How the lighting calculator works
Lighting levels are measured in foot-candles in the US (lux in metric). One foot-candle is one lumen spread over one square foot, so the lumens a room needs are simply its floor area times the target foot-candles.
Dividing by the lumens each bulb or fixture gives, and rounding up, gives the count. The diagram spreads that many fixtures evenly so you can picture the layout; the breakdown shows the extra light from rounding up.
Formula and assumptions
- foot-candles
- lumens per square foot for the room’s use
- lumens per fixture
- printed on the bulb box or fixture listing
Assumptions and limits
- Estimates general (ambient) light from the fixtures’ rated lumens.
- Does not model beam angle, fixture placement, or light loss over time.
Worked example
A 14 × 12 ft living room at 20 foot-candles with 800-lumen bulbs (example).
- Area = 168 sq ft.
- 168 × 20 = 3,360 lumens.
- 3,360 ÷ 800 = 4.2; round up to 5 bulbs.
- Five bulbs give 4,000 lumens, about 23.8 foot-candles.
These are the calculator’s default values, so the live result above matches this example until you change an input.
How to measure correctly
- Measure the floor area you want to light; split open-plan spaces into zones.
- Use lumens, not watts. LED bulbs list lumens on the box.
- Plan task lighting (counters, desks, mirrors) as its own zone at a higher level.
- Dark walls and high ceilings absorb light; aim toward the top of the range.
Brightness by room
| Room | Foot-candles | Lux (approx.) |
|---|---|---|
| Hallway | 5–10 | 54–108 |
| Living room | 10–20 | 108–215 |
| Bedroom | 10–20 | 108–215 |
| Kitchen (general) | 30–40 | 323–431 |
| Kitchen work areas | 70–80 | 753–861 |
| Bathroom | 70–80 | 753–861 |
Lighting Calculator FAQ
How many lumens do I need for a room?
Multiply the floor area in square feet by the foot-candles the room needs. A 168 sq ft living room at 20 foot-candles needs about 3,360 lumens.
What is a foot-candle?
A unit of illuminance: one lumen falling on one square foot. The metric equivalent is lux, one lumen per square meter; one foot-candle is about 10.76 lux.
How many lumens is a 60-watt equivalent LED?
Many 60-watt-equivalent LED bulbs are rated around 800 lumens, but check the box: lumens, not watts, tell you brightness.
How many recessed lights do I need?
Divide the room’s lumen target by the lumens of each downlight and round up, then space them evenly. The diagram shows an even spread for your count.
Should bathrooms and kitchens be brighter?
Yes. Home Depot’s guide suggests 70–80 foot-candles for bathrooms and kitchen work areas, versus 10–20 for living rooms and bedrooms.
Do dimmers change the calculation?
Plan for the brightest level you need, then dim down. Dimmers make it safe to land at the top of the range.