How to Use the Radiator BTU Calculator
- Choose units. UK mode uses metres; USA mode uses feet. Results show both BTU/h and watts.
- Enter the room's length, width and ceiling height.
- Choose the room type. Living rooms and bathrooms are kept warmer than bedrooms and hallways, so they need more heat per cubic metre.
- Choose the insulation level, from poor (old, single glazed, no cavity insulation) to excellent (new build).
- Count outside walls and windows. Each outside wall and each window adds heat loss.
- Tick north-facing and room above a heated room if they apply.
- Choose your heating system's flow temperature. Standard gas boilers use ΔT50. Heat pumps and low-temperature boilers need bigger radiators, and the calculator shows how much bigger.
The result is the radiator output you should look for in the manufacturer's data, quoted at ΔT50, which is how UK and European radiators are rated.
How Radiator BTU Is Calculated
The calculator follows the same method most UK radiator suppliers and heating engineers use for a quick estimate:
Step 1: Room volume × room-type factor
| Room type | Watts per m³ | Target temperature |
|---|---|---|
| Living room, dining room | 40 | 21°C |
| Bathroom, shower room | 45 | 22°C |
| Bedroom, study | 35 | 18–19°C |
| Kitchen | 30 | 18°C (appliances add heat) |
| Hallway, landing | 35 | 18°C |
| Conservatory | 60–80 | 21°C |
Step 2: Multiply by insulation
| Insulation level | Multiplier |
|---|---|
| Poor: pre-1980s, solid or uninsulated walls, single glazing | × 1.5 |
| Average: partial cavity insulation, older double glazing | × 1.2 |
| Good: full cavity insulation, modern double glazing | × 1.0 |
| Excellent: new build, triple glazing | × 0.85 |
Step 3: Add for outside walls and windows
- +10% for each outside wall
- +20% for each single-glazed window
- +10% for each double-glazed window
- +20% for a patio door or large bay window
Step 4: Final adjustments
- +10% if the room is north-facing
- −5% if there's a heated room directly below
Step 5: Convert. 1 watt = 3.412 BTU per hour.
Worked example: living room
A living room of 5 m × 4 m × 2.4 m with average insulation, two outside walls and one double-glazed window:
- Volume: 5 × 4 × 2.4 = 48 m³
- Base heat: 48 × 40 = 1,920 W
- Insulation: 1,920 × 1.2 = 2,304 W
- Walls and window: 2,304 × 1.30 (two walls at 10% each plus one window at 10%) = 2,995 W
- In BTU: 2,995 × 3.412 = 10,220 BTU/h
So you'd look for one radiator rated at least 3,000 W (10,200 BTU) at ΔT50, or two smaller radiators that add up to it, for example two 1,500 W double panels.
Worked examples: bedroom and bathroom
- Bedroom 3.5 m × 3 m × 2.4 m, average insulation, one outside wall, one double-glazed window, upstairs above a heated room: about 1,207 W (4,117 BTU/h).
- Bathroom 2.5 m × 2 m × 2.4 m, good insulation, one outside wall, one window: about 648 W (2,211 BTU/h).
What Is ΔT50, and Why Heat Pumps Need Bigger Radiators
Radiator outputs are published at a standard condition called ΔT50 (Delta T 50) under BS EN 442: the average radiator temperature is 50°C above a 20°C room. That matches a gas boiler running at about 75°C flow and 65°C return.
If your water runs cooler, the radiator gives out less heat. Output falls faster than the temperature, following an exponent of about 1.3. To find the ΔT50 rating you need at a lower temperature, multiply the room's heat requirement by (50 ÷ your ΔT)^1.3:
| System | Typical flow / return | Approximate ΔT | Multiply ΔT50 requirement by |
|---|---|---|---|
| Older gas boiler, high temperature | 82 / 71°C | 60 | 0.79 |
| Standard gas boiler | 75 / 65°C | 50 | 1.00 |
| Condensing boiler, low temperature | 65 / 55°C | 40 | 1.34 |
| Heat pump | 50 / 45°C | 30 | 1.94 |
| Heat pump, very low temperature | 45 / 40°C | 25 | 2.46 |
Example: the 2,995 W living room above needs radiators rated about 5,800 W at ΔT50 if it's heated by a heat pump running at ΔT30. That's why heat pump installs often include several radiator upgrades. Many manufacturers publish ΔT30 outputs directly; use those if they're available.
If you're replacing your boiler too, the Boiler Size Calculator adds up the whole house.
