Work out the heat a room needs
Fabric loss (U-value × area × temperature difference) plus air change loss (0.33 × air changes × volume × temperature difference). A quick estimate — not a full BS EN 12831 heat loss calculation.
−3 °C is a common UK design temperature; colder areas use lower figures. Room temperature and air changes fill in from the room type and can be changed.
| System | Factor | Watts | BTU/h |
|---|
An estimate for sizing radiators, not a full BS EN 12831 heat loss calculation. Check radiator outputs against the manufacturer's data; heat pump installs need a proper room-by-room calculation.
Figures used
Room temperatures, air change rates and typical U-values behind the calculator.
| Room | Design temp | Air changes / h |
|---|---|---|
| Living room | 21 °C | 1.5 |
| Bedroom | 18 °C | 1 |
| Kitchen | 18 °C | 2 |
| Bathroom | 22 °C | 3 |
| Hall / landing | 18 °C | 2 |
Air change rates are the older CIBSE Domestic Heating Design Guide figures, suited to draughty, naturally ventilated homes. A modern, airtight house may be well under 1 air change per hour — edit the figure if you know better.
| Element | U-value |
|---|---|
| Solid brick 220 mm, plastered | 2.10 |
| Solid brick 105 mm (half brick) | 3.00 |
| Cavity wall, unfilled | 1.47 |
| Cavity wall, insulated | 0.50 |
| Timber frame / modern insulated | 0.29 |
| Single glazed (wood / PVCu) | 5.00 |
| Single glazed, metal frame | 5.80 |
| Double glazed (older) | 2.90 |
| Low-E double glazed | 1.70 |
| Triple glazed (about 1.0 or less) | 1.00 |
| Floor: suspended timber, ventilated void | 0.82 |
| Floor: solid floor on earth, uninsulated | 0.82 |
| Ceiling: loft, 270 mm+ insulation | 0.16 |
| Ceiling: loft, older / thin insulation | 0.71 |
| Ceiling: loft, uninsulated | 2.30 |
| Ceiling: flat roof, insulated | 0.70 |
| Ceiling: flat roof, uninsulated | 2.19 |
Typical manufacturer heat-loss figures for UK construction. Real U-values vary with thickness, age and condition.
Radiator sizes for low-temperature systems
Radiators are rated at ΔT50 (BS EN 442 test). At a lower water temperature they give out less heat, so the catalogue figure has to be bigger.
| System ΔT | Factor | ΔT50 rating for 1,000 W | in BTU/h |
|---|---|---|---|
| ΔT60 | 1.268 | 789 W | 2,691 |
| ΔT55 | 1.132 | 883 W | 3,014 |
| ΔT50 | 1.000 | 1,000 W | 3,412 |
| ΔT45 | 0.871 | 1,148 W | 3,917 |
| ΔT40 | 0.747 | 1,339 W | 4,568 |
| ΔT35 | 0.627 | 1,595 W | 5,442 |
| ΔT30 | 0.513 | 1,949 W | 6,651 |
| ΔT25 | 0.404 | 2,475 W | 8,446 |
Divide the heat the room needs by the factor to get the ΔT50 rating to look for in the catalogue. Example: a room needing 1,000 W on a heat pump running at ΔT30 needs a radiator rated about 1,950 W at ΔT50.
How the BTU figure is worked out
Heat leaves a room two ways: through the walls, windows, floor and ceiling (fabric loss), and with the air that leaks in and out (ventilation loss). The calculator adds both at the design conditions — the room at its comfort temperature and −3 °C outside.
Fabric loss
For each outside surface: U-value × area × (room temperature − outside temperature). The outside wall area is the wall length × room height, less the windows. Floors and ceilings next to another heated room are taken as no loss; unheated floors and lofts use the outside temperature, as in common manufacturer calculators.
Ventilation loss
0.33 × air changes per hour × room volume (m³) × temperature difference. 0.33 W is roughly the heat needed to warm one cubic metre of air by 1 °C in an hour.
Watts to BTU
1 W = 3.412 BTU/h, so 1 kW = 3,412 BTU/h. Radiator catalogues in the UK quote both.
ΔT50, ΔT40, ΔT30
ΔT is the mean water temperature minus the room temperature. A traditional boiler system at 75 °C flow and 65 °C return in a 20 °C room is ΔT50. Condensing boilers run more efficiently cooler, and heat pumps typically run ΔT30 or lower, so the radiators need to be larger — roughly double the ΔT50 size at ΔT30.
What this does not do
It ignores heat lost to unheated rooms next door, thermal bridges and the reduced loss to the ground for large floors, and it uses typical U-values. For a heat pump design, an MCS installation or a whole-house survey, use a full room-by-room heat loss calculation.
Questions heating engineers ask
How many BTU do I need for a room?
It depends on the size of the room, how many outside walls and windows it has, the insulation and the temperature you want. As an example, a 4 × 3.5 × 2.4 m living room at 21 °C with one unfilled cavity outside wall, 2 m² of older double glazing and heated rooms above and below needs about 810 W, roughly 2,750 BTU/h, on this method.
How do I convert watts to BTU?
Multiply watts by 3.412 to get BTU per hour. 1,000 W is about 3,412 BTU/h, and 2,000 BTU/h is about 586 W.
What size radiator do I need for a heat pump?
Work out the heat the room needs, then divide by the correction factor for the system ΔT: about 0.75 at ΔT40, 0.63 at ΔT35 and 0.51 at ΔT30. At ΔT30 the radiator's ΔT50 catalogue rating needs to be about twice the room's heat loss.
Is this a proper heat loss calculation?
No. It is a quick room estimate with typical U-values and air change rates. A heat pump design or an MCS installation needs a full room-by-room heat loss calculation to BS EN 12831 using the actual construction.
More free tools for plumbers & heating engineers
Boiler size calculator — combi, system or regular — kW guide› All free calculators for the trades›Quotes, jobs and certificates from the van
Quote on site, dispatch callouts, keep CP12s and installation certificates against each job, and give the crew a 4-button app for timesheets.
See Graftum for plumbing & heatingAn estimate, not a BS EN 12831 heat loss calculation; check radiator outputs with the manufacturer.