Radiator BTU Calculator — Room Heating Requirements

This calculator estimates total steady-state heat loss from the room and converts it to BTU/h. It uses the standard CIBSE Guide A method: fabric loss Q = U × A × ΔT for each surface, plus ventilation loss Q = 0.33 × N × V × ΔT (where 0.33 W·h/m³·K is the volumetric heat capacity of air). The total heat loss determines the radiator output needed on a design-cold day.

Heat loss components

• Wall loss: Heat conducted through external walls, based on U-value and area (window area subtracted)
• Floor & ceiling loss: Heat conducted through the floor and ceiling, based on their U-values and the room footprint
• Window loss: Heat conducted through glazing, based on window U-value
• Ventilation loss: Heat lost to air changes, based on room volume and air change rate

Floor & ceiling assumption: floor and ceiling loss is derated by 50% because most rooms sit between heated spaces (mid-floor of a house, or above/below another heated room). If your room is on the ground floor over an unheated void, or on the top floor under a cold loft, the real floor or ceiling loss is higher — size the radiator upward, or get a full room-by-room heat-loss survey.

Choosing a radiator

UK radiator outputs are rated at ΔT50 (BS EN 442: flow 75°C, return 65°C, room 20°C — mean water 70°C, the legal rating basis since 2013). The recommended sizes use real ΔT50 catalogue outputs, so the suggested unit meets or exceeds the computed load. If you run a heat pump or low-temperature system, flow temperatures are cooler (ΔT30–40), which derates output by roughly 30–50% — choose a larger radiator accordingly.

Conversion

1 Watt = 3.412 BTU/h. The total watt heat loss is converted to BTU for radiator selection.