Choosing a Radiator from the BTU Figure
Once you know the output you need, compare it with the manufacturer's output table at ΔT50. Typical outputs for a 600 mm high, 1,000 mm long steel panel radiator:
| Radiator type | Description | Approx. output (ΔT50) |
|---|---|---|
| Type 11 (P1/K1) | Single panel, single convector | 1,000 W / 3,400 BTU |
| Type 21 (P+) | Double panel, single convector | 1,350 W / 4,600 BTU |
| Type 22 (K2) | Double panel, double convector | 1,750 W / 6,000 BTU |
| Type 33 (K3) | Triple panel, triple convector | 2,450 W / 8,400 BTU |
Figures vary by brand, so always check the specific model. Some practical tips:
- Place radiators under windows where possible. They counter the cold downdraught from the glass.
- Large rooms often work better with two radiators than one huge one, spreading heat more evenly.
- Allow for covers and shelves. A radiator cover or deep shelf can cut output by 10–20%.
- Size slightly up rather than down. A thermostatic radiator valve (TRV) will trim an oversized radiator, but nothing fixes an undersized one.
Bathroom radiators and towel rails
Towel rails are tricky. Their published output assumes they're bare, but in daily use they're covered in towels, which can cut effective output by 15–25%. Chrome rails also give out noticeably less heat than white or anthracite painted rails of the same size. If a towel rail is the room's only heat source, size it 20–25% above the calculator's figure, or add a small second radiator.
USA: Sizing Hydronic Baseboard and Radiators
In US homes with hot water (hydronic) heat, the same BTU figure tells you how much fin-tube baseboard a room needs. Standard residential baseboard gives roughly 500–600 BTU/h per linear foot at 180°F average water temperature, falling sharply at lower temperatures. In USA mode, the calculator divides the room's BTU requirement by 550 BTU per foot (adjustable) to suggest a baseboard length.
Example: a 12 × 14 ft bedroom with 8 ft ceilings, average insulation, two outside walls and one window needs about 7,090 BTU/h (2,078 W), or about 13 feet of baseboard.
For cast iron radiators in older homes, the US industry rates them in square feet of EDR (equivalent direct radiation), where 1 sq ft EDR gives about 240 BTU/h on steam, or roughly 150–170 BTU/h on hot water at 170°F.
Why Your Room Might Still Feel Cold
If a correctly sized radiator still leaves a room cold:
- Bleed the radiator. Air trapped at the top stops it heating fully.
- Balance the system. Radiators nearest the boiler can take all the flow, leaving distant rooms lukewarm.
- Check the TRV. A stuck pin keeps the valve closed.
- Flush the system. Sludge settles at the bottom of old radiators and cuts output.
- Fix draughts. Gaps around windows, doors and floorboards can double a room's heat loss.
Frequently Asked Questions
How many BTU do I need to heat a room?
Multiply the room's volume in cubic metres by about 35–45 watts (depending on room type), adjust for insulation, outside walls and windows, then multiply by 3.412 to get BTU per hour. A typical double bedroom needs 3,500–5,000 BTU/h and a living room 7,000–12,000 BTU/h.
How do I convert watts to BTU?
Multiply watts by 3.412. For example, 1,500 W × 3.412 = 5,118 BTU/h. To convert BTU to watts, divide by 3.412.
What size radiator do I need for a bathroom?
Most UK bathrooms need 1,500–3,500 BTU/h (450–1,000 W). If you're using a towel rail, add 20–25% to allow for towels covering it.
What does ΔT50 mean on a radiator?
It means the output was measured with the radiator's average water temperature 50°C above a 20°C room. It's the standard UK and European rating, so use ΔT50 figures when comparing radiators.
Do I need bigger radiators for a heat pump?
Usually, yes. At a typical heat pump ΔT30, a radiator gives out roughly half its ΔT50 rating, so you need about 1.9 times the ΔT50 output. Many rooms can be fixed by swapping single panels for double or triple panels.
Is it better to have one big radiator or two smaller ones?
In rooms over about 4 m long, or with windows on different walls, two radiators usually give more even heat and avoid cold spots.
Can a radiator be too big?
Not in a harmful way. With a thermostatic radiator valve, an oversized radiator simply runs for shorter periods, and it helps if you later lower the boiler's flow temperature or switch to a heat pump.
How much baseboard heat do I need per square foot?
For an average US room, roughly 1 foot of baseboard per 12–20 square feet of floor, depending on climate and insulation. Use the calculator for the room's BTU figure, then divide by your baseboard's rating per foot.
Upgrading radiators or switching to a heat pump?
A heating engineer can run a room-by-room heat loss and balance your system. Get local quotes first.