Electrical and solar
Lighting calculator
This calculator works out how many light fittings a room needs to be properly lit, based on what the room is for. A kitchen or study needs about twice the light of a bedroom, and a workshop more again. Enter the room size, choose the room type and the light fitting, and it gives the number of fittings, the total wattage and a simple grid to space them on.
What you need
This calculator works out the figures in your browser, which needs JavaScript switched on. The method below shows how to work it out by hand.
Estimates for planning and budgeting. Real quantities depend on the site and the workmanship, so check with your builder before you order.
Light is measured in lumens (what a lamp gives out) and lux (how much lands on each square metre). The calculator uses the standard lumen method: the lux the room needs, times its area, divided by the lumens of each fitting after allowing for light lost to walls and ceilings and for lamps dimming with age.
LED lamps are quoted in watts and lumens; always go by the lumens. A 9 W LED bulb gives about 800 lumens, roughly what an old 60 W bulb gave.
How it is worked out
- Fittings = (lux required × room area) ÷ (lumens per fitting × utilisation factor × maintenance factor), rounded up.
- The fittings are arranged on the grid closest to the room's proportions, with half a spacing to the walls.
What it assumes
- Average illuminance across the room at working height.
- Task lighting (under cupboards, at a desk) is extra.
Common questions
How many downlights for a bedroom?
A 4 × 3.5 m bedroom at 150 lux needs about 2,100 lumens after losses, so about four 600 lumen downlights or three 9 W bulbs.
How many lux does a kitchen need?
About 300 lux overall, with extra light over worktops.
What is the difference between watts and lumens?
Watts measure power used; lumens measure light given out. Compare LED lamps by lumens, then choose the one with the fewest watts for those lumens.
Want exact quantities for your plan?
A calculator works one wall or one slab at a time. A bill of quantities works from your actual drawings and counts everything, which is what builders price against and what stops you buying twice.