Pick your bedrooms for a tank size that meets the codes we checked. Then use the cost guides below: every price is a range, checked against published sources in October 2026.
Septic tank size by bedrooms: what the codes say
Minimum tank sizes differ by state. Here are four real codes side by side, and our recommendation, which meets all of them.
| Bedrooms | ICC | NC | WA | PA | We suggest |
|---|
| 1 | 750 | 1,000 | 1,000 | – | 1,000 |
| 2 | 750 | 1,000 | 1,000 | – | 1,000 |
| 3 | 1,000 | 1,000 | 1,000 | 900 | 1,000 |
| 4 | 1,200 | 1,000 | 1,000 | 1,250 | 1,250 |
| 5 | 1,425 | 1,250 | 1,250 | 1,400 | 1,500 |
| 6 | 1,650 | – | 1,500 | 1,550 | 1,750 |
Gallons. ICC = International Code Council model code; NC = North Carolina; WA = Washington; PA = Pennsylvania. "–" means the code sets that size another way. Washington adds 250 gallons per bedroom above four; North Carolina sizes homes over five bedrooms by design flow. Links to each code are in the sources below.
Once you know the size, see how often to pump it and how big the drain field needs to be.
Frequently asked questions
What size septic tank do I need for a 3-bedroom house?
1,000 gallons in most codes we checked (the ICC model code, North Carolina and Washington). Pennsylvania allows 900 gallons. A 1,000-gallon tank meets all of them.
What size septic tank for a 4-bedroom house?
Minimums range from 1,000 gallons (North Carolina, Washington) to 1,200 (ICC model code) and 1,250 (Pennsylvania). We recommend 1,250 gallons, which meets all four.
Why is tank size based on bedrooms and not people?
Bedrooms set how many people a house can hold over its life. Codes size for that, so the tank still works after the house is sold to a bigger family.
Does a garbage disposal need a bigger tank?
The EPA says a garbage disposal adds solids and can mean pumping more often. Some designers and codes add tank capacity for it; our calculator adds 50% as a safety margin.