Leave the second field empty to use the bedroom formula. The minimum design population is 5 either way.
Planning estimate for a 3-bedroom house
2,750 L
suggested minimum tank capacity
- Design population P
- 5
- Daily wastewater
- 750 L (5 × 150)
- Formula
- (P × 150) + 2,000
The formula
The commonly applied British Water approach starts from the number of people the system must serve:
- Design population P = bedrooms + 2, with a minimum of 5. So 1–3 bedrooms give P = 5, 4 bedrooms give 6, 5 give 7, and 6 give 8.
- Daily wastewater = P × 150 litres per person per day.
- Suggested minimum tank capacity = (P × 150) + 2,000 litres.
| Bedrooms | P | Daily wastewater | Minimum tank capacity |
|---|---|---|---|
| 1–3 | 5 | 750 L | 2,750 L |
| 4 | 6 | 900 L | 2,900 L |
| 5 | 7 | 1,050 L | 3,050 L |
| 6 | 8 | 1,200 L | 3,200 L |
Worked example (shown above without JavaScript): a 3-bedroom house gives P = 5, so 5 × 150 = 750 litres a day, and a suggested minimum tank of 750 + 2,000 = 2,750 litres.
Why this is a planning estimate, not a design
The formula sizes the tank on paper. The installed system also has to satisfy what the ground will accept and what the regulations require: ground conditions and percolation decide the drainage field, the invert level decides where the tank can sit, and Building Regulations or the manufacturer’s installation requirements can demand a larger unit than the minimum. Use this calculator to shortlist tanks and talk to installers, then let the site survey set the final size. See septic tank installation cost for what the install typically costs.
Size and emptying frequency
There is no fullness percentage here because a calculator cannot sense your tank — and a fake reading would be worse than none. The honest relationship is directional: a smaller tank with more people fills faster, so it sits nearer annual emptying (the England baseline) or the 12–24 month guidance in Wales. See how often to empty a septic tank for the full breakdown by nation and household.