Lighting Calculator
Calculate how many light fixtures you need for any room — bedrooms, kitchens, offices and more — based on IES illuminance targets and fixture lumens.
What this calculates
Calculates the number of fixtures needed to reach the recommended illuminance for the selected room type, the actual lux achieved, and the estimated total installed power assuming LED sources.
Formula used
Fixtures = ceil((target lux × area) ÷ (lumens × CU × FM)). CU (coefficient of utilization) = 0.65. FM (maintenance factor) = 0.80. Watts estimated at 1 W per 100 lm (LED).
Worked example
A 4 m × 3 m bedroom (12 m²) at 150 lux with 800 lm fixtures: fixtures = ceil((150 × 12) ÷ (800 × 0.65 × 0.80)) = ceil(1800 ÷ 416) = ceil(4.3) = 5 fixtures. Total: 5 × 8 W = 40 W.
IES illuminance targets
Hallway / storage: 100 lux. Bedroom: 150 lux. Living room: 200 lux. Bathroom: 300 lux. Office / kitchen / workshop: 500 lux. These are ambient (general) lighting targets; task areas benefit from additional local lighting.
When not to use this calculator
This calculator handles simple rectangular rooms with uniform ceiling fixtures. Complex layouts, architectural lighting, emergency lighting, retail or industrial spaces require a photometric design using dedicated lighting software.
Frequently asked questions
How many lumens do I need per m²?
Multiply the required lux by the room area to get total lumens needed (before CU/FM losses). A 150-lux bedroom of 12 m² needs a theoretical 1,800 lm, but about 2,900 lm accounting for losses.
What is the difference between lumens and lux?
Lumens = total light output of a source. Lux = lumens per m² at the work plane. 1 lux = 1 lm/m².
How many lumens does an LED produce?
Typical LEDs: 80–120 lm/W. A 10 W LED produces roughly 800–1,000 lm. This calculator estimates 100 lm/W for budget purposes.
What lux do I need for a home office?
400–500 lux for general ambient lighting. Supplement with a dedicated desk lamp for the work surface to reach 300–500 lux at the task plane.
Assumptions
- Coefficient of utilization (CU): 0.65 (average room with light-colored walls).
- Maintenance factor (FM): 0.80 (accounts for dust and lumen depreciation over time).
- LED efficacy estimate: 100 lm/W (1 W per 100 lm).
- Uniform ambient lighting in a rectangular room — no task or accent lighting modeled.
Actual illuminance depends on fixture type, ceiling height, room reflectances and lamp position. For critical applications use photometric software (DIALux, Relux, AGi32) with manufacturer IES files.
Common mistakes
- Using watts instead of lumens — newer LED equivalents make wattage misleading.
- Forgetting that ceiling height reduces effective lux at the work plane.
- Not adding task lighting in kitchens and offices where 500 lux is needed locally.