Lighting Calculator
Free lighting calculator. Get recommended lumens and bulb count for any room by size, type, and ceiling height. No signup.
A lighting calculator turns a room’s floor area and purpose into a target lumen figure and the number of bulbs needed, using foot-candle recommendations and a ceiling-height correction.
The lumen method
The calculation multiplies area by a light-level target, adjusts for ceiling height, then divides by the output of a single bulb:
area = length × widthlumens needed = area × foot-candlesheight factor = max(1, 1 + (height − 8) ÷ 2 × 0.10)adjusted lumens = lumens needed × height factor, thenbulbs = ceil(adjusted lumens ÷ bulb lumens)
A foot-candle is one lumen per square foot, so a 20 fc target is the same as asking for 20 lumens per square foot. The height factor adds about 10% per extra two feet above the 8-foot baseline the targets assume.
Worked example: a 12 × 10 ft living room
Defaults: length 12 ft, width 10 ft, height 8 ft, living/family room (20 fc), 800-lumen bulbs.
| Step | Calculation | Result |
|---|---|---|
| Floor area | 12 × 10 | 120 ft² |
| Target lumens | 120 × 20 | 2,400 lm |
| Height factor at 8 ft | 1 + (8 − 8) ÷ 2 × 0.10 | 1.00 |
| Adjusted lumens | 2,400 × 1.00 | 2,400 lm |
| 800 lm bulbs | 2,400 ÷ 800 = 3.0 | 3 bulbs |
Raise the ceiling to 10 ft and the factor becomes 1.10, lifting the target to 2,640 lumens and the count to 4 bulbs. That is the same room feeling dimmer purely because the light has farther to travel.
Reference: room type and light level
| Room type | Target | Total for 120 ft² | 800 lm bulbs |
|---|---|---|---|
| Hallway | 15 fc | 1,800 lm | 3 |
| Living / family | 20 fc | 2,400 lm | 3 |
| Bedroom | 20 fc | 2,400 lm | 3 |
| Kitchen | 40 fc | 4,800 lm | 6 |
| Bathroom | 40 fc | 4,800 lm | 6 |
| Office / study | 40 fc | 4,800 lm | 6 |
| Garage / workshop | 40 fc | 4,800 lm | 6 |
These are the exact targets the calculator applies. A 60-watt-equivalent LED is about 800 lumens, so the bulb count maps neatly onto common retrofit lamps.
General versus task lighting
The lumen figure covers general lighting for the whole room. Kitchens, offices, and workshops usually need task lighting on top: under-cabinet strips over a counter, a lamp at a desk, or a vanity light at a mirror. A common split is to supply roughly two-thirds of the target from ceiling fixtures and the rest from task lights, so the room reads as comfortable without every surface at maximum brightness.
Dimmers help too. They let you run lower at night without changing the fixture count, and they cost far less than adding circuits or fixtures.
| Lamp type | Approx. lumens per watt |
|---|---|
| Incandescent | 12–18 |
| Halogen | 15–20 |
| Compact fluorescent | 50–70 |
| LED | 80–120 |
This is why comparing watts misleads: a 9-watt LED and a 60-watt incandescent both land near 800 lumens, but the LED uses a fraction of the power.
Common mistakes
- Shopping by wattage. A 9 W LED and a 60 W incandescent can both emit around 800 lumens. Compare lumens, not watts.
- Under-lighting task areas. A room average cannot cover a kitchen counter or a desk. Add task lighting on top of the general figure.
- Ignoring ceiling height. The same fixture in a 10- or 12-foot room delivers noticeably less light at the working surface.
- Assuming the count is a fixture count. The result is bulbs, not fixtures; a five-bulb fixture covers a lot of the total.
How many lumens per square foot do I need?
Living rooms and bedrooms need about 20 lumens per square foot, kitchens, bathrooms, offices, and garages about 40, and hallways about 15.
How many 800-lumen bulbs for a 12 × 10 living room?
Three. The room is 120 ft², the target is 2,400 lumens, and 2,400 ÷ 800 = 3.
Does ceiling height change the answer?
Yes. The calculator adds roughly 10% to the lumen target for every two feet above 8 feet, so a 12-foot ceiling needs about 20% more light than the same room at 8 feet.
Lighting is the last layer of a room build. The drywall calculator and paint calculator cover the surfaces the fixtures mount to.