Septic tank size calculator
The minimum tank for your bedroom count, the size you'd actually buy, and what it means for how often you'll pump.
The short answer
Septic tanks are sized by bedrooms. Under the International Private Sewage Disposal Code, 1 or 2 bedrooms need at least 750 gallons, 3 bedrooms 1,000 gallons, 4 bedrooms 1,200 (so a 1,250-gallon tank), and 5 bedrooms 1,425 (a 1,500-gallon tank). Many codes require about 50% more with a garbage disposal, and your county may set higher minimums.
Tank to buy
1,000gallons
- Code minimum (IPSDC Table 802.7.1)
- 1,000 gal
- Required with disposal allowanceNo disposal
- 1,000 gal
- Design flow for the drain field150 gallons per bedroom per day
- 450 gal/day
- Pump-out interval at 4 peoplePenn State extension table
- about 2.6 years
Septic tank size chart by bedrooms
Minimums from IPSDC Table 802.7.1, rounded up to the sizes tanks are commonly sold in. The disposal column applies a 50% allowance, which many state and county codes require.
| Bedrooms | Code minimum | Tank to buy | With a disposal |
|---|---|---|---|
| 1 bedroom | 750 gal | 750 gal | 1,250 gal |
| 2 bedrooms | 750 gal | 750 gal | 1,250 gal |
| 3 bedrooms | 1,000 gal | 1,000 gal | 1,500 gal |
| 4 bedrooms | 1,200 gal | 1,250 gal | 2,000 gal |
| 5 bedrooms | 1,425 gal | 1,500 gal | 2,500 gal |
| 6 bedrooms | 1,650 gal | 2,000 gal | 2,500 gal |
| 7 bedrooms | 1,875 gal | 2,000 gal | 3,000 gal |
| 8 bedrooms | 2,100 gal | 2,500 gal | 3,500 gal |
Why bedrooms, not bathrooms or square feet
Codes size septic systems by bedrooms because bedrooms set how many people a house can hold over its life, whoever lives there now. Each bedroom is assumed to generate 150 gallons of wastewater a day, and the tank has to hold at least a day and a half to two days of that so solids have time to settle. The same design flow sizes the drain field.
That's also why a 2,000-square-foot house doesn't have a single answer: two bedrooms and a large living space need a smaller tank than four small bedrooms. A room that could be used as a bedroom — a den with a closet, for instance — is often counted as one.
Is a bigger tank better?
Within reason, yes. A larger tank holds sewage longer, settles more solids and goes longer between pump-outs: a 1,000-gallon tank with four people needs pumping about every 2.6 years, a 1,500-gallon tank about every 4.2. The extra cost of stepping up one size is usually a few hundred dollars. It doesn't replace an adequate drain field, which is sized separately from the same bedroom count.
Finding the size of an existing tank
Check the county permit record, a pumping receipt or a home inspection report. Failing that, a pumper can measure it: the inside length, width and liquid depth in feet, multiplied together and by 7.48, gives gallons for a rectangular tank.
Frequently asked questions
- What size septic tank do I need for a 3-bedroom house?
- At least 1,000 gallons under the model code; 1,500 gallons with a garbage disposal where the 50% allowance applies. Check your county's minimum.
- Can a 4-bedroom house have a 1,000-gallon septic tank?
- Not under the model code, which requires 1,200 gallons for 4 bedrooms. Older houses may have one from before current rules; a replacement or addition will usually have to be brought up to size.
- How many bedrooms can a 1,000-gallon septic tank support?
- Three, under IPSDC Table 802.7.1.
- How big a septic tank for a 2,000 sq ft house?
- It depends on bedrooms, not square feet: 1,000 gallons for 3 bedrooms, 1,250 for 4.
- How do I calculate the capacity of my septic tank?
- For a rectangular tank, multiply the inside length, width and liquid depth in feet, then multiply by 7.48 to get gallons.