Room lighting calculator
How many lumens a room needs and how many lamps that comes to.
Fill in the fields and the result will appear here automatically.
Estimate total lumens and whole identical lamps for an area and selected lux. Use the lamp output from its package; a retained-light factor raises initial flux to allow for reduced light over time. This simplified relation assumes 100% utilisation. It compares lamp sets without calculating distribution or guaranteeing illuminance at your work surface.
How it works
Formula and logic
Simplified flux estimate Φ=A×E/f lumens, with area A, selected illuminance E lux and retained-light factor f from 0.4 to 1. Lamps N=ceil(Φ/φ); installed flux N×φ. Utilisation is assumed 1: beam distribution, reflections, working plane and measured lux are not calculated. N rounds up to whole lamps without guaranteeing compliance with the illuminance target.
Example
18 m² at 150 lx with 800 lm lamps and a 0.8 factor needs 3,375 lm, which is five lamps. For 1 m² at 50 lx, f=1 and a 1000 lm lamp, estimated flux is 50 lm but whole lamp count 1 and installed flux 1000 lm.
Fields and units
- Room area, m² — m²
- Target illuminance, lx — lx
- Lamp output, lm — lm
- Retained-light factor — unitless
How to use
- — Enter the floor area of the room.
- — Choose the illuminance you are aiming for.
- — Enter the output of one lamp in lumens — it is on the box.
- — Set a retained-light fraction 0.4–1 for your scenario; it is a separate assumption, not compensation for all distribution losses.
Method and limitations
- Calculation method
- Formula and logic
- Data or methodology source
- CIE: maintenance factor compares light after time with initial light CIE: utilisation is the flux fraction on the reference plane; here assumed 1
- Limitation
- Utilisation is assumed 1: beam distribution, reflections, working plane and measured lux are not calculated. N rounds up to whole lamps without guaranteeing compliance with the illuminance target.
FAQ
What illuminance should I aim for?
Choose a target for the actual task. The retained 150 lux is an example, not a universal room requirement. Requirements depend on the work surface and applicable standard; verify the result by measurement or lighting design.
What is the maintenance factor for?
The field is the retained fraction of light, not a multiplier above one. With 0.8 the required initial flux is divided by 0.8, increasing 25%. It does not determine how much reaches the work plane or ensure compliance on its own.
Does the ceiling height matter?
Yes: actual distribution depends on distance to the work plane and luminaire characteristics. Height is absent here, so a high ceiling is not automatically compensated. An arbitrary percentage is no substitute for a lighting calculation.
Do wall and ceiling colours matter?
Yes. Surface reflectance changes the flux reaching the relevant plane. This estimate fixes utilisation at 1 and does not calculate those properties. Check distribution separately for dark surfaces and directional light.
Can I mix different lamps?
Yes: add their lumens manually and compare with the estimated flux. The displayed lamp count assumes identical lamps at the entered output. A lumen sum alone does not determine lux distribution.