Lighting Calculator
Estimate required lumens, fixture quantity, average spacing, installed wattage and annual electricity cost for a rectangular room.
Default project preview
6 fixtures
Design lumens
16,071
Average spacing guide
7.1 ft
Installed load
168 W
Annual electricity
367.9 kWh
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Room length
Enter this project assumption in ft. The current imperial default is 20 ft.
Room width
Enter this project assumption in ft. The current imperial default is 15 ft.
Target light level
Enter this project assumption in foot-candles. The current imperial default is 30 foot-candles.
Lumens per fixture
Enter this project assumption in lm. The current imperial default is 3000 lm.
Coefficient of utilization
Enter this project assumption in %. The current imperial default is 70 %. Minimum: 1. Maximum: 100.
Maintained-light factor
Enter this project assumption in %. The current imperial default is 80 %. Minimum: 1. Maximum: 100.
Watts per fixture
Enter this project assumption in W. The current imperial default is 28 W.
Operating hours
Enter this project assumption in hours/day. The current imperial default is 6 hours/day. Minimum: 0. Maximum: 24.
Electricity price
Enter this project assumption in per kWh. The current imperial default is 0.18 per kWh. Minimum: 0.
How to read the outputs
Design lumens
losses and utilization included. Default preview: 16,071.
Average spacing guide
square-grid planning guide. Default preview: 7.1 ft.
Installed load
28 W per fixture. Default preview: 168 W.
Annual electricity
6 hours per day. Default preview: 367.9 kWh.
Annual energy cost
entered electricity price. Default preview: $66.23.
Maintained target
entered design level. Default preview: 30 fc.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Raw lumens = room area × target light level. Design lumens divide raw lumens by utilization and maintained-light factors. Fixtures = round up (design lumens ÷ fixture lumens).
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 30, 2026
Use this calculator to estimate fixture quantity from target illuminance, room area, fixture lumens and an editable utilization factor. It supports preliminary lighting layouts, but glare, uniformity, controls and task-specific requirements still need a real design.
Calculation steps
- 1Calculate room area.
- 2Multiply area by the target illuminance.
- 3Reduce each fixture’s rated lumens by the entered utilization factor.
- 4Divide required lumens by effective lumens per fixture and round upward.
Worked numerical example
A 20 ft × 15 ft workshop is 300 ft². At a target of 50 foot-candles, 15,000 delivered lumens are needed. With 4,000-lumen fixtures and an 80% utilization factor, divide by 3,200 effective lumens and round up to 5 fixtures.
Purchasing and rounding
Fixtures are purchased as whole units. Keep lamp or integrated-LED output, color temperature, dimming protocol and mounting type consistent across the planned zone.
Interpret the result
A higher fixture count may be caused by a high task-light target, low-output fixtures or a conservative utilization factor. The result does not prove even light distribution.
Page-specific assumptions
- Uniform rectangular area.
- Fixture lumen output is current and accurate.
- One target illuminance for the whole zone.
- Utilization factor reasonably represents room surfaces and mounting height.
Common mistakes
- Using watts instead of lumens.
- Ignoring fixture spacing and glare.
- Applying one target to mixed-use areas.
- Rounding fixture count down.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
FAQ
Lighting Calculator questions
What is a foot-candle?+
One foot-candle is one lumen per square foot. One lux is one lumen per square metre.
Why are utilization and light-loss factors used?+
Not all fixture lumens reach the work plane, and output declines with dirt, ageing and operating conditions.
Does equal spacing guarantee uniform light?+
No. Fixture beam shape, mounting height and room surfaces determine uniformity.