BTU calculator for radiators
Work out how much heat a room needs in BTU per hour and watts, and the radiator output to look for, whether it runs off a boiler or a heat pump. Free for heating engineers quoting a job and for anyone choosing a radiator, and what you type stays in your own browser.
- A named method, with every factor shown
- BTU and watts, rounded up
- ΔT correction for heat pumps and low flow
Pick the room type, enter its length, width and ceiling height in metres, then the outside walls, any single-glazed windows, the walls and what is above and below.
The working
| 1,373.04 ft³ × room factor 5 | 6,865.2 |
|---|---|
| 1 outside wall(s) × 10% | +10% |
| Below the room | +6% |
| Total additions | +16% |
| Heat needed | 7,964 BTU/h |
A quick room estimate for choosing a radiator. Heat pumps and new systems need a full room-by-room heat loss calculation (BS EN 12831) from a heating engineer.
How the BTU sum works
The calculator takes the room's volume in cubic feet, multiplies it by a factor for the type of room, then adds a percentage for each thing that loses extra heat. That is the room-factor method set out by the builders' merchant Turnbull & Co in its radiator size guide: volume × room factor + feature adjustments = BTU per hour.
The same room factors (5 for a lounge, 4 for a bedroom, 3 for a kitchen) sit inside the Screwfix heat output calculator, so the base numbers are not one shop's invention. Turnbull's method does not cover the floor and ceiling, so for those the calculator uses the percentages published by Trade Radiators. Every factor is listed in the next section.
Here it is worked through for a ground-floor living room 4.5m by 3.6m with a 2.4m ceiling, one outside wall, a double-glazed window (so 0 single-glazed windows), filled cavity walls, a heated bedroom above and a solid concrete floor.
- Volume: 4.5 × 3.6 × 2.4 = 38.88m³, which is 1,373.03 cubic feet (one cubic metre is 35.3147 cubic feet).
- Living room factor of 5: 1,373.03 × 5 = 6,865.2 BTU/h.
- Adjustments: one outside wall adds 10%, a solid floor on the ground adds 6%. Double glazing, filled cavity walls and a heated room above add nothing. Total: 16%.
- Heat needed: 6,865.2 × 1.16 = 7,964 BTU/h, rounded up.
- In watts: 7,963.6 ÷ 3.412 = 2,334W.
- Radiator to buy, with a gas boiler: one rated at least 7,964 BTU/h at ΔT50, or two of at least 3,982 BTU/h each.
- The same room on a heat pump at ΔT30: 7,963.6 ÷ 0.515 = 15,464 BTU/h (4,533W) of ΔT50-rated radiator. Nearly twice the size, which is why the next sections matter.
Every factor the calculator uses
Each room type has a factor, and each feature adds a fixed percentage of that starting figure. The percentages are added together before they are applied, exactly as in Turnbull's own worked example: a 10 by 10 by 8 foot bedroom with one single-glazed window and one outside wall is 800 × 4 = 3,200 BTU/h, plus 20%, which is 3,840 BTU/h.
The room factors follow the temperature each room is heated to. CIBSE's Domestic Heating Design Guide, as reproduced in the MCS Heat Load Calculator's design conditions, sets 21°C for living rooms, 22°C for bathrooms and 18°C for bedrooms, kitchens and halls in older homes, and 21°C almost everywhere in newer, well-insulated ones.
Room factor (per cubic foot)
Bathroom or en-suite 6. Living room, dining room or study 5. Hallway or landing 5, for the draught from the front door. Bedroom 4. Kitchen 3, because appliances give off heat. All from Turnbull & Co. Screwfix puts bathrooms with bedrooms at 4, so a bathroom here comes out higher.
Outside walls: +10% each
Turnbull adds 10% for every wall that faces outside. An internal room with no outside walls adds nothing. A corner room with two outside walls adds 20%.
Windows: +10% per single-glazed window
Turnbull treats double glazing as already in the room factor, so it adds nothing. Each single-glazed window adds 10%. Triple glazing is counted the same as double: Turnbull gives no figure for it, and Trade Radiators knocks off only 5%.
Walls with no insulation: +15%
Turnbull adds 15% if the room has solid brick walls or cavity walls that have not been filled. Filled cavity walls add nothing.
Above the room
A heated room adds nothing. An insulated loft adds 5%, an unheated loft or roof void 8%, an uninsulated roof 10%. These are Trade Radiators' figures, because Turnbull's method only mentions loft insulation in passing.
Below the room
A heated room adds nothing. A solid concrete floor on the ground adds 6%, a suspended timber floor over a ventilated void 10%, and a garage or other unheated space 8%. Also from Trade Radiators.
Delta T, and why heat pumps need bigger radiators
A radiator's output is quoted at ΔT50 (Delta T 50): the gap between the average water temperature in the radiator and the room. Zehnder, which tests its radiators to EN 442, defines it as flow plus return divided by two, minus the room temperature. ΔT50 is roughly 80°C flow, 60°C return and a 20°C room.
Run the water cooler and the radiator gives out less heat. A heat pump might run at 55°C flow and 45°C return: an average of 50°C in a 20°C room is ΔT30. The radiator makers' association MARC, in a statement published by the Hot Water Association, wants output stated at both ΔT50 and ΔT30 for exactly this reason.
The calculator's system temperature setting offers ΔT50, ΔT40 and ΔT30, using the operating factors from Stelrad's radiator book. Divide the heat the room needs by the factor to get the ΔT50 rating to look for in a catalogue. Stelrad's own example: a radiator rated 2,000 BTU/h at ΔT50 gives 1,030 BTU/h at ΔT30. For a ΔT the calculator does not offer, take the factor from the list and divide by hand.
