What Size Septic Tank Do I Need?

What is a Septic Tank?

A septic tank is an underground wastewater treatment system commonly used in rural and semi-rural UK properties. It stores wastewater, allowing solids to settle and undergo a natural breakdown process using anaerobic bacteria. The separated liquid, or effluent, then flows to a drain field where it soaks into the soil.

The main components of septic tanks are:

A tank that stores wastewater and separates solids from liquids.
Pipes that carry wastewater from your home to the tank and then to the drain field.
A drain field (or soakaway) that disperses treated effluent into the soil.

UK Septic Tank Size Chart

The size of your septic tank is typically based on the number of bedrooms and expected daily water usage. Below is a general UK size chart:

Responsive Table

Number of Bedrooms	Estimated Occupants	Tank Capacity (Litres)	Tank Capacity (Gallons)
1–2	1–3	2,700L	600 gal
3	4–5	3,800L	850 gal
4	5–6	4,500L	1,000 gal
5+	6–8	5,700L+	1,250+ gal

Tip: The Environment Agency recommends septic tanks no smaller than 2,700 litres for a single bedroom home.

How to Choose the Right Septic Tank Size in the UK

1. Property Size & Bedrooms

The number of bedrooms is a key factor in estimating your household’s peak water usage. More bedrooms = more potential occupants = a larger tank needed. On average, one person produces 100–150 litres of wastewater per day.

2. Water Usage

If you use washing machines, dishwashers, or take long showers daily, your water usage will be higher, requiring a bigger tank.

3. Soil Type & Drainage

If your soil drains poorly, your system will need a larger tank and possibly a bigger drainage field. A percolation test can help determine this.

4. UK Rules & Regulations

Under the Environment Agency’s General Binding Rules, septic tanks installed in England must:

Discharge only into a drainage field, not a watercourse.
Be regularly maintained and emptied.
Meet minimum size requirements based on property occupancy.
Failure to comply can result in fines of up to £20,000.

5. Future Planning

If you plan to extend your home or have more occupants in the future, choose a tank one size larger than your current needs. This saves on costly upgrades later.

Why Septic Tank Size Matters

Too small → Overflows, foul odours, and costly repairs.
Too big → Wasted money, inefficient breakdown of solids.
Correct size → Efficient operation, longer lifespan, and compliance with UK regulations.

Common Mistakes to Avoid

Ignoring regulations before installation.
Choosing the cheapest option rather than the right size.
Placing the tank too close to trees or driveways, risking root damage and compaction.

Conclusion & Expert Help

Septic tanks are essential for wastewater management in many UK homes. Choosing the right size ensures efficient operation, saves money, and prevents environmental harm. For expert septic tank advice, installation, and maintenance, contact Fast Drains — we handle everything from sizing to compliance checks.