As a rough guide, a stove needs about 1kW of heat output for every 14 cubic metres of room volume. Multiply length by width by ceiling height in metres, divide by 14, and choose the stove nearest that figure: a 5m x 4m room with a 2.4m ceiling is 48 cubic metres, so about 3.4kW, which points to a 4kW or 5kW stove.
Older, draughty rooms with big windows or high ceilings need more; new, well-insulated rooms need less. Your installer's heat-loss check gives the right answer for your house, but the method below gets you to the right shortlist in five minutes.
The quick calculation
- Measure the room: length x width x ceiling height, in metres. That is the volume in cubic metres.
- Divide the volume by 14. The answer is the approximate heat output you need in kW.
- Adjust for the room (see below), then pick the stove with the nearest nominal output.
Prefer not to do the maths? Our stove size calculator works it out for you: enter your room measurements and how well it holds heat, and it suggests the heat output you need in kW.
Worked example: a living room 5m long, 4m wide with a 2.4m ceiling. 5 x 4 x 2.4 = 48 cubic metres. 48 / 14 = 3.4kW. A 4kW stove such as the FDC4 Eco suits a well-insulated version of that room; in an older house with original sash windows, a 5kW stove such as the Carron ECO 5kW is the safer choice.
If the room is L-shaped, split it into rectangles, work out each volume and add them together. If an archway or open stairs connect it to another space, include the part that heat will drift into.
Stove size by room size
| Room (2.4m ceiling) | Volume | Volume / 14 | Typical choice |
|---|---|---|---|
| 3.5m x 3.5m | 29 m³ | 2.1kW | A small stove of around 3kW to 4kW |
| 4m x 4m | 38 m³ | 2.7kW | 3kW to 4kW |
| 5m x 4m | 48 m³ | 3.4kW | 4kW to 5kW |
| 6m x 4m | 58 m³ | 4.1kW | 5kW |
| 6m x 5m | 72 m³ | 5.1kW | 5kW to 6kW |
| 7m x 5m | 84 m³ | 6.0kW | 6kW to 8kW |
| 8m x 6m (open plan) | 115 m³ | 8.2kW | 8kW |
For a ceiling higher than 2.4m, use your real height: a 3m Victorian ceiling adds a quarter to the volume.
What changes the answer
| Needs more heat | Needs less heat |
|---|---|
| Solid walls, single glazing or draughty sash windows | Cavity or external wall insulation, modern double or triple glazing |
| High ceilings, or open stairs that let heat escape upwards | A new build or extension built to current standards |
| Large glazed doors or a conservatory opening off the room | Other heating in the room (radiators, underfloor heating) |
| Open plan, with heat drifting into other rooms | A small, enclosed room |
The rule of thumb gets you to the right range. A heat-loss check by your installer, which accounts for the walls, windows and insulation, gives the right answer for your home. If you sit between two sizes, think about how you will use the stove: as the main heat on cold evenings, lean towards the larger; as a cosy extra alongside central heating, lean towards the smaller.
Snugs and smaller rooms
Up to 5kW
Compact stoves for rooms up to about 70 cubic metres on the rule of thumb, or larger rooms in well-insulated homes. Usually no air vent needed in an older house.
Most living rooms
5kW stoves
A popular size for typical UK living rooms, and the largest that normally avoids a permanent air vent in a home built before 2008.
Large and open-plan rooms
7kW to 8kW
For big rooms, open-plan living and older homes with high ceilings. An air vent will be needed: 550mm² per kW above 5kW in an older home, or per kW of the whole output in an airtight one.
Room heat plus radiators
Boiler stoves
Send part of their output to water for radiators and hot water. Size them on the heat they give the room as well as the total.
Why bigger is not better
An oversized stove overheats the room, so owners turn the air right down to tame it. A starved fire smoulders: it blackens the glass, tars the flue, makes smoke and wastes fuel. Modern Ecodesign stoves burn cleanest when run close to their nominal output, so it is better to choose a stove you can run properly than one you have to choke.
Run it lively, not choked. A 5kW stove running well gives a better fire, cleaner glass and less smoke than an 8kW stove turned down to its minimum in the same room.
The 5kW line and air vents
In England, Approved Document J does not normally require a permanent air vent for a stove with a rated output of 5kW or less in a home with an air permeability above 5.0 m³/(h.m²), which covers most houses built before 2008. Above 5kW, a vent of 550mm² per kW over 5kW is needed. That is why 4.9kW and 5kW stoves are so popular in older homes. Newer, airtight homes need a vent whatever the size. See do I need a vent for a log burner?
Nominal output and output range
Stoves quote a nominal (rated) output, the figure tested for efficiency and emissions, and sometimes a range. Size by the nominal output. A range tells you a stove can be turned down for mild evenings or pushed briefly for a cold one; it is not a target to run at all night.
