Radiator Sections Calculator

Room area, m²

Ceiling height, m

Section output, W

Aluminium 170–190, bimetal 150–180, cast iron 110–140.

Climate

Windows in the room

Corner room (1 for yes)

Result

Sections12
Heat required, W2,000
Exact section figure11.76
Split by window12 sections under each window
Section output, W170
Climatetemperate

Room demand divided by one section

Works out how many radiator sections a room needs from its area, ceiling height, climate and the output of a single section.

The calculation runs in two steps: work out the room's heat demand, then divide by the output of one section. That second figure depends heavily on material — aluminium gives 170 to 190 watts a section, bimetal 150 to 180, cast iron only 110 to 140.

One detail matters more than people expect: rated output is quoted at a 70-degree temperature difference, meaning 90-degree water against 20-degree air. On a real system running at 70 degrees, the same section delivers roughly half of that.

What this calculator shows

The section count rounded up, and the exact figure behind it
The room's heat demand in watts
How the sections split across the windows in the room

What to keep in mind

The baseline is 100 W per m² at a 2.7 m ceiling, corrected for climate, corner position and window count.
Section output is taken as rated. If your system runs cooler than 90 degrees, enter the real output rather than the catalogue one.

FAQs

How many radiator sections does a room need?

Divide the room's demand in watts by the section output. Twenty square metres in a temperate climate is about 2,000 watts, which is 12 aluminium sections.

What is the output of one section?

Aluminium 170 to 190 W, bimetal 150 to 180, cast iron 110 to 140 — all rated at a 70-degree temperature difference.

Why does my radiator underperform its rating?

Because the rating assumes 90-degree water. At a 70-degree flow the output roughly halves, and that is normal for every radiator on the market.

Round up or down?

Always up. Excess heat can be throttled at a valve or a thermostatic head; missing heat has nowhere to come from.

Worked example

A twenty-metre room on aluminium

Input: 20 m², 2.7 m ceiling, 170 W sections, temperate climate

Output: 12 sections for 2,000 watts

Note: The exact figure is 11.76 and it has to round up: too little heat can only be fixed by replacing the radiator, while an extra section can simply be throttled at the valve.

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Radiator Sections Calculator

Result

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