Radiator BTU calculator
A quick estimate of the BTU output a room needs, by size, room type, glazing and insulation. Set the dimensions and drag the rest.
Your room
Wall to wall.
Wall to wall.
Floor to ceiling.
Required output
— a double panel, or split across two radiators.
Illustrative only. A standard trade rule-of-thumb, not a room-by-room heat loss survey. For an exact spec — especially on extensions or unusual rooms — get a Gas Safe engineer to run the full calculation.
Undersize a radiator and a customer's back with you again in a month complaining the room never gets warm. Oversize it and you've quoted more radiator than the room needs. This calculator uses the standard trade rule of thumb — room volume times a BTU-per-cubic-metre factor for the room type, adjusted for glazing and insulation — to give you a sizing figure to work from in seconds.
Why the factor changes by room
A living room and a hallway of the same volume don't need the same output. Rooms people sit still in for long stretches — lounges, dining rooms, bathrooms — get a higher factor, because comfort matters more when nobody's moving around. Hallways and landings get a lower one; people pass through, they don't linger. Conservatories sit well above everything else, because a wall of single-pane glass loses heat far faster than a plastered cavity wall ever will.
Glazing and insulation
The base figure assumes double glazing and average insulation for a typical UK home. Single glazing pushes the requirement up — more heat escapes through the glass, so the radiator has to work harder to hold the room at temperature. Triple glazing and better-than-average insulation (modern cavity fill, a well-insulated loft, retrofitted internal wall insulation) pull it back down. If a room's had recent insulation work, or it's an older solid-wall property with none, set the insulation control accordingly rather than leaving it on average.
A worked example
A 4m × 3.5m living room with a 2.4m ceiling is 33.6m³. At the living room factor of 140 BTU/m³, with double glazing and average insulation, that's 4,704 BTU/hr — comfortably a double panel radiator, or a longer single. Swap to single glazing and poor insulation — an older bay-windowed lounge, say — and the same room jumps to around 6,760 BTU/hr, likely needing two radiators or a larger double.
Where this falls short
This is a quick sizing tool, not a heat loss survey. It doesn't account for the number of external walls, floor type, extractor fans, or how exposed the property is. For a straightforward domestic radiator swap it's a solid starting point. For a new install, an extension, or anything where getting it wrong is expensive, run a proper room-by-room calculation (BS EN 12831) before you order radiators.
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