Radiator datasheets and heat-loss calculators use two units and a temperature basis that is easy to miss. This page explains how watts and BTU/h relate, what ΔT50 means, and what happens to a radiator's output when the temperature difference is not 50.
Watts and BTU/h measure the same thing
Both units describe a rate of heat output. Watts (W) are the SI unit; BTU/h (British thermal units per hour) is the alternative unit you will also see on radiator figures. The calculator uses one conversion factor everywhere:
1 W = 3.412142 BTU/h
The International Table BTU (BTU_IT) is defined as 1055.05585262 J. A watt is one joule per second, or 3600 J per hour, so 1 W = 3600 ÷ 1055.05585262 BTU/h ≈ 3.412142 BTU/h, rounded to six decimal places. Source: the International Table definition of the BTU, with tabulated values in NIST SP 811 (Appendix B).
To convert:
- watts to BTU/h: multiply by 3.412142;
- BTU/h to watts: divide by 3.412142.
| Watts | Exact BTU/h | Shown as |
|---|---|---|
| 500 W | 1706.071 | 1706 BTU/h |
| 550 W | 1876.6781 | 1877 BTU/h |
| 850 W | 2900.3207 | 2900 BTU/h |
| 1000 W | 3412.142 | 3412 BTU/h |
Going the other way, 3412.142 BTU/h divided by 3.412142 is exactly 1000 W. The calculator includes a two-way converter for this.
Why BTU/h is shown as a whole number
The calculator rounds the room total up to the nearest 50 W first, and only then converts it to BTU/h. The BTU/h figure is rounded to the nearest whole number, with .5 rounded up, and shown without decimals. It is never calculated from the unrounded total, so the watt and BTU/h figures on screen always describe the same rounded value.
When you compare a result with a datasheet, use the unit the datasheet uses. If one figure is in watts and the other in BTU/h, convert with the factor above rather than estimating.
What ΔT50 means
ΔT (delta T) is the difference between the average water temperature in the radiator and the room temperature:
ΔT = (flow temperature + return temperature) ÷ 2 − room temperature
Here flow is the temperature of the water entering the radiator and return is the temperature of the water leaving it. ΔT50 means the average water temperature is 50 °C above the room.
Under BS EN 442, radiator heat output is quoted at ΔT50. Because every manufacturer uses the same basis, ΔT50 figures let you compare one radiator with another directly. It is also the basis for the calculator: the room figure is meant to be compared with radiator outputs at ΔT50.
The practical point: check that a datasheet's output really is quoted at ΔT50. A figure at a different ΔT is not directly comparable with a ΔT50 figure from another catalogue.
How output changes at a different ΔT
A radiator gives out less heat when the temperature difference is smaller. The relationship used in BS EN 442 is:
Q = Q50 × (ΔT ÷ 50)^n
- Q50 is the catalogue output at ΔT50;
- ΔT is the temperature difference you want to estimate for;
- n is the radiator exponent from the manufacturer's datasheet. If the datasheet does not give it, the calculator uses 1.3.
Because the exponent is greater than 1, output falls faster than the ΔT itself. Halving the temperature difference more than halves the output.
Worked example
Take a radiator rated at 1000 W at ΔT50, with n = 1.3, and estimate its output at ΔT 30:
- ΔT ÷ 50 = 30 ÷ 50 = 0.6
- 0.6^1.3 ≈ 0.51475
- 1000 × 0.51475 ≈ 514.75 W
That 514.75 W is an intermediate, continuous value. The calculator applies the same rounding rule it uses for rooms, rounding up to the nearest 50 W, so it shows 550 W, and the BTU/h figure is calculated from that: 550 × 3.412142 = 1876.6781, shown as 1877 BTU/h. It does not show 515 W as the result.
What the correction does and does not change
The ΔT correction in the calculator is an educational tool. It shows how a catalogue figure scales with the temperature difference. It does not change the room heat-loss estimate, which is always meant to be compared with outputs at ΔT50. The Methodology page describes both calculations.
The result also depends on the exponent. A datasheet value of n is better than the 1.3 default, so use it when the manufacturer provides one.
A quick checklist for reading a datasheet
- Find the output figure and its unit: W or BTU/h.
- Confirm the figure is quoted at ΔT50 (BS EN 442).
- If it is in a different unit from your room estimate, convert with 3.412142.
- If it is quoted at a different ΔT, use the exponent from the same datasheet to scale it, or use the calculator's correction tool.
- Compare it with the room figure from the calculator, which is an indicative estimate rather than a design value.
Sources
Figures and formulas on this page come from the calculator's specification, which is based on: the International Table definition of the BTU (1055.05585262 J) and NIST SP 811 (Appendix B) for the watt to BTU/h conversion (1 W = 3.412142 BTU/h); BS EN 442 for the ΔT50 rating basis and the output relationship Q = Q50 × (ΔT ÷ 50)^n; and manufacturers' datasheets for the exponent n.
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).