Radiator BTU Calculator
BTU or watt output needed to heat a room.
Enter your values
Enter the room length in ft.
Enter the room width in ft.
Enter the ceiling height in ft.
Choose one of: Well insulated, Average, Poor.
Choose one of: Modern double glazing, Standard, Large or single glazing.
Choose one of: Living room, Bedroom, Kitchen / bathroom.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Heat output needed
- 5,376 BTU/hr
- In watts
- 1,576 W
- In kilowatts
- 1.58 kW
- Room volume
- 1,344 cubic feet
Working
- 11,344 ft3 x 4 x 1 x 1 = 5,376 BTU/hr
Quick answer
Room heat requirement scales with volume and heat loss. This tool uses a volume factor adjusted for insulation, glazing and room type.
BTU = volume x factor x insulation x glazing
At a glance
| What it does | BTU or watt output needed to heat a room. |
|---|---|
| Category | Home & DIY |
| Inputs needed | Room length, Room width, Ceiling height, Insulation, Glazing, Room type |
| Main output | Heat output needed |
| Formula | BTU = volume x factor x insulation x glazing |
| Cost | Free — no sign-up, no download |
How to use the Radiator BTU Calculator
- 1Measure the room and enter its volume dimensions.
- 2Adjust for insulation quality and how much glass the room has.
- 3Match or slightly exceed the BTU figure when picking a radiator.
Inputs explained
- Room length(ft)
- Enter the room length in ft.
- Room width(ft)
- Enter the room width in ft.
- Ceiling height(ft)
- Enter the ceiling height in ft.
- Insulation
- Choose one of: Well insulated, Average, Poor.
- Glazing
- Choose one of: Modern double glazing, Standard, Large or single glazing.
- Room type
- Choose one of: Living room, Bedroom, Kitchen / bathroom.
Worked example
Using the values the calculator loads with:
Inputs
- Room length14 ft
- Room width12 ft
- Ceiling height8 ft
- InsulationAverage
- GlazingStandard
- Room typeLiving room
Results
- Heat output needed5,376 BTU/hr
- In watts1,576 W
- In kilowatts1.58 kW
- Room volume1,344 cubic feet
- 1,344 ft3 x 4 x 1 x 1 = 5,376 BTU/hr
Frequently asked questions
Should I oversize a radiator?
A modest 10 to 15% margin lets you run lower water temperatures, which is more efficient with a condensing boiler or heat pump.
How do I convert BTU to watts?
Divide BTU per hour by 3.412. A 5,000 BTU/hr radiator is about 1,465 watts.
What do I need to enter into the Radiator BTU Calculator?
Just 6 values: room length, room width, ceiling height, insulation, glazing, room type. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Radiator BTU Calculator work out the answer?
Room heat requirement scales with volume and heat loss. This tool uses a volume factor adjusted for insulation, glazing and room type. It applies the formula BTU = volume x factor x insulation x glazing and shows the working so you can check each step by hand.
Is the Radiator BTU Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.