Output can also depend on fuel. The Woolly Mammoth 5, for example, gives 4.6kW on wood and 4.9kW on anthracite. Gas stoves usually quote a range: the Opus Trio Scala gives 4.3kW to 6.9kW. For a multi-fuel stove, the air supply rules use the fuel that gives the highest output.
Sizing a boiler stove
A boiler stove splits its output between the room and a water jacket that feeds radiators or a hot water cylinder. The headline figure is the total, so check both parts. The Woodfire CX8 is rated 8kW, with 5.4kW going to water and 2.6kW to the room. Inset boiler stoves such as the Woodfire EX10 send most of their heat to water: 8.5kW to water and 1.4kW to the room. Choose the room output to suit the room, and let a heating engineer size the water side against your radiators and cylinder. Browse boiler stoves.
Freestanding or inset?
Size is not only about kilowatts. A freestanding stove stands on a hearth in front of or inside the fireplace, with its stove pipe on show, and suits almost any room with a chimney or twin wall flue. An inset stove is built into the fireplace opening or a purpose-made chamber, so it takes up less floor space and gives a cleaner, framed look, but the opening must be built to the maker's instructions.
Whichever you choose, check the physical size as well as the output: the stove's height, width and depth against the fireplace opening, the clearances to combustible materials in the manual, and the depth of hearth needed in front. A small, efficient stove that fits the space neatly will usually look and work better than a larger one squeezed in. Browse inset wood burning boiler stoves.
Stoves to match each size
- 4kW: FDC4 Eco, a compact multi-fuel stove, Defra exempt, with a 125mm flue.
- Around 5kW: Carron ECO 5kW, cast iron, 78.6% efficient and approved for smoke control areas; FDC5 Eco and the wider-window FDC5W Eco; Woolly Mammoth 5; and the Opus Melody LS, a 5kW cylindrical wood stove with a log store.
- 7kW to 8kW: the Opus Harmony at 7kW; FDC8 Eco, Woolly Mammoth 8 and Carron ECO Revolution 8kW for large and open-plan rooms (an air vent will be needed).
Browse small log burners up to 5kW and all stoves. If you live in a smoke control area, choose from our stoves exempt for smoke control areas.
Before you buy
- Smoke control area? Check your council's map; if you are in one and want to burn wood, choose an exempt stove.
- Flue size. Many 4kW to 8kW stoves have a 125mm outlet, others 150mm. Your chimney or liner must suit it.
- Hearth and clearances. Check the stove's manual for the hearth it needs and the distance to combustible materials.
- Log length. Make sure the firebox takes the logs you can buy or cut.
- Installer. Book a HETAS registered installer, who will also fit the carbon monoxide alarm required in the room.
Common questions
How do I calculate what size stove I need?
Multiply the room's length, width and ceiling height in metres to get the volume, then divide by 14. The answer is the approximate kW you need for an average room. Older or draughty rooms need more, well-insulated rooms less.
What size room will a 5kW stove heat?
Using the volume divided by 14 rule, about 70 cubic metres, for example a 6m x 5m room with a 2.4m ceiling, in an average house. A well-insulated room of that size may need less.
Is it better to get a bigger stove?
No. An oversized stove overheats the room, so it gets run with the air turned down, which makes it smoulder, blacken the glass and tar the flue.
Do I need a vent for a 5kW stove?
In England, not normally in a home built before 2008, as long as the rated output is 5kW or less. Newer, airtight homes need a vent whatever the size.
What size stove for an open plan room?
Calculate the whole open area, including any space heat will drift into. Large open plan rooms often need 6kW to 8kW, which will need an air vent.
What happens if my stove is too small?
You will run it flat out to keep warm, burning more fuel and risking overfiring, which damages baffles and firebricks. Check the room volume again, or add heat from radiators on the coldest days.
Can one stove heat the whole house?
A room heater stove mainly heats the room it is in, with some warmth drifting through open doors and up stairs. To heat radiators around the house, look at a boiler stove sized with a heating engineer.
Should I size my stove for the coldest day of the year?
Not if you have central heating as well. Size for a normal cold evening and let the radiators help on the few very cold days. A stove sized for the worst week will be too big, and turned down, for most of the winter.
Does ceiling height make a difference?
Yes. Use the real ceiling height in the calculation. A 3m ceiling holds a quarter more air than a 2.4m one, so the same floor area needs a larger stove.
Does a 4.9kW stove avoid the need for a vent?
In an older home in England, yes, a stove rated 5kW or less does not normally need a permanent vent. For a multi-fuel stove, use the output on the fuel that gives the highest figure.
Find the right size stove
Send us your room measurements and a photo of the fireplace or wall, and we will suggest stoves that suit the space and your chimney.
