Calculate how many light bulbs you need for any room. Enter your room dimensions, choose the room type, and specify your bulb lumens — the Lighting Calculator tells you the total lumens required and the number of bulbs needed to properly illuminate the space. Also try the use the Lux to Lumens Calculator.
Bulbs Needed
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Total Lumens Required
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Room Area
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Recommended Foot-Candles
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Effective Lumens per Bulb
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It depends on the room type. General living spaces typically need 10–20 foot-candles (lumens per sq ft), kitchens and offices need 30–50 foot-candles for task lighting, and hallways need around 5–10 foot-candles. This calculator applies the correct standard automatically based on the room type you select. See also our Emergency Lighting.
The basic formula is: Total Lumens = Room Area (sq ft) × Required Foot-Candles. Then divide the total lumens by the lumens your chosen bulb produces to get the number of bulbs needed. Don't forget to account for a light loss efficiency factor (typically around 80%) to compensate for fixture and surface absorption.
A 4 m × 5 m studio has an area of 20 m², which is about 215 sq ft. For a general living or studio space needing ~15 foot-candles, you'd need roughly 3,230 lumens. With standard 800-lumen LED bulbs at 80% efficiency, that comes out to about 5 bulbs.
Bedrooms benefit from layered lighting. For ambient (general) lighting, 10–15 foot-candles is recommended, while reading areas may need 30–50 foot-candles. Warm-toned bulbs (2700–3000K) are best for creating a relaxing atmosphere. You might also find our Watts to Lumens Calculator useful.
Lumens measure the total amount of visible light emitted by a source. Lux measures how much of that light falls on a specific area — one lux equals one lumen per square meter. Foot-candles are the imperial equivalent: one foot-candle equals one lumen per square foot, or approximately 10.764 lux.
Not all light emitted by a bulb reaches the surfaces you want to illuminate. Some is absorbed by the fixture housing, lampshade, or room surfaces like dark walls. An efficiency factor of 80% is a common estimate for typical residential installations — lower it if your fixtures are enclosed or your room has dark finishes.
Always use lumens. Watts measure energy consumption, not brightness. A modern LED bulb might produce 800 lumens using only 8–10 watts, while an old incandescent needed 60 watts for the same output. Lumens are printed on all bulb packaging and are the reliable measure of actual light output.
Yes — higher ceilings mean light has to travel farther, reducing the effective illuminance on floor-level surfaces. For ceilings above 10 feet, consider increasing your lumen estimate by 10–25%. This calculator uses a standard ceiling height baseline; for tall rooms, use a lower efficiency factor to compensate.