Enter the number of bedrooms in the house and choose a state rule. The calculator gives the minimum septic tank size in gallons, liters and cubic feet. Tank size is set by your local health code, so use this to plan and then confirm with your county.
How to use the septic tank size calculator
- Count the bedrooms, including any room that could be used as one, such as a den or office with a closet. Codes size tanks by bedrooms because they predict how many people live in the house.
- Choose the Washington or Virginia rule. They are two common ways states set minimum sizes.
- Check the result against your own state and county rules before you buy or replace a tank.
Septic tank sizing rules
Washington (single-family home): up to 4 bedrooms = 1,000 gal; each additional bedroom = +250 gal Virginia (dwelling units): 1–2 bedrooms = 750 gal, 3 = 900 gal, 4 = 1,200 gal, 5 = 1,500 gal Cubic feet = gallons × 231 ÷ 1,728; liters = gallons × 3.785
Washington’s rule is in WAC 246-272A-0232, Table VII. Virginia’s table is in 12VAC5-610-815, which also says no septic tank may be smaller than 750 gallons and that tanks are sized for at least 48 hours of detention at the daily design flow.
Worked example: a 3-bedroom house
- Washington: 3 bedrooms is within the first 4, so the minimum is 1,000 gallons.
- Virginia: the table gives 900 gallons for 3 bedrooms.
- A 1,000 gallon tank holds 1,000 × 231 ÷ 1,728 = 133.7 ft³, or about 3,785 liters.
For a 6-bedroom house, Washington requires 1,000 + 2 × 250 = 1,500 gallons.
Minimum tank size by bedrooms
| Bedrooms | Washington | Virginia |
|---|---|---|
| 1 | 1,000 gal | 750 gal |
| 2 | 1,000 gal | 750 gal |
| 3 | 1,000 gal | 900 gal |
| 4 | 1,000 gal | 1,200 gal |
| 5 | 1,250 gal | 1,500 gal |
| 6 | 1,500 gal | Ask your health department |
Why tanks are sized by bedrooms
A septic tank needs to hold wastewater long enough for solids to settle and scum to float before the liquid flows to the drainfield. Codes use bedrooms rather than the current number of people because a house can change hands, and the system has to work for the next owner too. A bigger tank also means a longer time between pumpings.
Things that can change the size you need
- Local rules. Counties often add their own requirements on top of state rules; the permit sets the final size.
- Extra flow. Home businesses, very large houses or accessory dwellings may be sized on design flow rather than bedrooms.
- Two-compartment tanks. Some codes require them, or require an effluent filter on the outlet.
- Soil and drainfield. The tank size is only one part of the system; the drainfield is sized from a soil evaluation.
Questions
What size septic tank do I need for a 3-bedroom house?
Usually about 1,000 gallons. Washington requires 1,000 gallons for up to four bedrooms, and Virginia requires at least 900 gallons for three.
How many bedrooms can a 1,000 gallon septic tank handle?
Under Washington’s rule, up to four bedrooms. Under Virginia’s table, three bedrooms; four bedrooms need 1,200 gallons.
What size septic tank for a 4-bedroom house?
1,000 gallons in Washington and 1,200 gallons in Virginia. Check your own state, since the rules differ.
Is a bigger septic tank better?
A larger tank gives solids more time to settle and stretches the time between pumpings, but the system is permitted as a whole, so size it with your installer and health department.
Sources
- WAC 246-272A-0232: Design requirements, septic tank sizing, Washington State Legislature.
Used for: Table VII: 1,000 gal for up to 4 bedrooms, plus 250 gal for each additional bedroom. - 12VAC5-610-815: Septic tank capacity, Virginia Administrative Code.
Used for: 750 gal minimum; 48-hour detention; Table 5.2 tank sizes for 1–5 bedrooms. - NIST Guide to the SI, Appendix B.8: Factors for units listed alphabetically, National Institute of Standards and Technology.
Used for: US gallon = 231 in³; gallon to liter conversion.