BTU Calculator

Size every radiator in your home in minutes. No sign-in, no ads, works offline.

BTU Calculator — Size Your Radiators with Confidence

Stop guessing. BTU Calculator uses the UK standard heat-loss formula to tell you exactly how powerful a radiator you need for any room in your home.

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.

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. Switch back to metric and your original values are restored.

SAVE & REVIEW Name and save as many calculations as you like — one per room, or compare different insulation scenarios side by side. Swipe left to delete saved calculations. 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 17 or later required.