What this calculator does
A septic tank is a holding and settling vessel, so its size follows from how much water goes in and how long it stays. This calculator converts residents into a daily flow, applies the retention time your code requires, adds the sludge allowance and selects the next standard size.
How to use it
- Enter the number of residents and the daily flow per person — 130 to 180 litres is the usual range.
- Set the retention time required by the local code, commonly 48 hours for a single dwelling.
- The sediment factor of 1.3 allows for the third compartment where solids accumulate.
- The leach field must be sized separately from a percolation test.
Governing formula
V = (n × q) × (t/24) × factor Where:
-
n= Number of residents [–] -
q= Flow per person [L/day] -
t= Retention time [h]
Worked example — Five residents at 150 L/day
The figures below were produced by running this page's own engine with the example inputs (Residents, Flow per person (L/day), Required retention time (h), Sediment storage factor (–)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Required tank volume | 1.95 m³ |
|---|---|
| Recommended standard size | 2 m³ |
| Daily flow | 0.75 m³/day |
| Assessment | At 150 L/p/d the tank needs ≈ 2.0 m³ → specify a 2 m³ tank. Check the treatment/soil-percolation requirement for the leach field. |
- Daily flow is 5 × 150 = 750 L, or 0.75 m³/day.
- Two days of retention gives 1.5 m³, and the sediment factor raises it to about 1.95 m³.
- The next standard size is 2.0 m³.
Assumptions & limitations
Engineering calculation. Sizes the septic tank from daily flow, retention time and a sediment-storage factor. The leach field is not sized — it depends on soil percolation.
- Tank sizing only — the drain field area depends on soil percolation, which this tool does not assess.
- Local codes set minimum tank volumes that may exceed the calculated figure for small installations.
- Assumes no commercial or industrial inflow.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- ASCE 24 — septic tank and leaching field design.
- Local building codes — minimum septic tank capacity requirements.
- EPA — onsite sewage treatment systems guidance.