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Radiator BTU calculator: heat loss for a room and the rad size to fit
Work out how much heat a room loses on a cold day and what size rad replaces it. It uses the room-by-room method MCS and CIBSE use, the same approach as TradeCalc in the Hard Hat Way app: fabric loss through the outside wall, windows, floor and ceiling, plus ventilation, at a design temperature for the type of room. It then converts that to a ΔT50 catalogue figure for the flow temperature you run, so it works for a condensing boiler at 55 °C or a heat pump as well as a boiler at 75 °C.
What you need
Heat loss
- Design temperature
- 21 °C in, -3 °C out ΔT 24 K, 1.0 air changes an hourMIS 3005 room temperatures; CIBSE design day
- Heat required
- 916 Wabout 3,126 BTU/hr, including a 10% margin
- Heat required
- 3,126 BTU/hr
- Heat per m² of floor
- 76 W/m²
- Fabric loss: outside wall
- 274 W7.6 m² of outside wall after the windows at U 1.5U-values: OpenLearn energy in buildings; Part L 2021
- Fabric loss: windows
- 130 W2 m² at U 2.7
- Fabric loss: ceiling
- 0 Wheated room above, no loss
- Fabric loss: floor
- 202 WU 0.7
- Vent loss
- 228 W0.33 × 28.8 m³ × 1 ACH × 24 KCIBSE-style room air-change rates
Radiator
- ΔT50 equivalent output
- 916 Wthe catalogue figure to buy against (75 °C flow, 65 °C return, 20 °C room)BS EN 442 ΔT50; Stelrad / Zehnder correction factor (ΔT/50)^1.3
- Suggested rad
- 600 × 600 K2 (1,020 W at ΔT50)double panel, double convector (Type 22), 600 mm high, next catalogue length upK2 ≈ 1,700 W/m at 600 mm high, typical; check the maker’s table
- Or a K1 (Type 11, single panel)
- 600 × 1000 K1 about 980 W/m at 600 mm high
- Or a K3 (Type 33, triple panel)
- 600 × 400 K3 about 2,300 W/m at 600 mm high, typical
- K2 at other heights
- 500 × 700 or 700 × 500 output scales roughly with height
Fittings
- Pipe size to the rad
- 15 mm15 mm carries about 6 kW at 1 m/ssquote pipe sizing (15 mm 6–7 kW, 22 mm 14–16 kW)
- Rad valves
- 1 TRV + 1 lockshield per rad, plus tails
- Brackets
- 4 bracketsas supplied with the rad
- Add to the boiler load
- 0.92 kWthis room’s share of the whole house
Round up to whole packs and bags. Figures include the wastage you chose and match TradeCalc in the app.
For plumbers: turn this into a quote
In the app, TradeCalc hands these quantities straight to ProQuote with your regional material prices, and the customer gets a branded PDF. Free to download.
Or claim your free profile link nowHow the maths works
- Design temperatures are 21 °C living and dining rooms, 18 °C bedrooms, kitchens and halls, 22 °C bathrooms (MCS / CIBSE) against a −3 °C design day, so a living room is designed for a 24 K difference and a bedroom for 21 K.
- Fabric loss is area × U-value × temperature difference. Wall U-values in W/m²K: solid 2.1, unfilled cavity 1.5 (most 1930–1990 houses), filled cavity 0.55, 2006-on or insulated 0.3, timber frame 0.4 (OpenLearn; Part L 2021). Glazing: single 4.8, pre-2002 double 2.7, modern low-E 1.4, triple 1.0.
- The outside wall area is the length of wall facing outside × ceiling height, less the windows. Ceiling and floor loss depend on what is above and below: nothing for a heated room, 0.16 for a 270 mm loft, 2.0 for a bare loft, 0.7 for an uninsulated solid floor, 0.8 for suspended timber.
- Vent loss is 0.33 × room volume × air changes an hour × temperature difference, with CIBSE-style rates of 1.0 for living rooms, 0.7 for bedrooms and 1.5 for kitchens, bathrooms and halls. A north-facing room gets 10% more; the optional 10% margin covers warm-up after a setback.
- Watts × 3.412 gives BTU per hour.
- Catalogue outputs are at ΔT50 (BS EN 442: 75 °C flow, 65 °C return, 20 °C room). At other flow temperatures the output is scaled by (ΔT/50)^1.3 (Stelrad, Zehnder): at 55 °C flow a rad gives about half (× 0.51); at 45 °C about a third (× 0.30). The ΔT50 equivalent is the figure to buy against.
- Rad lengths at 600 mm high: K1 about 980 W/m, K2 about 1,700 W/m (a Stelrad Compact K2 600 × 1000 is 1,600–1,730 W), K3 about 2,300 W/m, rounded up to the next catalogue length (400 to 2,000 mm). Check the maker’s own table before you order.
- 15 mm pipe is fine to about 6 kW; use 22 mm above that.
Worked example
A 4 m × 3 m living room with 2.4 m ceilings, 4 m of unfilled cavity wall, 2 m² of older double glazing, an uninsulated solid floor and a heated room above loses about 916 W with the 10% margin, which is 3,126 BTU/hr. A 600 × 600 K2 (1,020 W at ΔT50) covers it on a boiler at 75 °C; at 55 °C flow you would buy against 1,780 W and go to a 600 × 1100 K2.
Questions people ask
How many BTU does my room need?
It depends on the room type, how much wall faces outside, what the wall and windows are made of and what is above and below, which is why the calculator asks. As a rough check, the example room works out at about 76 W (260 BTU) per m² of floor with unfilled cavity walls and older double glazing; a solid-walled room with single glazing needs roughly double, and a modern insulated room half.
Why is this different from other BTU calculators?
Many quick calculators assume every room is the same. This one uses the MCS room temperatures (21 °C living, 18 °C bedroom, 22 °C bathroom), asks what the wall really is and what is above and below, and no longer assumes an insulated floor and ceiling in every room. The defaults are a typical 1930–1990 house: unfilled cavity walls, pre-2002 double glazing and a bare solid floor. If your house is insulated, pick those options and the number drops a lot.
What does ΔT50 mean on a radiator?
Catalogue outputs are tested to BS EN 442 at ΔT50: water at a mean of 70 °C (75 °C flow, 65 °C return) in a 20 °C room. Run cooler and the same rad gives less: about 75% at 65 °C flow, 51% at 55 °C and 30% at 45 °C, using the (ΔT/50)^1.3 correction that Stelrad and Zehnder publish. Pick the flow temperature above and the calculator turns your heat loss into the ΔT50 figure to buy.
What size rad for a heat pump?
A heat pump runs at 45 to 55 °C flow, so a rad gives roughly a third to a half of its catalogue output. Divide your heat loss by 0.30 (45 °C) or 0.51 (55 °C) and buy a rad rated at that figure at ΔT50, which usually means going one or two sizes up in the same range, or from a K1 to a K2 or K3 on the same brackets. An MCS installer will do this room by room as part of the survey.
K1, K2 or K3?
K1 (Type 11) is a single panel with one convector; K2 (Type 22) is two panels and two convectors and gives about 1.7 times the heat for the same wall space; K3 (Type 33) gives about 2.3 times but sticks out further. If the length you need is longer than the wall, go up a type or a height rather than along.
Do I need a heating engineer to fit a radiator?
Swapping or adding a rad on an existing wet system is plumbing work, though the system needs draining, refilling and balancing, and on a sealed system repressurising. Anything on the gas boiler itself must be done by a Gas Safe registered engineer.
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