Septic Tank Size Calculator
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Septic Tank Size Calculator
Work out the minimum septic tank capacity for a property from the number of users or bedrooms, plus the daily wastewater flow and organic load. Built on UK Building Regulations (Approved Document H2) and British Water Flows & Loads / BS 6297.
Minimum septic tank capacity
Approved Document H2 (1.18): at least 2,700 litres for up to 4 users, plus 180 litres for each additional user. Cross-checked against the British Water / BS 6297 formula.
Daily flow & organic load
The figures you need for a discharge/permit assessment: hydraulic flow at 150 litres per person per day and organic load at 60 g BOD per person per day (British Water Flows & Loads).
A septic tank only settles solids — it must discharge to a drainage field for secondary treatment (it cannot discharge directly to a watercourse). Drainage field floor area At = p × Vp × 0.25 (m²), where p = population and Vp = the percolation value from a soil test.
Reference & rules
The sizing bases and the key compliance points for off-mains foul drainage.
| Users | AD H min. capacity | BS 6297 (150P+2000) |
|---|
Sizing bases. Approved Document H: 2,700 L for ≤4 users, +180 L per extra user. British Water / BS 6297: capacity = 150 × population + 2,000 L, minimum 2,700 L (assumes annual de-sludging). The larger figure governs.
Population. Single dwelling = bedrooms + 2, minimum 5 people. Size for maximum occupancy, not current residents; children count as adults.
Compliance. A septic tank must discharge to a drainage field/soakaway, never directly to a watercourse (England & Wales General Binding Rules). It needs two chambers, ventilation, and at least annual emptying by a licensed contractor. Install ≥7 m from a habitable building where possible and keep clear of water supplies. A percolation test and drainage-field design are required.
This septic tank size calculator works out the minimum septic tank capacity for a property from the number of users or bedrooms, plus the daily wastewater flow and organic load. It follows Approved Document H2 and the British Water Flows & Loads / BS 6297 method. The rules for using a septic tank without a permit are the Environment Agency's general binding rules.
Minimum septic tank capacity
Approved Document H2 sets a minimum of 2,700 litres for up to four users, plus 180 litres for each additional user, cross-checked against the British Water / BS 6297 formula. Enter the design population directly, or estimate it from bedrooms (population = bedrooms + 2, minimum 5). The calculator returns the minimum capacity.
Daily flow and organic load
For a discharge or permit assessment you also need the hydraulic flow (150 litres per person per day) and the organic load. The calculator gives both, so you have the figures the Environment Agency or your designer will ask for.
After the tank — the drainage field
A septic tank only settles and partially treats the effluent; it then has to discharge to a drainage field sized from a percolation test. Use our drainage field calculator for that, and check whether you need a percolation (infiltration) test first.
Further reading: general binding rules for small sewage discharges · BS 6297 infiltration testing for foul drainage fields · do you need infiltration testing?.
Frequently asked questions
At least 2,700 litres for up to four people, plus 180 litres per extra person, under Approved Document H2. The calculator confirms the figure from your population or bedrooms.
Not if you meet the Environment Agency's general binding rules. If you cannot, you need an environmental permit. The rules also require discharge to a drainage field, not a watercourse.
British Water Flows & Loads takes population as bedrooms plus two, with a minimum of five. The calculator uses this when you size by bedrooms.
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