Use the liquid depth from the outlet pipe down to the floor, since the space above it holds air, not wastewater.
How the septic tank size calculator works
SepticTankSize sizes the tank by bedrooms: 1,000 gallons for one to three bedrooms and 250 gallons for each extra bedroom, the most common minimum in US state codes, increased by half and rounded up to the next 250 gallons when there is a garbage disposal. The daily design flow is bedrooms × gallons per bedroom per day, 150 by default; many codes use 120 to 150. The drain field needs enough trench bottom to absorb that flow: area = flow ÷ the soil's long-term loading rate. The pumping estimate assumes the tank needs pumping when sludge and scum fill a third of it, and that each person adds about 30 gallons of solids a year (about 50% more with a disposal). Tank volume is cubic feet of liquid × 7.48 gallons per cubic foot.
Septic tank size and drain field by bedrooms (loam, 3 ft trenches)
| Bedrooms | Tank | With disposal | Daily flow | Trench bottom | Trench length |
|---|---|---|---|---|---|
| 1 | 1,000 gal | 1,500 gal | 150 gpd | 300 sq ft | 100 ft |
| 2 | 1,000 gal | 1,500 gal | 300 gpd | 600 sq ft | 200 ft |
| 3 | 1,000 gal | 1,500 gal | 450 gpd | 900 sq ft | 300 ft |
| 4 | 1,250 gal | 2,000 gal | 600 gpd | 1,200 sq ft | 400 ft |
| 5 | 1,500 gal | 2,250 gal | 750 gpd | 1,500 sq ft | 500 ft |
| 6 | 1,750 gal | 2,750 gal | 900 gpd | 1,800 sq ft | 600 ft |
Typical soil loading rates
| Soil | Loading rate | Area for 450 gpd |
|---|---|---|
| Sand | 1.2 gpd/sq ft | 375 sq ft |
| Loamy sand | 0.9 gpd/sq ft | 500 sq ft |
| Sandy loam | 0.7 gpd/sq ft | 643 sq ft |
| Loam | 0.5 gpd/sq ft | 900 sq ft |
| Silt loam | 0.4 gpd/sq ft | 1,125 sq ft |
| Clay loam | 0.25 gpd/sq ft | 1,800 sq ft |
Local rules differ: some states size tanks by daily flow, require two compartments or effluent filters, or use other loading rates. A permit, a soil or perc test and a licensed designer are normally required. Results are estimates for planning.
Frequently asked questions
How do I calculate septic tank gallons?
Length × width × liquid depth in feet × 7.48. For a round tank, 3.1416 × radius² × depth × 7.48.
What are the dimensions of a 1,000 gallon septic tank?
Commonly about 8 × 4.5 ft inside with 4 ft of liquid depth, though shapes vary by maker.
How do I find my septic tank size?
Check the permit or as-built drawing at the county, the pumping company records, or measure the inside when it is pumped.
Is SepticTankSize free?
Yes, with no sign-up or ads.
Are my numbers saved?
Yes, in this browser only.