Radiator Watts Calculator: How Many BTU Do I Need?

This radiator watts calculator works out the heat output you need from a radiator to warm a room to a target temperature. You enter the room length, width, height, your desired indoor temperature, and the coldest outdoor temperature, and it returns the required heat output in watts (and BTU/hr). Use the result to choose a radiator with a wattage rating that matches or slightly exceeds the calculated figure.

Formula

The calculator uses the widely quoted “35 watts per cubic metre” rule, adjusted for the temperature difference between inside and outside. The formula is:

Required watts = Room volume (m³) × 35 × (ΔT / 24)

Where:

• Room volume = length × width × height, in cubic metres (m³)
• ΔT = target indoor temperature − outdoor design temperature, in °C
• 35 = base watts per m³ for a typical insulated room with a 24°C temperature difference
• 24 = the reference temperature difference used to derive the 35 W/m³ figure

The result is in watts (W). To convert to BTU/hr (British Thermal Units per hour), multiply watts by 3.412.

Worked Example

Using the default values from the calculator: room 4 m × 3 m × 2.4 m, target indoor temperature 21°C, outdoor design temperature −1°C.

1. Room volume = 4 × 3 × 2.4 = 28.8 m³
2. ΔT = 21 − (−1) = 22°C
3. Required watts = 28.8 × 35 × (22 / 24) = 28.8 × 35 × 0.9167 = 924 W
4. Required BTU/hr = 924 × 3.412 = 3153 BTU/hr

The calculator rounds the wattage to the nearest whole number, giving 924 W and 3153 BTU/hr.

How to Read the Result

The calculated figure is the heat output the radiator must provide when the outdoor temperature is at its coldest design value. In practice, radiators are sized to meet the heat loss on the coldest day of the year, so you should choose a radiator with a rated output at least equal to the calculated watts. If the radiator is rated for a different water temperature (e.g., 50°C instead of 70°C), its output will be lower than the nominal rating, so you’ll need a larger radiator.

Common Mistakes and Misconceptions

• Using floor area instead of volume: The 35 W/m³ rule is based on cubic metres, not square metres. A room with a 3 m ceiling needs more heat than one with a 2.4 m ceiling.
• Forgetting the outdoor temperature: The formula is temperature-difference dependent. If you use a mild outdoor temperature (e.g., 5°C) the result will be too low for winter.
• Ignoring insulation and glazing: The rule assumes average insulation and double glazing. A poorly insulated room with single-glazed windows can lose 50% more heat, so multiply the result by 1.5. A very well-insulated modern room might need only 25 W/m³.
• Mixing up watts and BTU: Radiator output is often listed in both, but if you compare a wattage figure to a BTU figure you’ll be off by a factor of 3.4.

When to Use This Calculator

This calculator is a first-pass sizing tool for a typical room in a house or flat. It works best for rooms with standard ceiling heights (2.3–2.7 m), average insulation, and double glazing. Use it when you are replacing a radiator, planning a new heating system, or estimating the heat output needed for an extension. It is not a substitute for a full heat-loss calculation (often called a “heat loss survey”) that accounts for wall construction, window sizes, floor type, and air infiltration. For a new build or major renovation, a professional heating engineer should do a detailed calculation (e.g., using the CIBSE or ASHRAE methods).

Alternatives and Adjustments

If you want a more precise estimate, you can adjust the base factor:

• Poor insulation or single glazing: use 50–60 W/m³
• Average insulation, double glazing: use 35–40 W/m³
• Good insulation, modern windows: use 25–30 W/m³
• High ceiling (above 2.7 m): increase the volume calculation by using the actual height, then add 10% for heat rising.

For rooms with large windows or an unheated space above, add 10–20% to the final figure. For bathrooms, many people prefer a slightly higher temperature (24°C), so adjust the target temperature accordingly.

Frequently Asked Questions

What does a radiator wattage rating mean?

A radiator’s wattage (or BTU) rating tells you how much heat it emits per hour when the water inside is at a standard temperature, usually 70°C average (flow 75°C, return 65°C). If your heating system runs at a lower temperature, the radiator will produce less heat, so you need a higher-rated radiator to achieve the same room temperature.

How do I convert watts to BTU/hr?

Multiply the wattage by 3.412. For example, 1000 W = 3412 BTU/hr. Conversely, divide BTU/hr by 3.412 to get watts.

What is the 35 watts per cubic metre rule?

It’s a quick sizing rule that assumes a typical room needs 35 watts of heat per cubic metre of space when the temperature difference between inside and outside is 24°C. The rule is a rough guide; actual heat loss depends on insulation, glazing, and ventilation.

Can I use this calculator for underfloor heating?

No. Underfloor heating operates at lower water temperatures and heats the room differently. It requires a heat-loss calculation that accounts for floor construction and coverage area. Use a dedicated underfloor heating design method or consult a professional.

What if the calculated watts don’t match a radiator size I can buy?

Choose the next size up. It’s better to have a radiator that is slightly oversized than undersized, because you can always turn it down with a thermostatic valve. An undersized radiator will struggle to reach the target temperature on cold days.

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in length | Room length | m | 4 | 0
in width | Room width | m | 3 | 0
in height | Room height | m | 2.4 | 0
in target | Target indoor temp | °C | 21 | 0
in outdoor | Outdoor design temp | °C | -1 | 0
out watts | Required heat output | W | length * width * height * 35 * (target - outdoor) / 24 | 0
out btu | Required output | BTU/hr | (length * width * height * 35 * (target - outdoor) / 24) * 3.412 | 0