How to use the lighting calculator
Getting the amount of light right is one of the first lighting design decisions on any interior. Too little and a kitchen or office feels gloomy and fails the brief; too much and you waste energy, blow the lighting power budget and create glare. This calculator uses the lumen method, the standard hand calculation for general lighting, to turn a target light level into a number of fittings.
- Choose a room type to load a typical target illuminance, or choose Custom and type your own. Switch units to work in feet and footcandles.
- Enter the room length and width and the rated output of one fitting in lumens, taken from the manufacturer's data sheet (use the fitting output, not the bare LED chip).
- Adjust the factors if you know them. The defaults (utilisation 0.6, maintenance 0.8) suit a pale room with LED downlights or panels.
- Read the results: total lumens needed, number of fittings, the light level you'll actually get with that number, installed watts, power density and a suggested grid with spacings.
Lumen method formulas
Lumens needed = E × A ÷ (UF × MF)
Fittings N = Lumens needed ÷ lumens per fitting (round up)
Achieved E = N × lumens per fitting × UF × MF ÷ A
Installed watts = N × lumens per fitting ÷ efficacy
1 footcandle = 1 lm/ft² = 10.764 lux
E is the target illuminance on the working plane (lux or footcandles), A the floor area (m² or ft²), UF the utilisation factor and MF the maintenance factor. Because a lux is a lumen per square metre and a footcandle a lumen per square foot, the formula works in either system as long as you keep area and light level in matching units.
| Space | Typical lux | Footcandles |
|---|---|---|
| Bedroom, hallway, stairs | 100 | 10 |
| Living room | 150 | 15 |
| Dining room, bathroom | 200 | 20 |
| Kitchen (general), classroom | 300 | 30 |
| Kitchen worktop, office desk, retail | 500 | 50 |
| Drawing board, detailed work | 750 | 75 |
These are typical design values drawn from EN 12464-1, the SLL Code for Lighting and IES recommendations. Always check the standard or client brief that applies to your project.
Worked examples
A living room
A 5 × 4 m living room (20 m²) at 150 lux, with UF 0.6 and MF 0.8, needs 150 × 20 ÷ 0.48 = 6,250 lumens. With 800-lumen downlights that's 7.8, so 8 fittings. At 90 lm/W they draw about 71 W, or 3.6 W/m². In practice you'd put some of those lumens into lamps and wall lights on dimmers.
A kitchen in footcandles
A 12 × 10 ft kitchen (120 ft²) needs 30 footcandles of general light. Lumens = 30 × 120 ÷ 0.48 = 7,500. Using 1,000-lumen recessed lights gives 7.5, so 8 fittings. For even spacing the calculator suggests a 3 × 3 grid of 9 lights at about 4 × 3.3 ft centres, giving about 36 fc. Under-cabinet strips then lift the worktops toward 50 fc.
An open-plan studio
A 10 × 8 m architecture studio (80 m²) at 500 lux needs 500 × 80 ÷ 0.48 ≈ 83,300 lumens. With 4,000-lumen LED panels that's 20.8, so 21 panels. The calculator suggests a 6 × 4 grid (24 panels, roughly 1.7 × 2 m centres), which gives about 576 lux and a tidy, even ceiling.
Who this calculator helps
- Architects fixing ceiling layouts, coordinating fittings with grids and services, and checking lighting power density at concept stage.
- Interior designers working out how many downlights, pendants or tracks a scheme really needs.
- Lighting and electrical designers doing a quick lumen-method check before modelling in DIALux or AGi32.
- Electricians and contractors pricing jobs and checking that a drawing has enough fittings.
- Homeowners choosing LED bulbs and deciding how many recessed lights to put in a room.
Practical tips
- Layer the light. Use the calculator for the general (ambient) layer, then add task lighting at worktops and desks and accent lighting for art and texture.
- Pale surfaces raise the utilisation factor. A dark ceiling or deep-coloured walls can cut useful light by a third, so lower the UF if your room colour palette is dark.
- Daylight comes first. Check daytime light levels with the daylight factor calculator and put perimeter fittings on separate, daylight-linked circuits.
- Measure irregular rooms with the square footage calculator first.
- Keep downlights about half their spacing from walls, and line them up with furniture, not just the room outline.
Want to see how a room looks with warmer, cooler or evening light before choosing fittings? Upload a photo and try Relight.
