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Heating Calculator
Calculate your BTU for each room
ΔT basics
What does Delta T mean: Delta T (ΔT) is the temperature difference used to rate radiator output. The UK standard domestic specs use ΔT50, which means the radiator average ~70°C, room 20°C. Higher ΔT like ΔT60 makes outputs look larger but isn’t UK‑standard and can lead to undersized radiators and cold rooms. If it is not clear always ask what Delta T ratings the company is advertising at, we quote all our outputs at ΔT50. Remember if a quoted output is much larger for a given size than one on our site it will be most likely due to the competitor wrongly advertising using a higher ΔT rating. Always compare at the same ΔT (UK: ΔT50).
What affects heat loss most?
- Insulation & walls: Older solid or uninsulated cavity walls lose heat faster than insulated cavity or modern timber frame.
- Windows: Big window areas and single glazing push outputs up; double glazing helps.
- Exposure: Corner rooms, rooms under unheated roofs, or floors over garages/voids need more output.
- Room use: Bathrooms and busy kitchens often run hotter; living rooms need steady comfort.
Our calculator uses ΔT50 (the UK standard) and lets you refine for windows, walls and what’s above/below the room.
Delta T Output Converter
Adjusted output: — BTU
Using factors from ΔT50 table: factor(target) ÷ factor(current).
How to use the Delta T converter
Start by entering the radiator’s rated output into “Rated output” and choose the unit (BTU or Watts).
Select the Current ΔT that the spec uses; our product pages list outputs at ΔT50 as standard.
Choose your Target ΔT (for example ΔT30 for a low-temperature system). The widget instantly shows the adjusted output.
Unlike BTU (British Thermal Unit), which measures the total heat required to warm a room, the Delta T (ΔT) value is a measurement that represents the temperature difference between the radiator’s output and the surrounding room temperature.
Think of it this way:
- BTU will indicate how much heat is needed.
- BTU/hr or Watts are the units of output.
- ΔT is the test condition used to calculate the heat output.
Electric radiators and ΔT
Electric radiators are rated by electrical input in watts and do not use ΔT test ratings (e.g., ΔT50). If you want a BTU figure for comparison, convert watts to BTU/h (BTU/h ≈ watts × 3.412). Use our heating calculator to size electric radiators by the required watts for your room, then pick products whose element wattage meets or exceeds that value.Delta T Conversion Information
The standard Delta T value for domestic gas boilers is typically ΔT50. This provides a reliable estimate of how well a radiator will heat a room in standard conditions. Be sure to look for this Delta T value, and also look out for the MARC logo (Manufacturers Association of Radiators and Convectors). You can also request the Declaration of Performance, which is done by an accredited body.
ΔT60, was previously used but is no longer permitted for domestic sales in the UK.
ΔT30, is sometimes used for low-temperature heating systems like underfloor heating. Below are common Delta T values and their correction factors:
How to Calculate Delta T Manually
To calculate Delta T manually, subtract the room temperature from the radiator's output temperature. For example, if the radiator output is 70°C and the room temperature is 20°C, the Delta T is 50°C.
Correction factors relative to ΔT50
ΔT50 is the current UK standard. All of our radiators on offer are compliant with this. To find a different ΔT rating simply multiply the ΔT50 rating on our website, by the correction factor. For ΔT30 (low temperature system) for example, take the ΔT50 rating and *0.51.
A simple sense check, a radiator with 5000BTU at ΔT50 * 0.51 = 2550 BTU at ΔT30. The resulted output is lower at ΔT30, which is expected, meaning you will need a bigger radiator to omit the required amount of heat.
When buying a radiator, always make sure it will supply the correct amount of heat for your room, and that it is within ΔT50 standard. Our heating calculator can help calculate this by giving you the heat output of your radiator.
| Delta T | Correction factor (vs ΔT50) |
|---|---|
| 75° | 1.69 |
| 70° | 1.55 |
| 65° | 1.41 |
| 60° | 1.27 |
| 55° | 1.13 |
| 50° | 1.00 |
| 45° | 0.87 |
| 40° | 0.75 |
| 35° | 0.63 |
| 30° | 0.51 |
| 25° | 0.41 |
| 20° | 0.30 |
| 15° | 0.21 |
| 10° | 0.12 |
| 5° | 0.05 |
| Delta T | Correction factor (vs ΔT60) |
|---|---|
| 60° (reference) | 1.00 |
| 55° | 0.906 |
| 50° | 0.787 |
| 45° | 0.705 |
| 40° | 0.605 |
| 35° | 0.519 |
| 30° | 0.430 |
| 25° | 0.344 |
| 20° | 0.262 |
| 15° | 0.185 |
What does BTU at Delta T65 mean?
This indicates the radiator’s heat output in BTUs when there’s a 65°C difference between the radiator temperature and room temperature. Delta T65 is rare in the UK, as it’s above the standard limits.
What does BTU at Delta T50 mean?
BTU at Delta T50 shows the heat output when there’s a 50°C difference between the radiator and the room. This is the UK’s standard for domestic heating.
What does BTU at Delta T30 mean?
This measures heat output at a 30°C difference, often used in low-temperature systems like underfloor heating where a lower temperature gradient is preferred.