Septic Tank Sizing Calculator — What Size Septic Tank Do I Need?

This calculator uses the Approved Document H 2015 §1.18 formula to determine the minimum septic tank capacity and also calculates the required drainage field area based on the soil percolation rate.

Septic tank capacity formula (AD H 2015 §1.18)

C = 2,700 + 180 × (P − 4) litres, with a minimum of 2,700 litres (P = users)

Where P is the population equivalent (bedrooms + 1). A tank serving up to 4 users must be at least 2,700 litres; each user beyond 4 adds 180 litres. Desludging frequency does not reduce this minimum (the older BS 6297 "÷2 for biannual" rule is not in Approved Document H and would under-size the tank).

Drainage field sizing

Drainage field area = P × Vp × 0.25 m²

Where Vp is the percolation value (seconds per mm) determined by a percolation test — work yours out with our percolation test calculator, which also covers what a percolation test costs in the UK. The area is divided into trenches, 300–900mm wide at the base, with a minimum of 2 trenches (this tool uses ~15m per trench as a conservative default; BS 6297 permits up to 30m per leg, and AD H 2015 sets no trench-length limit). Per AD H 2015 §1.42, 2m of undisturbed ground must be maintained between parallel trenches; §1.40 limits the gradient to no steeper than 1:200.

Standards

Tank capacity and drainage-field area are set by Approved Document H 2015 (§1.18 capacity and §1.44 drainage-field floor area). Drainage-field design and installation should also follow BS 6297 (Code of Practice for the Design and Installation of Drainage Fields for Use in Wastewater Treatment). The installation must meet the General Binding Rules (England) 2020 for properties not connected to mains drainage, and the tank itself must comply with BS EN 12566-1.