Room Lighting Lumens Calculator
How many lumens a room needs and how many bulbs or fixtures that takes: room size or area, room use or a custom foot-candle or lux level, bulb presets, an optional ceiling-height and wall-color correction, a layout grid, wattage and yearly running cost.
How it works
Quick method: lumens = area (ft²) × foot-candles, or area (m²) × lux · Lumen method: lumens = area × target ÷ (utilization factor × light-loss factor), utilization factor from the room cavity ratio 5 × (ceiling − 2.5 ft) × (length + width) ÷ area · Bulbs = lumens ÷ lumens per bulb, rounded up
One foot-candle is one lumen falling on one square foot (one lux is one lumen per square meter), so the light a room needs is its floor area times the recommended level for its use. That quick method is what most home guides use, and it is the default here. It assumes all the light lands on the floor. In a real room some light is absorbed by the walls and ceiling, more so in tall, narrow or dark rooms, and bulbs dim with age and dust. The optional lumen method accounts for that with a utilization factor (from the room cavity ratio and wall color) and a light-loss factor, which is how lighting designers size fixtures. The calculator also suggests a ceiling grid for the bulbs, a color temperature for the room, and shows the wattage and yearly running cost.
Worked example
A 12 × 12 ft room is 144 ft². Lit as general living space at 30 foot-candles it needs 144 × 30 = 4,320 lumens. With 800-lumen LED bulbs that is 4,320 ÷ 800 = 5.4, so plan for 6 bulbs, laid out as 2 rows of 3. At about 90 lumens per watt they draw 53 W. With the lumen method on (8 ft ceiling, light walls, 85% light kept), the room cavity ratio is 5 × 5.5 × 24 ÷ 144 = 4.58, the utilization factor is about 0.59, and the room needs 4,320 ÷ (0.59 × 0.85) ≈ 8,600 lumens, or 11 bulbs.
Sources: Foot-candle (Wikipedia) · Super Bright LEDs — IES residential recommended light levels · The Home Depot — lumens per square foot by room
Frequently asked questions
Should I buy bulbs by watts or lumens?
By lumens — lumens measure actual brightness, while watts only measure power draw. LEDs produce the same light as old incandescents at a fraction of the wattage, so a "60 W-equivalent" LED near 800 lumens draws under 10 real watts.
How many foot-candles does a room need?
Roughly 5–10 for hallways, 10–20 for bedrooms and lounges, 20–30 for living and dining rooms, 40 for a home office or laundry, 50 for kitchens and bathrooms, and 75–100 for workbenches, counters and sewing. In lux, multiply by about 10.76.
Why does the ceiling-height option need so many more lumens?
Because the quick method pretends every lumen lands on the floor. In practice a fixture sends light sideways and up too, and the walls and ceiling absorb part of it. Tall ceilings, small or narrow rooms and dark walls lose the most. A utilization factor of 0.5–0.7 is common in homes, so the lumen method often asks for 1.5 to 2 times the quick figure.
How far apart should ceiling lights be?
A common rule for recessed lights is spacing equal to about half the ceiling height (4 ft apart under an 8 ft ceiling), with the outer lights about half that distance from the walls. The suggested layout spreads your bulbs evenly over the room in rows.
What color temperature should I choose?
Warm white (2700–3000 K) suits bedrooms, living rooms and dining rooms. Neutral white (3000–4000 K) suits kitchens, bathrooms and offices. Cool white (4000–5000 K) suits garages, workshops and detailed tasks.
Is it better to use one bright bulb or several?
Several. Splitting the total lumens across multiple fixtures produces softer, more even illumination and fewer harsh shadows than a single very bright source, and it keeps working if one bulb fails.
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This calculator is for educational purposes. Double-check important results before acting on them.