Septic Tank Size Calculator

Septic Tank Size formula

Design flow (gal/day) = Bedrooms x 150
Tank size = 1,000 gal for 1 to 3 bedrooms, 1,250 for 4, 1,500 for 5, 1,750 for 6; next size up with a disposal
Absorption area (sq ft) = Design flow / soil rate  (sand 1.2, loam 0.6, silty clay 0.3, clay 0.2 gal/sq ft/day)
Trench length (ft) = Absorption area / trench width (3 ft)

Worked example

A 4-bedroom house with a garbage disposal on sandy soil: design flow is 4 x 150 = 600 gal per day. The minimum tank for 4 bedrooms is 1,250 gal, and the disposal moves it up one size to 1,500 gal. Absorption area is 600 / 1.2 = 500 sq ft, which at 3 ft wide is 500 / 3 = 167 ft of trench, laid as three runs of about 56 ft.

The same house on clay at 0.2 gal per sq ft per day needs 600 / 0.2 = 3,000 sq ft of absorption area and 1,000 ft of trench. That is why clay sites usually end up with a mound, chamber or aerobic system instead of a conventional drain field.

Quick reference

How septic tank size is decided

Health departments size a septic system by the number of bedrooms, not by who lives there today, because a 3-bedroom house can hold a family of six tomorrow. Each bedroom is assigned 150 gallons per day, and the tank must hold roughly two days of that flow so solids have time to settle and grease can float. That is where the 1,000 gallon minimum for up to 3 bedrooms comes from, stepping up 250 gallons per bedroom beyond that.

A garbage disposal sends ground food waste into the tank, which builds sludge faster. Most codes respond by requiring the next tank size up, and some require 50 percent more capacity. A larger tank costs a few hundred dollars more and buys years between pumpings, so oversizing is rarely a mistake.

Drain field and the perc test

The drain field, not the tank, is usually what limits a lot. Effluent leaves the tank and must soak into the soil, and each soil type can only absorb so many gallons per square foot per day. Sandy soil takes 1.2, loam about 0.6, and clay only 0.2, so the same house needs six times the trench length on clay as on sand. The county tests this with a percolation test before issuing a permit and assigns the real rate for your site.

This calculator is a planning tool for sizing a lot, budgeting, or checking an existing system. The installer's design, the perc test results and the local health code govern the actual tank, trench layout, setbacks from wells and property lines, and whether a conventional system is allowed at all.

Key facts

• Most state codes size septic systems at 150 gallons per bedroom per day, which assumes 2 occupants per bedroom at 75 gallons each.
• A 1,000 gallon tank is the most common minimum in the US; a 3-bedroom house on 450 gallons a day gets about 2.2 days of retention in it.
• The EPA recommends inspecting a septic tank every 3 years and pumping it every 3 to 5 years, more often with a garbage disposal.
• Soils that percolate slower than 60 minutes per inch generally fail a perc test for a conventional drain field.
• Conventional drain field trenches are 1 to 3 ft wide, 18 to 36 in deep, and limited to about 100 ft per run with at least 6 ft between trench centers.

Frequently asked questions

Sources and references

• •How Septic Systems Work (EPA)
• •Appendix 75-A.6, Septic Tanks and Enhanced Treatment Units (New York State Department of Health)
• •WAC 246-272A-0232, Septic tank sizing (Washington State Department of Health)
• •Soil Evaluation for Septic System (University of Illinois Extension)