BTU Calculator

Heat loss breakdown by component. Enter dimensions and get BTU/hr, Watts, and kW. Save, reload, and share calculations. Metric or imperial. No sign-in needed.

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.

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 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: 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/inches. Dimensions you haven't changed are converted automatically; any value you've edited by hand is kept exactly as entered.

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, or share it as text. Swipe to delete. 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. iOS 26 or later required.