Approximate BTU calculator by build era.
Get a quick, era-aware estimate of the heat output your radiators or heat-pump emitters need to keep a room warm — without measuring every window and door. Pick your room dimensions, the decade your house was built, and the flow temperature your heat source runs at.
1Room dimensions
m
m
m
2Room type
3Heat-loss adjustments
4When was the property built?
2003 onwards — Part L 2002/2006 era.
Improved cavity insulation, double glazing standard, basic loft top-up.
Wall
0.35 W/m²K
Roof
0.20 W/m²K
Window
2.00 W/m²K
Floor
0.25 W/m²K
5Heat output required
Heat the room needs
5,840BTU/h
1,712 W · actual heat loss — unchanged by ΔT
Catalogue rating to buy@ ΔT50
5,840BTU/h
1,712 W · figure to look for on the spec sheet
Room volume38.9 m³
ΔT factor× 1.00
6Radiator flow temperature (ΔT)
ΔT50×1.00
ΔT40×0.75
ΔT30×0.51
ΔT25×0.41
Default
ΔT50 — gas & oil boilers. Standard rating. Use for combi, system and conventional boilers at ~70–75°C. Heat pumps should not be sized at ΔT50.
Room volume38.9 m³
Era multiplier× 95 BTU/m³ (2000s)
Room-type factor× 1.00 (Lounge)
Adjustments× 1.00
ΔT correction× 1.00 (ΔT50)
Catalogue rating5,840 BTU/h
Your room list
0 rooms- No rooms saved yetConfigure a room above, name it, and tap Add this room.
Important
This is a rough indicative estimate only. It uses generalised volumetric coefficients and does not perform a room-by-room heat-loss calculation to BS EN 12831 / MCS 3005.
- U-values shown are typical Building Regulations minimums of the era, not your actual fabric.
- Assumes 21°C internal and –3°C external (South of England design temperature).
- For heat pumps, always size at the actual design ΔT (ΔT25–30), never at ΔT50.
- Retrofitted insulation or single-glazed bays will materially change the result.
/01
Why decade matters
The U-values legally required of UK homes have tightened dramatically since 1965. A pre-war solid-wall semi can lose nearly three times the heat of a 2015-built equivalent of the same volume.
/02
Why ΔT matters
Radiators are rated at the temperature difference between the water inside and the air outside them. Drop that difference and output falls fast — which is why heat-pump-ready emitters are physically larger.
/03
Need a real survey?
Our trade partners offer full MCS heat-loss surveys from £180. Get a room-by-room report with emitter schedule, pipework sizing and pump duty — book through your trade account.
