BTU Radiator Calculator

Now with a radiator guide and heat pump support. Enter room dimensions, get BTU/hr, Watts and kW, then see which radiator sizes fit — corrected for your flow temperature.

Stop guessing which radiator to order — get the exact BTU/hr, Watts, and kW a room needs before you buy.

BTU Calculator uses the UK standard heat-loss formula, covering conventional boilers and heat pump upgrades.

HOW IT WORKS Enter your room dimensions and a few details about the room's construction — wall type, glazing, what's above and below the room, and how many external walls are exposed. The app calculates heat loss through every surface and accounts for air changes, then adds a 15% safety margin to give you three figures: BTU/hr, Watts, and kilowatts.

RADIATOR GUIDE Every calculation comes with suggested radiator sizes: the smallest Type 11 (single panel), Type 21 and Type 22 (double panel) 600mm-high radiators that meet the requirement, using typical ΔT50 outputs. Always confirm against the manufacturer's datasheet.

HEAT PUMPS & FLOW TEMPERATURE Radiator outputs are quoted at ΔT50, but heat pumps run much cooler. Pick your system's ΔT — from ΔT50 for a conventional boiler down to ΔT25 for a low-temperature heat pump — and the radiator guide applies the EN 442 correction so the sizes it suggests will actually deliver the heat your room needs.

HEAT LOSS BREAKDOWN Tap the chart icon on the result card to see a breakdown of heat loss by component — glass, walls, ceiling, floor, and ventilation — with a percentage bar for each. Useful for spotting where insulation improvements would have the most impact.

ROOM PRESETS Start from a typical UK room — modern lounge, 1930s semi, Victorian terrace bedroom, new-build bedroom, kitchen extension, or family bathroom — then adjust the details.

ROOM TYPES • Living rooms (22 °C target temperature) • Bedrooms (19 °C target temperature) • Hallways and kitchens (17 °C target temperature) • Bathrooms (23 °C target temperature)

WALL & GLAZING OPTIONS Wall types: brick cavity, insulated brick cavity, 105mm solid brick, 220mm solid brick, timber frame Glazing: add as many windows as the room has, each with its own area and glazing type — double glazed (wood/plastic or metal frame), single glazed (wood/plastic or wood frame) Ceiling: heated room above, roof space (uninsulated, 50mm or 100mm+), flat roof (insulated or uninsulated) Floor: heated room below, wooden suspended floor, solid concrete floor

ENVIRONMENTAL ADJUSTMENTS

• North-facing rooms (+15% heat loss allowance)
• French / patio doors (+20% heat loss allowance)
• 1, 2, or 3 exposed external walls

METRIC & IMPERIAL Switch freely between metres and feet — every dimension and window area converts automatically, so the room stays the same size.

SAVE, RELOAD & SHARE Name and save as many calculations as you like — one per room, or compare different insulation scenarios side by side. Tap any saved entry to see its full inputs and result, reload it straight back into the calculator and update it in place, copy it, or share it as text. The History tab totals every saved room into a whole-house figure for boiler or heat pump sizing, and can export the lot as a CSV file. Your history is stored locally and persists between sessions.

NO SIGN-IN. NO ADS. NO DATA COLLECTED. All calculations run entirely on your device. Nothing is sent to any server. The app has no analytics, no tracking, and no network requests of any kind.

Built for iPhone and iPad, with a split-view layout on iPad. iOS 26 or later required.