- ΔT50: 1.000 (standard rating, a typical older boiler system).
- ΔT45: 0.872.
- ΔT40: 0.748.
- ΔT35: 0.629.
- ΔT30: 0.515 (the low temperature rating, typical of a heat pump system).
- ΔT25: 0.406.
- ΔT60: 1.267. Some sellers quote ΔT60 because it makes a radiator look about a quarter more powerful. MARC has warned against exactly that, so always compare at ΔT50.
What insulation does to the answer
The same room can need a fifth more heat in an older house that has not been improved. Take a back bedroom 3.5m by 3m with a 2.4m ceiling, upstairs, at the corner of the house so it has two outside walls, with a heated room below.
As it might be before any work: cavity walls that were never filled, one single-glazed window, and an unheated loft above. The volume is 25.2m³, or 889.93 cubic feet. At the bedroom factor of 4 that is 3,559.7 BTU/h. The adjustments are 20% for the two walls, 10% for the window, 15% for the empty cavity and 8% for the loft: 53% in all. The answer is 5,447 BTU/h, or 1,597W.
Now the same room after cavity wall insulation, double glazing and loft insulation. The adjustments drop to 20% for the walls and 5% for the insulated loft. The answer is 4,450 BTU/h, or 1,305W, nearly a fifth less.
That is also the limit of a method like this. It does not know how thick the loft insulation is or how draughty the room is, and it only allows for a north-facing room if you set North-facing to Yes (Turnbull adds 10% for that). For anything unusual, such as a conservatory, an extension with a glass roof, a big open-plan room or a ceiling well over 2.4m, treat the answer as a rough guide and get the room measured properly.
When you need a proper heat loss calculation
This calculator is fine for swapping a radiator on a gas boiler system. It is not good enough for designing a heating system, and it must not be used to size a heat pump. That needs a room-by-room heat loss calculation to BS EN 12831, done by a heating engineer who has measured the walls, windows, floors and roof.
For heat pumps this is a requirement, not a nice extra. The MCS heat pump standard, MIS 3005-D, says the heat loss calculation must comply with BS EN 12831 and use set room and outside temperatures (clause 5.5, shown in MCS's own consultation document). MCS gives installers a free Heat Load Calculator that works to BS EN 12831-1:2017.
Two things a room-factor sum cannot do: it does not use U-values (how fast heat passes through each wall, window and floor), and it does not use the outside design temperature for where you live. A survey does both. Adding up every room from this calculator also does not size a boiler: MCS notes that a building's total heat load is not simply the sum of its rooms. For the boiler, use the boiler size calculator, then have the engineer confirm it.
Using it when you quote a radiator job
If you fit radiators, the useful part is the working. Put the room size, the factors and the result on the quote, and the customer can see why you specified a double panel and not the single they saw online. It also answers the call a month later about the cold back bedroom.
Say on the quote which ΔT you sized for. A radiator sized at ΔT50 for today's boiler will be too small if the customer moves to a heat pump in five years, and some customers will pay for the bigger radiator now if you tell them. The quote template has room for that note, while your labour figure can come from the day rate calculator.
Common questions
How many BTUs do I need for my room?
Multiply the room's volume in cubic feet by the room factor (5 for a living room, 4 for a bedroom, 3 for a kitchen, 6 for a bathroom), then add 10% for each outside wall and each single-glazed window, and 15% for uninsulated walls. A 4m by 3.5m bedroom with a 2.4m ceiling and one outside wall is 1,186.57 cubic feet × 4 = 4,746.3, plus 10% = 5,221 BTU/h.
How do I convert watts to BTU?
Multiply watts by 3.412 to get BTU per hour, and divide BTU by 3.412 to get watts. So a 1,500W radiator gives about 5,118 BTU/h, and 4,000 BTU/h is about 1,172W. Both examples are Turnbull's.
What size room will 12,000 BTU heat?
As a living room with no extra heat loss, about 2,400 cubic feet: 68m³, or a floor of roughly 28m² with a 2.4m ceiling. With one outside wall it drops to about 25.7m². If you are asking about air conditioning, this is the wrong calculator: cooling is sized differently.
Is this the same as an air conditioning BTU calculator?
No. This one sizes radiators for heating. Air conditioning is also rated in BTU, but it is sized for cooling, with different inputs. Use the air conditioner maker's own sizing guide.
What does ΔT50 mean on a radiator?
It means the output was measured with the water in the radiator, on average, 50°C hotter than the room. Lower water temperatures mean less heat: at ΔT30, Stelrad's factor is 0.515, so a radiator gives about half its ΔT50 output.
Do heat pumps need bigger radiators?
Usually, yes. A heat pump runs at a lower water temperature, often around ΔT30, where a radiator gives about half its rated output. To get the same heat, the radiator needs roughly twice the ΔT50 rating, unless the house has been insulated enough to need less heat. The heat pump installer's room-by-room heat loss calculation decides it.
Is it bad to fit a radiator bigger than the BTU needed?
Not on its own. Turnbull says a slightly bigger radiator is fine with a thermostatic radiator valve, which cuts the flow once the room is warm. A radiator that is too small is the bigger problem: the room never reaches temperature.
Should I fit one big radiator or two smaller ones?
In a long or large room, two radiators that add up to the total usually heat it more evenly. The calculator gives the total for the room. Split it between the radiators yourself, for example half each for two.
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