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

For a 3-bedroom house the population equivalent is 4 (bedrooms + 1). Per Approved Document H 2015 §1.18 a septic tank must be at least 2,700 litres for up to 4 users (C = 2700 + 180×(P−4)), so a 4-person tank = 2,700 litres. In practice you would install the nearest standard manufactured size (e.g. 2,800 or 3,000 litres). Desludging frequency does not reduce the AD H minimum capacity.

Approved Document H 2015 §1.18: a septic tank should have a capacity below the inlet of at least 2,700 litres for up to 4 users, increased by 180 litres for each additional user — i.e. C = 2700 + 180×(P−4), with a 2,700 L floor. (This is the current statutory rule and supersedes the older 'C = 2000 + 180P' form sometimes attributed to BS 6297; desludging frequency does not reduce the required capacity.)

No. Since 1 January 2020 under the General Binding Rules (England), septic tanks can no longer discharge directly to a watercourse (stream, river, ditch). The effluent must be treated through a drainage field (soakaway) or you must upgrade to a sewage treatment plant. Existing septic tanks that discharge to a watercourse must be replaced or upgraded when the property is sold. See our Sewage Treatment Plant Calculator if you need to upgrade.

Septic tanks should be desludged at least once per year. Note the Approved Document H minimum capacity (2,700 L for up to 4 users, +180 L per extra user) is not reduced by more frequent desludging — under-sizing for a desludging schedule is the unsafe direction. Regular desludging prevents overflow that can block the drainage field and cause expensive repairs. A tanker visit typically costs £150-£300.

A drainage field (also called a soakaway system) is a network of perforated pipes buried in gravel-filled trenches that allows septic tank effluent to percolate into the surrounding soil for final treatment. The size depends on the number of residents and the soil percolation rate (Vp value). The formula is: Area = P × Vp × 0.25 m². A percolation test must be carried out to determine the Vp value before designing the drainage field.