Septic Tank Size Calculator

How to use the septic tank size calculator

1. Enter the bedroom count the house is (or will be) permitted for, not just the number of people living there now. Codes size one- and two-family homes by bedrooms because a room can be occupied later.
2. Treat the result as a planning minimum. Confirm it with the local health department or the authority that issues the permit before you buy a tank.
3. Do not use this page to size a drain field or to price a tank install. Soil, setbacks and daily flow rules are local, and this calculator does not cover them.

The EPA / IPSDC bedroom table, and how states differ

Table 4-13 in the EPA Onsite Wastewater Treatment Systems Manual lists minimum septic-tank liquid capacities for one- and two-family dwellings: 750 gallons for one or two bedrooms, 1,000 for three, 1,200 for four, 1,425 for five, 1,650 for six, 1,875 for seven and 2,100 for eight, then 225 gallons per bedroom above eight. The same manual notes that many jurisdictions have raised the minimum to 1,000 gallons or more, so a 750-gallon result is flagged on the page.

State codes are not identical. Illinois Department of Public Health’s Private Sewage Disposal Code, Appendix A, Illustration F, uses 750 / 1,000 / 1,250 / 1,500 gallons at two-or-fewer, three, four and five bedrooms, then 250 gallons per extra bedroom — so a four-bedroom Illinois minimum is 1,250 gallons, not the 1,200 in the EPA table. Penn State Extension and Pennsylvania’s Chapter 73 set a 900-gallon floor, with a three-bedroom house in Pennsylvania required to have a 900-gallon or larger tank. Use the table on this page as a starting point, then read the code that actually applies.

A worked example: three- and four-bedroom houses

A three-bedroom house reads 1,000 gallons in the EPA/IPSDC table, which also meets the Illinois 1,000-gallon three-bedroom minimum and is larger than Pennsylvania’s 900-gallon floor. A four-bedroom house reads 1,200 gallons here; Illinois would require 1,250. A nine-bedroom house is 2,100 + 225 = 2,325 gallons on this table.

Sources & assumptions

How this calculator gets its numbers. Full method: our methodology.

• Planning tank gallons follow Table 4-13 of the U.S. EPA Onsite Wastewater Treatment Systems Manual (EPA/625/R-00/008), which reproduces the International Private Sewage Disposal Code table: 750 / 750 / 1,000 / 1,200 / 1,425 / 1,650 / 1,875 / 2,100 gallons for 1–8 bedrooms, then +225 gallons per bedroom above 8.
• A result under 1,000 gallons is flagged because the same EPA manual notes that many jurisdictions have raised the minimum to 1,000 gallons or more.
• State codes differ (Illinois Appendix A Illustration F uses 1,250 gallons at four bedrooms and +250 thereafter; Pennsylvania’s floor is 900 gallons). Local code always governs. This is not a drain-field design and not an install-cost estimate.

Sources

1. U.S. Environmental Protection Agency, Onsite Wastewater Treatment Systems Manual (EPA/625/R-00/008). February 2002: Table 4-13 septic-tank capacities for one- and two-family dwellings by bedrooms; many jurisdictions have raised the minimum to 1,000 gallons or more.
2. Illinois Department of Public Health / JCAR, 77 Ill. Adm. Code 905 Appendix A, Illustration F. Minimum tank volumes: 2 or fewer bedrooms 750 gal; 3: 1,000; 4: 1,250; 5: 1,500; then +250 gal per extra bedroom (larger still with a garbage grinder).
3. Penn State Extension, Septic System Basics. A three-bedroom house in Pennsylvania must have a 900-gallon or larger septic tank; tanks are sized by bedrooms under state code.

Last reviewed November 12, 2026. Installed prices vary by region, contractor and job conditions; a written local bid wins.