There is a simple calculation that gets you most of the way, and then a few rooms where it needs adjusting. Both are below.
The quick calculation
Take the room's length and width in metres, add them together, and multiply by ten. The result is a sensible fitting diameter in centimetres.
A room measuring 3 m by 4 m gives you 3 + 4 = 7, so around 70 cm. A small bedroom at 2.5 m by 3 m gives 55 cm.
That rule comes from the world of chandeliers and hanging fittings, and it is generous. For a flush ceiling light, which sits tight against the ceiling and is seen as a disc rather than an object, most people are happier taking 15–20% off that figure. The 3 m by 4 m room is then looking at 55–60 cm rather than 70.
Treat the number as the top of your range, not the target.
Room by room
| Room | Typical floor area | Flush fitting diameter |
|---|---|---|
| Hallway, landing | 3–6 m² | 25–35 cm |
| Small bedroom, study | 7–10 m² | 35–45 cm |
| Double bedroom | 11–15 m² | 45–55 cm |
| Living room | 16–25 m² | 55–75 cm |
| Open-plan living or kitchen | 25 m² and up | 75 cm and up, or two fittings |
Past about 25 m², a single fitting stops being the right answer however wide it is. Light falls away with distance, so one very large fitting in the centre leaves the edges of a long room dim while the middle is over-lit. Two smaller fittings spaced along the room give a far more even result.
Ceiling height changes the answer
A standard British ceiling is around 2.4 m, and the figures above assume that. Higher ceilings need more fitting, because the light has further to travel before it reaches anything.
A rough adjustment: for every 30 cm of ceiling height above 2.4 m, add about 5 cm to the diameter. A Victorian room with 3 m ceilings wants roughly 10 cm more than the chart suggests.
Low ceilings work the other way, but the constraint there is depth rather than width. Below 2.4 m, a flush fitting is the only comfortable choice. Anything that drops will be in someone's eyeline, and in a hallway, in their way.
The figure that matters more than diameter
Diameter is about proportion, whether the fitting looks right on the ceiling. It says surprisingly little about whether the room will actually be bright enough.
For that, read the lighting coverage on the product page, given in square metres. It is the floor area the fitting is designed to light on its own, and it is the one number you should check against your room before you order.
Most of our ceiling lights cover 5–30 m². Smaller decorative fittings cover less. Amia is rated 5–15 m², which is a hallway or a small bedroom rather than a sitting room. A wide multi-head fitting like Amsel, at up to 94 cm with seven heads, is built for the other end of that range.
If the coverage figure is below your room's floor area, the honest answer is not a bigger ceiling light. It is a second light source: a table lamp in the corner, or a portable lamp where there is no socket.
A few cases the chart does not cover
Long, narrow rooms. The calculation assumes something roughly square. In a 2.5 m by 6 m room it will suggest an 85 cm fitting that physically dominates the width. Use the narrower dimension as your limit, and use two fittings along the length instead.
Rooms with a strong centre. If there is a dining table or an island below, you are no longer sizing to the room. You are sizing to the furniture, and a hanging fitting is usually the better choice. That has its own rules, which we cover in our guide to pendant lighting.
Home offices. A ceiling light alone rarely lights a desk well, whatever its size, and you end up working in your own shadow. There is more on getting that right in how to light your home office properly.
In short
- Add the room's length and width in metres, multiply by ten, take 15–20% off for a flush fitting.
- Add about 5 cm for every 30 cm of ceiling height above 2.4 m.
- Check the coverage in m² against your floor area. That is the brightness question, and diameter does not answer it.
- Past 25 m² or in a long room, use two fittings rather than one large one.
Every fitting in our ceiling lights collection lists its available diameters and its coverage in square metres. If you want the wider picture first, covering flush against semi-flush, light temperatures and materials, start with everything that matters when buying a ceiling light.