Radiator size calculator
A quick way to size a radiator is to multiply the floor area by a heat demand per square metre that depends on how well the building is insulated. The calculator gives the output in watts and an equivalent number of cast-iron sections.
Your measurements
Result
- Heat output needed
- 1,260 W
- Radiator sections (cast iron, guide)
- 9 pcs
Based on 150 W per section. For other radiators, use the output from the manufacturer's catalogue at your system's water temperature.
How the calculation works
- Heat output (W) = room area × demand per m²: about 100 W/m² in an uninsulated house, 70 W/m² with standard insulation and 50 W/m² with high-performance insulation.
- Sections = heat output ÷ 150 W (a typical cast-iron section), rounded up.
Worked example
An 18 m² room with standard insulation needs 18 × 70 = 1,260 W. At 150 W per section that is 1,260 ÷ 150 = 8.4, so a radiator with 9 sections.
What the calculator assumes
- The values per m² are for 2.5–2.7 m ceilings and ordinary winter temperatures.
- The output of a section (or a steel panel radiator) comes from the manufacturer's catalogue, at your system's water temperature.
When to adjust the result
- Corner rooms, rooms with large windows or high ceilings need 10–20% more.
- Catalogue outputs are usually given for water at 70–75 °C; with a heat pump at 40–45 °C a radiator gives only a third to a half of it.
- A proper heat-loss calculation replaces this rule of thumb for new systems.
Choosing and placing radiators
Radiator output is quoted at a standard temperature difference; at the lower water temperatures of a condensing boiler or heat pump, real output can be far lower. Check the correction factor for your actual flow and return temperatures before choosing sizes.
Placing radiators under windows counters cold downdraughts, but anywhere works in a well-insulated room. Leave space below and above for airflow, avoid covering them with furniture or thick curtains, and fit thermostatic valves plus a balanced system so every room gets the right flow.
Frequently asked questions
How many watts per square metre for heating?
As a rough guide about 50 W/m² in a well-insulated house, 70 W/m² with standard insulation and 100 W/m² or more in an uninsulated one.
Are radiators enough with a heat pump?
Often only if they are larger or the house is well insulated, because a heat pump runs at lower water temperatures and radiators then give less heat.
Single or double-panel radiator?
Double panels with fins give more output in the same wall space, which helps at low water temperatures; single panels suit small rooms.
Why is my radiator cold at the bottom?
Usually low flow because the system is unbalanced or sludge has settled; cold at the top means trapped air.