Lighting Calculator
Calculate how many lumens a room needs and how many fixtures deliver them, using the lumen method with published footcandle targets for each room type.
By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Fixtures needed
- 6 fixtures
- 1,600 lumens each, led bulb in a fixture
- Lumens required
- 8,400 lumens
- 35 fc target x 168 sq ft, divided by a 0.7 room efficiency factor
- Light delivered
- 40 footcandles
- Target band for a kitchen, general is 30 to 40 fc
- Lumens per square foot
- 57 lumens per sq ft
- Installed lumens before the room efficiency factor is applied
- Connected load
- 96 watts
- About 105 kWh a year at 3 hours a day. Enter a rate to see the cost
Lighting formula
Lumens required = Footcandle target x Area (sq ft) / Efficiency factor
Fixtures = Lumens required / Lumens per fixture, rounded up
Footcandles delivered = Fixtures x Lumens x Efficiency factor / Area
Watts = Fixtures x Lumens x Watts per 1,000 lumens / 1,000Worked example
A 20 x 22 ft garage workshop at the 60 footcandle target: 440 sq ft x 60 = 26,400 lumens before losses. At a 0.7 efficiency factor the requirement is 37,714 lumens.
Four foot LED shop lights at 4,400 lumens each cover it in 9 fixtures, which is 39,600 installed lumens and 63 footcandles delivered. At 7.5 watts per 1,000 lumens that is 297 watts for the room, or about 325 kWh a year at 3 hours a day.
Quick reference
| Room | Footcandles | Lumens per sq ft at 0.7 efficiency |
|---|---|---|
| Hallway or stairs | 5 to 10 | 7 to 14 |
| Living or family room | 10 to 20 | 14 to 29 |
| Bedroom | 10 to 20 | 14 to 29 |
| Kitchen, general | 30 to 40 | 43 to 57 |
| Home office | 40 to 50 | 57 to 71 |
| Garage or workshop | 50 to 75 | 71 to 107 |
| Kitchen counter or vanity | 70 to 80 | 100 to 114 |
The lumen method in one pass
Lighting a room is three numbers: the footcandle target for what happens there, the floor area, and an efficiency factor for everything that eats light on the way down. Multiply the first two, divide by the third, and you have the lumens the room needs. Divide by the lumens on the box and you have the fixture count.
The efficiency factor is the part people skip. It rolls together the coefficient of utilization, which depends on ceiling height and how reflective the walls are, and the light loss factor for dirt and lumen depreciation over the life of the lamp. Seven tenths suits a normal room with light walls and an 8 to 9 foot ceiling. A dark room with a 12 foot ceiling is closer to a half, and that alone changes the fixture count by 40 percent.
Layers, not one big number
A single footcandle figure lights a room evenly, which is right for a hallway and wrong for a kitchen. Real rooms want layers: ambient light at the general target, task light at 70 to 80 footcandles where someone chops or shaves, and accent light for contrast. Run this calculator once for the ambient layer and again for the counter or vanity, treating the task area as its own small room.
Then decide the control. Anything at or above the top of its band belongs on a dimmer, because a kitchen at 40 footcandles is right at dinner and harsh at midnight. Keep one color temperature per room, usually 2700 K to 3000 K in living spaces and 4000 K in a shop, and check the color rendering index on anything over a work surface.
What this calculator leaves out
Fixture placement and spacing, which decide whether the light is even or blotchy, and the beam angle of a directional lamp. It does not account for daylight, window size or shading, the reflectance of specific paint colors, or the difference between a bare bulb and the same bulb behind a shade. It also leaves out switching and dimmer compatibility, the circuit the fixtures land on, and any local energy code requirement for high efficacy lamps or controls.
Mistakes that cost money
- Buying by watts. A 9 W LED can be 800 or 1,100 lumens depending on the model, and only the lumen number sizes a room.
- Ignoring wall color. Dark walls and a high ceiling drop the efficiency factor from 0.7 to about 0.5, a 40 percent swing in fixture count.
- One central fixture for a kitchen. General light at 35 fc does not reach 75 fc at the counter, and the cook stands in their own shadow.
- Mixing color temperatures in one room; 2700 K and 4000 K side by side reads as a fault, not a design choice.
- Sizing to the maximum band everywhere, then having no way to turn it down. Put high-output rooms on dimmers.
- Forgetting that lumens fall over life. Fixtures are rated L70, meaning 70 percent output at end of life, which the loss factor covers.
Key facts
- A footcandle is one lumen falling on one square foot, which is the unit US lighting targets are published in.
- Published targets run about 10 to 20 footcandles for a living room, 30 to 40 for general kitchen light, 70 to 80 at a work counter or vanity, and 5 to 10 in a hallway.
- Lumens replaced watts on the box: 800 lumens is a 60 W incandescent, 1,100 is a 75 W and 1,600 is a 100 W.
- Household LED bulbs run about 80 to 100 lumens per watt, and 4 ft LED shop lights reach 120 to 140.
- The efficiency factor combines the coefficient of utilization with light loss; 0.7 is a normal room and 0.5 a dark one with a high ceiling.
Frequently asked questions
How many lumens do I need for a room?
Multiply the footcandle target by the floor area and divide by about 0.7. A 14 x 12 ft kitchen at 35 footcandles needs 8,400 lumens, or roughly six 1,600 lumen fixtures.
What is a footcandle?
One lumen falling on one square foot. It is the unit US lighting targets are published in, and it measures light arriving at a surface rather than light leaving a lamp.
How many lumens replace a 60 watt bulb?
About 800 lumens. A 75 W incandescent is about 1,100 lumens and a 100 W is about 1,600. Buy by the lumen number on the box, since LED wattage varies between models at the same output.
Why does my kitchen feel dark with plenty of fixtures?
General light at 35 footcandles does not do task work. Counters want 70 to 80 footcandles from under-cabinet or directly overhead fixtures, and a single ceiling light puts the cook in their own shadow.
Sources and references
- High Efficacy Lighting, lumens, efficacy and room lighting levels (Pacific Northwest National Laboratory)
- Lighting Fundamentals, footcandles, lumens and the lumen method (Lighting Design Lab)
- Solar Energy Technologies Office, household electricity use and rates (US Department of Energy)
Next steps for this project
The usual order for a electrical & plumbing project. See all electrical & plumbing calculators.
Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.