evaluations, State of Oregon Department of Environmental Quality, Portland OR, 800 452-4011. PDF document. We recommend this excellent document that offers detail about soil perc tests, deep hole tests, safety, and septic design. Readers should also see Soil Percolation Tests and for testing an existing septic system, also see Dye Tests
Grass is Always Greener Over the Septic Tank, Bombeck, Erma: $ 5.99; FAWCETT; MM;
This septic system classic whose title helps avoid intimidating readers new to septic systems, is available new or used at very low prices.
It's more entertainment than a serious "how to" book on septic systems design, maintenance, or repair. Not recommended -- DF.
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
Water Wells and Septic Systems Handbook, R. Dodge Woodson. This book is in the upper price range, but is worth the cost for serious septic installers and designers.
Quoting Amazon: Each year, thousands upon thousands of Americans install water wells and septic systems on their properties. But with a maze of codes governing their use along with a host of design requirements that ensure their functionality where can someone turn for comprehensive, one-stop guidance? Enter the Water Wells and Septic Systems Handbook from McGraw-Hill. Written in language any property owner can understand yet detailed enough for professionals and technical students this easy-to-use volume delivers the latest techniques and code requirements for designing, building, rehabilitating, and maintaining private water wells and septic systems. Bolstered by a wealth of informative charts, tables, and illustrations, this book delivers: * Current construction, maintenance, and repair methods
* New International Private Sewage Disposal Code
* Up-to-date standards from the American Water Works Association
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.
The NSFC Products List has an excellent list of design manuals/modules available from their website or by telephone 800-624-8301
Submissions welcome. send us a suggested document link or request an exchange of website links
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