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

This calculator estimates annual oil consumption and recommends a standard tank size based on your boiler output, estimated annual usage hours, and delivery frequency.

The formula

Litres per hour = Boiler output (kW) ÷ Boiler efficiency (90%) ÷ Energy content of kerosene (10.35 kWh/L)

Annual consumption = Litres per hour × Heating hours per year × Seasonal load factor (0.4)

Litres per delivery = Annual consumption × (Delivery weeks ÷ 52)

Why a load factor?

An oil boiler does not fire at its full rated output for every hour the heating is switched on — it cycles on and off (and modern condensing boilers modulate) to match demand. Metered hour-counter data shows the burner typically fires only around 40% of the time the heating is on across a heating season — more on the coldest days, much less in mild weather. Multiplying the full-rate burn by every “on” hour would over-state real consumption by roughly 2.5×, so this calculator applies a 0.4 seasonal load factor. The result for a typical 25 kW boiler running ~2,000 hours a year is around 2,000 litres — in line with real UK supplier and metered figures (and with our FAQ below). It is an estimate; actual use depends heavily on insulation, climate and thermostat settings.

Assumptions

• Boiler efficiency: 90% (modern condensing oil boiler)
• Kerosene energy content: 10.35 kWh per litre (BS 2869 Grade C2, 28-second kerosene)
• Seasonal load factor: 0.4 — the burner fires ~40% of the hours the heating is on
• Heating hours: Typically 1,500-2,500 hours/year depending on climate and property
• Tank buffer: 20% added to delivery volume for safety margin

Bund sizing

A bund (secondary containment) must hold 110% of the tank's full capacity. This is required by OFTEC Technical Book 3 and Building Regulations Approved Document J. Bunded tanks have this containment built in; for single-skin tanks, a separate bund wall or tray must be constructed.

Standards

Oil tank installations must comply with OFTEC standards, Building Regulations Approved Document J (Combustion appliances and fuel storage systems), and BS 5410 (Code of practice for oil firing). Installation must be carried out by an OFTEC registered technician.