Sizing a radiator comes down to two numbers: an estimate of how much heat the room loses, and the output a radiator catalogue quotes at ΔT50. This guide walks through what to measure, what to enter in the calculator and how to read the result, using one worked example from start to finish. You can follow along in the calculator.
Step 1: Measure the room
You need the length, width and ceiling height of the room in metres. If you only know the height roughly, the calculator starts from 2.4 m.
The method covers one rectangular room at a time. Every dimension has to be greater than zero; a zero or negative length is rejected rather than producing a misleading figure.
Step 2: Count the walls that face outside
Heat escapes mainly through surfaces that face the outdoors or an unheated space, so the calculator needs to know how much external wall there is.
In Simple mode you only choose how many walls are external, from 0 to 4. The calculator turns that into a length of external wall:
- 0 walls: no external wall;
- 1 wall: the longer side of the room;
- 2 walls: length + width;
- 3 walls: length + twice the shorter side;
- 4 walls: the whole perimeter.
Simple mode also assumes the windows take up 25% of the external wall area. For a 4 m × 3 m room, 2.4 m high, with two external walls, that gives 7 m of external wall and 4.2 m² of glazing.
In Advanced mode you enter the external wall length and the actual glazing area yourself. The glazing area cannot be larger than the external wall area.
Step 3: Choose the room type and glazing
The room type sets the indoor temperature the estimate aims for and the default air-change rate. A living room, for example, starts at 21 °C and 1.0 air change per hour; a bathroom at 22 °C and 2.0. The page What affects a room's heat loss explains these defaults and when to change them.
Then pick single, double or triple glazing, or enter your own window U-value in Advanced mode.
Step 4: Say what is above and below
A heated room above or below contributes nothing to the heat loss. An unheated space, a roof open to the outside or the ground does. Simple mode assumes a heated room above and the ground below; Advanced mode lets you change both.
Step 5: Read the result — a worked example
Here is the reference example from the calculator's specification: a living room 4 m × 3 m × 2.4 m, with 7 m of external wall containing 3 m² of double glazing, a heated room above and a ground floor below. The indoor temperature is 21 °C, the outdoor design temperature is −3 °C and the ground is taken as 10 °C. Default U-values apply: wall 0.60, double glazing 2.8 and floor 0.80 W/(m²·K).
| Component | Calculation | Heat loss |
|---|---|---|
| External wall (13.8 m² after removing the window) | 13.8 × 0.60 × 24 | 198.72 W |
| Window | 3 × 2.8 × 24 | 201.6 W |
| Floor | 12 × 0.80 × (21 − 10) | 105.6 W |
| Ceiling (heated room above) | — | 0 W |
| Ventilation (28.8 m³, 1.0 ACH) | 0.33 × 28.8 × 1.0 × 24 | 228.096 W |
| Total | 734.016 W |
The temperature difference of 24 is simply 21 − (−3). Next, the calculator adds the 10% planning allowance: 734.016 × 1.10 = 807.4176 W. Nothing has been rounded so far.
Only now is the total rounded, and always up to the nearest 50 W: 807.4176 W becomes 850 W. A total that is already an exact multiple of 50 stays as it is, so exactly 500 W remains 500 W.
Finally, the rounded figure is converted: 850 × 3.412142 = 2900.3207, shown as 2900 BTU/h. The BTU/h figure is always calculated from the rounded watts, never from the unrounded total.
Step 6: Compare with radiator outputs at ΔT50
UK radiator catalogues quote heat output at ΔT50 under BS EN 442. Compare the room figure, 850 W or 2900 BTU/h in the example, with that ΔT50 figure, and make sure you are comparing like with like: a watt figure with a watt figure, or BTU/h with BTU/h.
If a datasheet quotes output at a different temperature difference, convert it first. The page Watts, BTU/h and ΔT50 explains how and includes the calculator's educational correction tool.
Checks before you rely on the figure
Look at the breakdown. In the example, ventilation is the largest single item. If one component dominates, check the input behind it first.
Decide on the allowance. The 10% allowance is a planning margin, adjustable from 0% to 25% in Advanced mode. Set to 0%, the same room gives 750 W (2559 BTU/h) instead of 850 W.
Check the units. All dimensions are entered in metres. Confirm that each figure is in metres before you read the result.
Use real values when you have them. The wall, floor and ceiling U-values are indicative averages. An EPC or builder's specification may give you better figures to enter in Advanced mode.
Know the limits. The estimate covers one rectangular room and does not count heat gains. For anything beyond planning a purchase, use a professional heat-loss calculation. The full method and its assumptions are on the Methodology page.
Sources
Figures and formulas on this page come from the calculator's specification, which is based on: CIBSE Guide A and the Open University's OpenLearn material for the heat-loss and ventilation formulas; BRE RdSAP 10 Table 24 as the reference for window U-values (2.8 is the table's value for older 12 mm double glazing; that row also covers an unknown installation date); MCS references, based on CIBSE guidance, for the indoor and outdoor design temperatures; NIST SP 811 for the conversion 1 W = 3.412142 BTU/h; and BS EN 442 for the ΔT50 basis of radiator outputs.
Disclaimer
This calculator provides an indicative room heat-loss estimate for planning radiator purchases. It is not a substitute for a qualified heating engineer’s design, a Gas Safe registered installer’s advice, or an MCS-compliant heat-pump system design. Radiator outputs should be compared at ΔT50 (BS EN 442).