How this calculator works
A wood burner has to give out about as much heat as the room loses. The rule used by stove suppliers is to divide the volume of the room in cubic metres by 14 to get the kW needed for an average room with double glazing and cavity walls. A very well insulated room loses less heat, so the divisor is higher (about 25), and a draughty room with poor insulation loses more, so the divisor is lower (10). The nearest common stove size above that is the one to look at. Approved Document J of the Building Regulations also requires a permanent air vent of 550 mm² for each kW of output above 5 kW (and for each kW in a very airtight house) to supply the combustion air.
Worked example: a living room 5 × 4 m with a 2.4 m ceiling is 48 m³, which needs 3.4 kW, so a 4 kW stove. If the room is poorly insulated it needs 4.8 kW (5 kW), and if it is very well insulated 1.9 kW (3 kW). A larger 6 × 5 m lounge needs 5.1 kW and a long 8 × 4 m open-plan room with a 2.8 m ceiling needs 6.4 kW (7 kW stove, with 1,100 mm² of air vent). A 4 kW stove needs no vent in an older house, but 2,200 mm² in a very airtight one.
Stove size for common rooms (2.4 m ceiling)
The output in kW for average, very well insulated and poorly insulated rooms.
| Room | Volume | Average | Very good | Poor |
|---|---|---|---|---|
| 3 × 3 m | 22 m³ | 1.5 kW | 0.9 kW | 2.2 kW |
| 4 × 3.5 m | 34 m³ | 2.4 kW | 1.3 kW | 3.4 kW |
| 5 × 4 m | 48 m³ | 3.4 kW | 1.9 kW | 4.8 kW |
| 6 × 4 m | 58 m³ | 4.1 kW | 2.3 kW | 5.8 kW |
| 6 × 5 m | 72 m³ | 5.1 kW | 2.9 kW | 7.2 kW |
| 8 × 4 m | 77 m³ | 5.5 kW | 3.1 kW | 7.7 kW |
Tips before you buy
- A stove that is too big is a common mistake: it makes the room too hot, and then is run on a low, smoky burn that blackens the glass. Choose the size for the room, not the biggest.
- Better insulation and draught-proofing make a smaller stove work. Fix the heat loss first.
- Open doorways let heat flow into the next room. If the stove is in an open-plan space, use the volume of the whole space.
- Use dry, seasoned wood (below 20% moisture) or approved smokeless fuel, and have the chimney swept every year.
- A carbon monoxide alarm is a must, and the stove needs a proper flue, a hearth and clearances to combustible materials.
Tools & materials for this project
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Frequently asked questions
What size wood burner do I need for my room?
Divide the room volume in cubic metres by 14. A room of 5 × 4 × 2.4 m is 48 m³ and needs 3.4 kW, so a 4 kW stove.
What size room will a 5kW stove heat?
About 70 m³ of average room (29 m² of floor with a 2.4 m ceiling). It is more in a well-insulated room, and less in a draughty one.
Does my log burner need an air vent?
A stove above 5 kW needs a permanent air vent of 550 mm² for each kW above 5 kW in an older house, which is 1,100 mm² for the 7 kW stove for the long room. In a very airtight house every stove needs one, 2,200 mm² for a 4 kW stove.
Is it bad to have a stove that is too big?
Yes. A stove that is too big overheats the room, and is then run on a low, slow burn that makes smoke and soot and blackens the glass. Choose the size for the room.
Do I need a HETAS installer?
A stove must be installed by a HETAS-registered installer, or with building control approval, so that the work is signed off. In a smoke control area the stove must be DEFRA-exempt.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.