What size septic tank do I need for a 3-bedroom house?

For a 3-bedroom house, the population equivalent is 4 (bedrooms + 1). Using the BS 6297 formula: C = 2000 + (180 × 4) = 2,720 litres. In practice, you would install a 2,800-litre or 3,000-litre tank as the nearest standard manufactured size. This assumes annual desludging. For biannual desludging, the minimum is C = 2000 + (90 × 4) = 2,360 litres.

What is the BS 6297 formula for septic tank sizing?

The BS 6297 formula for minimum septic tank capacity with annual desludging is: C = 2000 + (180 × P) litres, where P is the number of people (population equivalent). The 2000 litres provides the base settlement volume, and 180 litres per person accounts for annual sludge accumulation. For biannual desludging, the sludge factor reduces to 90 litres per person.

Can a septic tank still discharge to a watercourse?

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.

How often should a septic tank be desludged?

Septic tanks should be desludged at least once per year for the standard BS 6297 sizing formula. If you desludge more frequently (biannually), a smaller tank capacity is acceptable. Regular desludging is essential to prevent the tank from overflowing, which can block the drainage field and cause expensive repairs. A tanker visit typically costs £150-£300.

What is a drainage field and how big does it need to be?

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.

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

How We Calculate This

This calculator uses the BS 6297 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

C = 2000 + (180 × P) litres (for annual desludging)

C = 2000 + (90 × P) litres (for biannual desludging)

Where P is the population equivalent (number of people). The 2,000-litre base provides the minimum settlement volume, and the per-person factor accounts for sludge accumulation between desludging visits.

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. The area is divided into trenches, each a maximum of 15m long and 600mm wide at the base, with a minimum of 2 trenches. Trenches should be spaced at least 2m apart (centre to centre).

Standards

Septic tank sizing must comply with 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. Septic tanks must comply with BS EN 12566-1.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.