Estimate Your Ideal Radiator Output
Enter your room details below to estimate the heating power you need in both watts and BTU/h, plus a practical radiator size guide.
How this radiator size calculator works
This tool gives you a practical estimate of radiator output based on your room volume and heat-loss related factors. It is designed for fast planning, whether you are replacing a radiator, upgrading to a heat pump, or checking if a room is currently under-heated.
The calculator uses a baseline heat demand per cubic meter, then adjusts it for insulation level, window quality, exposed external walls, temperature difference, and your heating system temperature. The final output is shown in both watts and BTU/h, which are the two most common radiator rating units.
Why correct radiator sizing matters
- Comfort: rooms heat up faster and stay stable in colder weather.
- Efficiency: properly sized radiators help boilers and heat pumps run at better operating conditions.
- Lower bills: avoiding oversized or undersized units reduces wasted energy and cycling losses.
- Future-proofing: if you plan to run lower flow temperatures later, selecting appropriate radiator output now is helpful.
Quick step-by-step guide
1) Measure the room accurately
Measure internal length × width × height in meters. Even small measuring errors can change the result enough to affect radiator selection.
2) Choose realistic building factors
Use the insulation and window settings that best reflect the room. Older properties with poor insulation or single glazing need more output.
3) Set your temperature assumptions
The larger the difference between target indoor temperature and winter outdoor design temperature, the more heat the room needs.
4) Match system temperature
If your system runs lower flow temperatures (common with heat pumps), radiator output per unit size drops, so bigger radiators are often needed. The calculator applies a multiplier to reflect this.
Understanding the results
Your output result is the recommended total heating power for that room. You can meet it using one radiator or split it across multiple radiators.
- Watts (W): standard modern engineering unit for heat output.
- BTU/h: common unit on radiator product listings, especially in older catalogs.
- Type suggestions: the calculator estimates approximate total width for common 600 mm high radiator panel types.
Radiator panel types at a glance
Type 11 (single panel)
Lower output per width, slim profile. Useful where wall depth is limited.
Type 21 (double panel, single convector)
Balanced option for many rooms, with stronger output than Type 11.
Type 22 (double panel, double convector)
Very common choice in homes due to good output-to-size ratio.
Type 33 (triple panel, triple convector)
High output in limited wall width, though deeper projection from the wall.
Common sizing mistakes to avoid
- Choosing a radiator by “what fits the wall” without checking output.
- Ignoring outside wall exposure and glazing quality.
- Forgetting low-temperature operation when moving to a heat pump.
- Using one whole-house rule of thumb instead of room-by-room sizing.
- Not allowing a small safety margin for cold snaps.
FAQ
Should I size radiators room by room?
Yes. Each room has different heat loss due to orientation, windows, wall exposure, and use patterns.
Is this enough for final installation design?
This is a high-quality estimate tool for planning. For final specification, especially in renovations or new heating systems, a detailed heat-loss survey by a qualified professional is recommended.
Can I use two smaller radiators instead of one large one?
Absolutely. Matching total output is key. Two units can improve heat distribution and comfort in larger rooms.
Disclaimer: This calculator provides an estimate only and does not replace professional design calculations for compliance or system commissioning.