Size a septic tank by number of users — daily flow, retention time and sludge storage — or by bedrooms using the US UPC table, and get two-compartment dimensions.
Septic tanks must meet the local health or municipal authority's rules for minimum size, materials, setbacks and the soakaway or drain field, which is sized separately from soil percolation tests.
A septic tank does two jobs: it holds wastewater long enough for solids to settle and scum to float, and it stores the settled sludge until the tank is emptied. Its volume is the sum of both:
| Bedrooms | Minimum capacity |
|---|---|
| 1–2 | 750 gal (2,840 L) |
| 3 | 1,000 gal (3,785 L) |
| 4 | 1,200 gal (4,540 L) |
| 5–6 | 1,500 gal (5,680 L) |
In the US, homes are sized by bedrooms rather than occupants because bedrooms predict the long-term occupancy. Many states set 1,000 gal as the minimum for any house. For larger dwellings or non-residential buildings use the flow method.
A long, narrow tank settles solids better than a square one, so a length-to-width ratio of 2:1 to 3:1 is typical, with a liquid depth of about 1.2–1.8 m. Two compartments, the first holding about two-thirds of the volume, keep floating scum and settled sludge from washing through to the outlet.
10 users at 150 L/day, 24 h retention, 40 L/person/year sludge, emptied every 2 years.
By the flow method: daily wastewater × retention time plus sludge storage — about 2.3 m³ for 10 users at 150 L/day with 2-year emptying. In the US, the UPC requires 1,000 gal for a 3-bedroom house and 1,200 gal for 4 bedrooms.
Typically every 2–3 years for a household tank, or when sludge and scum fill about a third of the tank. The calculator lets you set the interval; a longer interval needs a larger tank.
The first compartment catches most of the sludge and scum; the second gives a further quiet zone so fewer solids reach the outlet and clog the soakaway or drain field.
Commonly 100–200 litres in homes, depending on fixtures and climate, and 40–70 litres per user in offices and schools where only toilets and handwashing are used.
No. The soakaway or drain field depends on soil percolation tests and the local code. This tool sizes the septic tank only.
Found a number that looks wrong, or a case the tool does not cover? Say so here — corrections are welcome and get answered.
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