Lighting Calculator
Work out how much light a room needs — from its size and your target light level — in lumens, bulbs and watts.
How many bulbs does a room need? It is the room's area times the light level, divided by what one bulb gives. This calculator is that division, with the light level yours to set.
Enter the room and the target; the bulbs follow.
Your results
Enter your measurements and press Calculate.
Quantities are rounded up to whole tiles, planks, boxes or bags, after the wastage allowance. Cost appears only when you enter a price.
Formula
All lengths are converted to a single unit before the calculation: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 ft² = 0.09290304 m², 1 yd² = 9 ft². Material quantities are rounded up — you cannot buy part of a tile, plank or bag.
How to use this calculator
- Enter the room length and width.
- Enter the target light level in lux.
- Enter the lumens and watts of your bulbs.
- Read the total lumens, the bulbs needed and the total watts.
Worked example
| Room | 4 m × 5 m = 20 m² |
|---|---|
| Target | 300 lux |
| Total lumens | 20 × 300 = 6,000 lm |
| Bulbs (800 lm each) | ceil(6,000 ÷ 800) = 8 bulbs |
| Total watts | 8 × 10 W = 80 W |
Check it by hand: every step above uses the formula in the box. If your numbers match the example inputs but the calculator gives a different answer, please report it via the contact page.
Frequently asked questions
What is a lux?
One lumen per square metre. A room of 20 m² at 300 lux needs 6,000 lumens. Lux is the light level; lumens are the light itself.
How many lumens do I need per room?
It depends on the room: hallway about 100 lux, bedroom 200, living room or kitchen 300, task lighting 500. Enter your own target — this calculator does the rest.
How many lumens does an LED bulb give?
About 800 lumens for a 10 W LED (roughly the old 60 W bulb). Check the packaging — this calculator uses the figure you enter.
Why round the bulbs up?
Because you cannot buy 7.5 bulbs. Rounding up to 8 gives slightly more light than the target, which is better than slightly less.
Related calculators
Estimates only. Results are for planning. Measure twice, confirm box coverage and tile sizes on the packaging you buy, and follow the manufacturer's instructions. See the disclaimer.