Get the output right
BTU calculator
A calm, honest calculation for period Irish homes. Sash windows count. Ceiling height counts. Guessing costs money.
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Estimated output needed
3462 W
Comfortable range: 10632 – 12994 BTU
This is an estimate based on typical rooms. For heat pumps, unusual rooms or critical installations, have a qualified heating engineer confirm the output.
We recommend choosing a radiator within 10% of the calculated figure. Over-sizing costs to run; under-sizing never catches up on a cold morning.
Your rooms
Save each room as you go and we'll add up the total output for your home.
Total: 0 BTU · 0 W
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Suggested radiators
Working out suitable radiators…
How we calculate
BTU = room volume (m³) × heat rate for the room type × insulation × glazing × window area × external walls × target temperature. Measurements in feet are converted to metres first. Watts = BTU ÷ 3.412. The recommended range is 10% either side of the result, and results are rounded to the nearest whole BTU.
| Heat rate (BTU per m³) | Living room 140 · Bedroom 112 · Bathroom 168 · Kitchen 126 · Hallway 126 |
|---|---|
| Insulation | Period property 1.25 · Average 1.0 · Modern 0.85 |
| Glazing | Single or sash 1.25 · Double 1.0 · Triple 0.9 |
| Window area | Small 0.95 · Average 1.0 · Large or patio doors 1.1 |
| External walls | None 0.9 · One 1.0 · Two 1.1 · Three 1.2 · Four 1.3 |
| Target temperature | Cooler (about 18°C) 0.9 · Standard for the room 1.0 · Warmer (about 23°C) 1.1 |
Suggested radiators are matched on their heat output at ΔT50, taken from the manufacturer's data for each size. A radiator is suggested when its output is between 90% and 125% of your result. The result is an estimate for typical rooms and should be confirmed by a qualified heating engineer for heat pumps, unusual rooms, critical installations or a whole-house specification.