Pro Tool
Calculate the BTU heating requirement for any UK room. Radiator sizing, heat loss, energy costs. Factor in insulation, glazing, and room dimensions. Instant results.
Calculate Your SDLT — FreeBTU (British Thermal Unit) measures heat energy. One BTU is the energy needed to raise 1 pound of water by 1 degree Fahrenheit. In property terms, the BTU rating tells you how powerful a radiator or heater needs to be to warm a room to a comfortable temperature. Undersizing means a cold room; oversizing wastes energy and money.
The basic formula starts with room volume (length x width x height) and multiplies by a factor based on insulation and glazing quality. However, other factors matter too — number of external walls, floor type, room orientation, and room use. Our calculator handles all of these automatically.
A typical UK bedroom (3m x 4m, 2.4m ceiling, double glazed, one external wall) needs approximately 3,500-4,500 BTU. Larger bedrooms, poor insulation, or multiple external walls increase this. Our calculator gives you the exact figure for your specific room.
Significantly. Double glazing roughly halves the heat loss through windows compared to single glazing. For a room with a large window, upgrading from single to double glazing can reduce the BTU requirement by 15-25%, saving on both radiator size and running costs.
It depends on the room's BTU requirement, how many hours you heat it, and your fuel type. A typical living room (6,000 BTU) heated 8 hours/day on gas costs roughly £180-£250/year at 2025 tariffs. Electric heating costs 3-4x more per unit. Our tool calculates the exact figure for your rooms.