Room Lighting Calculator

Direct answer

Convert the measured room area to square metres, multiply by the illuminance target you choose, divide by stated lamp lumens and round up to a whole lamp. U.S. Department of Energy material directs consumers to compare bulb brightness in lumens; this calculator uses the entered lumen rating and makes no universal lighting recommendation.

Variables and formula

• Room area: length multiplied by width, converted from square feet to square metres.
• Target illuminance: a scenario input in lux.
• Lumens per lamp: light output stated for one lamp.

Lamps needed = ceiling(Room area in square metres * Target illuminance / Lumens per lamp)

Reproducible example

A 16 ft by 12 ft room is 192 sq ft, or about 17.84 square metres. At 200 lux, it needs 17.84 * 200 = 3,567.48 lumens. With 800-lumen lamps, ceiling(3,567.48 / 800) = 5 lamps.

Interpretation

The result is an area-and-output estimate. Check how fixtures distribute light before buying lamps, especially where a task area needs different light from the rest of the room.

Assumptions and limitations

• The room is rectangular and lamp ratings are comparable.
• The illuminance target is chosen by the user.
• Daylight, glare, fixture losses, reflectance, dimming and layout are excluded.

Common mistakes

• Treating watts as a measure of light output.
• Mixing lumen ratings from unlike products.
• Assuming a whole-room estimate replaces task lighting.

Assumptions

• The room is rectangular and the entered lumens are comparable lamp ratings.
• The illuminance target is a user-selected scenario input.
• Fixture distribution, reflectance, daylight, glare, dimming and task-specific lighting are excluded.

Sources and methodology

CalcMotive publishes the formula and assumptions so you can decide whether the estimate fits your use case. See our methodology standards.

• Energy 101: Lighting choices; U.S. Department of Energy; accessed Aug 6, 2026