Heating Power Calculator

Area, m²

Ceiling height, m

Climate

Insulation

External walls

Windows

Result

Required, kW2
With a 15% margin, kW2.3
Required, W2,000
Per square metre, W100
Room volume, m³54
Climatetemperate
Insulationordinary building, double glazing

A hundred watts a metre, and the corrections

Works out the kilowatts of heat a room or a house needs, with corrections for ceiling height, climate, insulation, external walls and windows.

The starting point is familiar: a hundred watts per square metre. The trouble is that the figure was derived for a temperate climate, a 2.7 metre ceiling and an ordinary building with double glazing. Change any of those and it stops holding.

The corrections multiply. A northern house with old single-glazed windows wants nearly twice the heat per square metre of an insulated southern one. A corner room with two windows against an internal one with a single window is another quarter on top.

What this calculator shows

The heat requirement in kilowatts and in watts
The same figure with a 15 percent margin, for sizing a boiler
The specific load per square metre after every correction

What to keep in mind

The baseline is 100 W per m² at a 2.7 m ceiling; height scales the figure proportionally.
Climate factors: southern 0.8, temperate 1.0, northern 1.4. Each external wall and each window beyond the first adds 10 percent.

FAQs

How many kilowatts for 100 m²?

In a temperate climate with ordinary insulation, about 10 kW. In a cold climate with poor insulation up to 18; in a well-insulated southern house, 8.

Does the 100 watts per metre rule work?

As a first approximation, yes — but only at a 2.7 metre ceiling, in a temperate climate, in a building with double glazing. Everything else needs correcting.

How much margin should a boiler have?

10 to 20 percent. More is actively harmful: an oversized boiler short-cycles, switching on and off in bursts that wear the burner and the heat exchanger.

How much does insulation matter?

More than anything else on the list. The gap between an old single-glazed building and a modern insulated one reaches a factor of one and a half at the same floor area.

Worked example

An ordinary temperate-climate room

Input: 18 m², 2.7 m ceiling, one external wall, one window

Output: 1.8 kW, or 2.07 kW with margin

Note: Exactly a hundred watts a metre — the case the rule of thumb was written for. Move the same room to a cold climate with old windows and it wants 3.3 kilowatts.

Useful links

All tools in Home & Utilities

Heating Power Calculator

Result

Related tools

Radiator Sections Calculator

Firewood Calculator

Air Conditioner Sizing Calculator

Useful links