Lighting Calculator
Estimate general-lighting fixture count from area, target lux, and usable lumens.
How the result changes
Only room floor area changes; all other inputs stay at your current values. The line illustrates the formula across the shown range. This is not a forecast of real-world outcomes.
About this calculation
Illuminance in lux means lumens reaching each square metre, while a fixture rating states lumens emitted. This calculator connects the two with a user-selected utilisation factor and rounds the resulting fixture count upward. Calculate lighting when comparing household options.
Formula
estimated fixtures = ceil(area × target lux ÷ (lumens per fixture × utilisation fraction))
Worked example
With Room floor area: 20 m², Target illuminance: 300 lux, Lumens per fixture: 800 lm, Utilisation factor: 70 %, lumens at task plane is 6,000 lm.
Inputs and units
Enter floor area in square metres and a target lux value chosen for the task. Provide lumens emitted by one proposed fixture and an estimated percentage that reaches the working plane after room geometry and surface effects.
- Room floor area (m²): Measure the floor area receiving general illumination.
- Target illuminance (lux): Choose a target appropriate to the room's actual task and applicable guidance.
- Lumens per fixture (lm): Use the luminaire's delivered lumen rating where available.
- Utilisation factor (%): Estimate the share of fixture lumens reaching the task plane.
Calculation and worked example
Multiply target lux by area for lumens needed at the task plane. Multiply lumens per fixture by the utilisation fraction for usable lumens per fixture, then divide and round upward to a whole fixture In compact notation, estimated fixtures = ceil(area × target lux ÷ (lumens per fixture × utilisation fraction)).
A 20 m² room targeted at 300 lux needs 6,000 lumens at the task plane. An 800-lumen fixture with 70% utilisation contributes an estimated 560 usable lumens, so 6,000/560 rounds upward to eleven fixtures.
How to use the answer
The count is a simplified average-illuminance estimate. Eleven fixtures placed badly may leave dark spots; actual lighting quality also depends on spacing, mounting height, glare, reflectance, and maintenance.
Compare fixture choices or see how a different utilisation assumption affects quantity. Use a proper lighting plan and applicable standards for workplaces, safety-critical spaces, or detailed architectural design.
Assumptions and checks
Fixtures are identical, contribution adds linearly, and the chosen utilisation factor adequately summarizes light reaching the task plane. The method does not simulate room distribution or daylight.
Lux and lumens are not interchangeable: 300 lux over 20 m² calls for 6,000 delivered lumens, not 300. Enter 70 for seventy percent, not 0.7, in the percentage field.
At 100% utilisation, 1,000 lumens per fixture and 1,000 lumens required, one fixture is enough. Lowering utilisation must never reduce the estimated fixture count.
Common questions
What does the lumens at task plane result represent?
It is the total light output estimated to reach the room's task plane for the selected area and average illuminance target.
What should I check before relying on this result?
Check target lux, actual fixture lumens, utilisation estimate, and distribution; use a lighting professional when design requirements are critical.
Inputs
- Room floor area (m²)
- Target illuminance (lux)
- Lumens per fixture (lm)
- Utilisation factor (%)