Enter your room dimensions (width, height, depth), then fill in window area, room type, wall construction, and window type — the Radiator Capacity Calculator works out your BTU required, watts, radiator area, and Delta T needed to heat the space.
BTU Required
--
Watts Required
--
Radiator Area Required
--
Delta T
--
BTU (British Thermal Unit) measures the heat output of radiators. It's approximately the energy required to heat 1lb of water by 1°F. Calculating the correct BTU output ensures your radiator provides adequate heating for your room size and conditions.
BTU calculation considers room volume, insulation quality, window area, construction materials, and external walls. Our calculator factors in all these variables to provide an accurate BTU requirement for optimal heating performance.
Both measure heat output but use different units. BTU is traditional in heating applications, while Watts is the international standard. The conversion is: 1 Watt = 3.412 BTU/hr. Both values help you select the right radiator size.
Better insulation reduces heat loss, lowering BTU requirements. Poor insulation in walls, windows, floors, or ceilings increases heat loss, requiring higher capacity radiators to maintain comfortable temperatures.
Type 11 (K1) radiators suit lower BTU needs, Type 21 and Type 22 (K2) handle medium requirements, while Type 33 (K3) radiators provide maximum heat output for high BTU demands or poorly insulated spaces.
Windows are major heat loss sources, especially single-glazed ones. Larger window areas and poor glazing significantly increase BTU requirements. Double-glazed windows reduce heat loss compared to single-glazed alternatives.
Mean water temperature typically ranges from 60-80°C depending on your boiler system. Higher water temperatures provide more heat output per radiator size. Most modern systems operate around 70°C mean water temperature.
Undersized radiators won't adequately heat your room, leading to discomfort and higher energy bills. Oversized radiators waste energy and money. Proper calculation ensures optimal comfort, efficiency, and cost-effectiveness.