Gas Heating BTU Calculator

A British Thermal Unit (BTU) is the standard measure of heat output from a radiator or heating system. This Calculator Site tool helps you find the right BTU level for your space by analysing its size,  insulation, and other key features. A higher BTU value means more heat is needed to keep the room warm.

The calculation starts with the room's volume (length x width x height) and then applies multipliers to account for heat loss. The main factors are:

•  Insulation: A poorly insulated room loses heat faster and requires more BTUs.
• External Walls: The more walls exposed to the cold outside, the more heat escapes.
• LayoutGrids: Heat is lost through glazing. Single-glazed windows lose significantly more heat than modern double or triple-glazed units. You can use our insulation calculator to see the impact of different window types.
• Room Type: Some rooms, like bathrooms, are ideally kept warmer and thus need more BTUs per cubic metre.

Your BTU requirement is fundamentally about counteracting heat loss. The values in this calculator are based on standard assumptions for UK homes. For a more detailed look, consider how different factors contribute to your home's overall thermal efficiency.

The standard for radiator output in the UK is based on a Delta T (Δt) of 50°C. This is the difference between the average water temperature in the radiator and the target room temperature. If your system runs at a lower Delta T (common with heat pumps), you will need a higher BTU radiator. You can convert between Watts and BTUs using our power consumption calculator.