How Many Lumens Per Square Foot Do I Need?
Choosing the right amount of light for each room in your home is easier than you think. Our lighting calculator (foot candle calculator) determines the optimal illumination level for any room and tells you exactly how many light fixtures you need to achieve it.
Understanding Foot-Candles and Lux
Illumination is measured in foot-candles (FC) for the American/Imperial system or lux for the metric system. One foot-candle equals 10.764 lux. Both measure the amount of light falling on a surface.
| Room / Activity | Foot-Candles (FC) | Lux |
|---|---|---|
| Bedroom (ambient) | 15 FC | 161 lux |
| Living Room | 15 FC | 161 lux |
| Kitchen (general) | 40 FC | 430 lux |
| Kitchen (counter / task) | 100 FC | 1,076 lux |
| Bathroom | 75 FC | 807 lux |
| Office / Study | 60 FC | 645 lux |
| Dining Room | 35 FC | 376 lux |
| Hallway | 7 FC | 75 lux |
| Closet | 20 FC | 215 lux |
Lighting Calculation Formula
The calculator uses the following equations:
- Imperial: Lumens needed = Foot-candles × Area (sq ft)
- Metric: Lumens needed = Lux × Area (m²)
- Fixtures needed = ⌈Lumens needed / (Lumens per fixture × Efficiency)⌉
The Coefficient of Utilization (CU / efficiency, typically 0.5–0.8) accounts for light lost due to fixture design, room shape, and surface reflectance. A value of 0.7 is a reliable default for most home environments.
Using the Foot-Candle Calculator: An Example
Suppose you want to light a bedroom that is 12 ft × 15 ft = 180 sq ft using LED bulbs rated at 800 lumens each with a CU of 0.7.
- Recommended illuminance for a bedroom: 15 FC
- Lumens needed: 15 × 180 = 2,700 lm
- Fixtures needed: ⌈2,700 / (800 × 0.7)⌉ = ⌈4.82⌉ = 5 fixtures
Electricity Cost Calculation
The calculator also estimates your monthly and yearly electricity costs. Simply enter the wattage of each bulb, the hours of use per day, and your electricity rate. You can select from 12 major world currencies including USD, EUR, GBP, CAD, AUD, JPY, INR, BRL, and more.
- Monthly kWh = Fixtures × Watts × Hours/day × 30 / 1,000
- Monthly cost = Monthly kWh × Rate per kWh
FAQs
- What is a foot-candle?
- A foot-candle is a unit of illuminance equal to the light intensity produced by one candle on a surface one foot away. It equals 10.764 lux.
- How many lumens do I need per square foot?
- It depends on the room. A bedroom needs roughly 15 lumens per sq ft, while a kitchen task area may need 70–150 lumens per sq ft.
- What is the coefficient of utilization?
- The CU (0–1) represents how efficiently the fixture delivers light to the work surface. A typical LED fixture in a standard room has a CU of about 0.65–0.75.
- Can I use this calculator for outdoor lighting?
- Yes — select "Custom" room type and enter your target foot-candle or lux value manually.