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Table of Required Septic Tank Sizes InspectAPedia®  -     Septic tank size table based on water usage How to compute septic tank capacity

On this page we provide a table to determine the required size of a septic tank. 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.

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 document is a chapter of Inspecting, Testing, & Maintaining Residential Septic Systems. Also see The Septic Information Website

© Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links 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.

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

How big does our septic tank need to be?

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 .

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.

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.

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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)
• Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.

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More Information on Building Diagnostic Inspections and Repairs

• Return to the main septic system text: Inspecting, Testing, & Maintaining Residential Septic Systems
• The Septic Information Website our main information page on onsite waste disposal inspection, testing, maintenance, repair
• SEPTIC SYSTEM DESIGN BASICS: Choosing Septic Tank Size, Absorption System Size - basic septic system volume and absorption system design guides.
• How Big Should the Leach Field Be? - table of soil percolation rate vs. field size
• Septic Tank Pumping Guide: When, Why, How to Pump A Septic Tank.
• Tank Location - How to Find the Septic Tank, how deep will the cover be, how to document its location
• Home & Outdoor Living Water Requirements
• The Home Buyer's Guide to Septic Systems
• Septic Systems - An Engineer's View - good explanation of what septic systems are, how they work, and why they fail
• Septic System Design Basics: Choosing Septic Tank Size, Absorption System Size

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