Lumen Guide: Lighting Design & Foot-Candle Calculator
Lumens, Foot-Candles & Lux
- Lumen (lm): Total light output from a source
- Foot-candle (fc): Light falling on a surface (1 fc = 1 lumen/sq ft)
- Lux (lx): Metric unit (1 lux = 1 lumen/m², 1 fc = 10.76 lux)
Foot-candles = Total lumens / Area (sq ft)
Recommended Foot-Candles by Space
| Space | Foot-Candles | Lumens/sq ft | Lux |
|---|---|---|---|
| Bedroom | 10–20 | 10–20 | 108–215 |
| Living Room | 10–20 | 10–20 | 108–215 |
| Kitchen (general) | 30–50 | 30–50 | 323–538 |
| Kitchen (counter) | 50–75 | 50–75 | 538–807 |
| Bathroom | 20–50 | 20–50 | 215–538 |
| Office (general) | 30–50 | 30–50 | 323–538 |
| Office (desk task) | 50–75 | 50–75 | 538–807 |
| Classroom | 30–50 | 30–50 | 323–538 |
| Workshop | 50–100 | 50–100 | 538–1,076 |
| Warehouse | 10–30 | 10–30 | 108–323 |
| Parking Garage | 5–10 | 5–10 | 54–108 |
| Exterior (security) | 1–5 | 1–5 | 11–54 |
LED vs Incandescent Lumens
| Incandescent (W) | LED (W) | Lumens | LED Savings |
|---|---|---|---|
| 25W | 4W | 250 | 84% |
| 40W | 6W | 450 | 85% |
| 60W | 9W | 800 | 85% |
| 75W | 11W | 1,100 | 85% |
| 100W | 14W | 1,600 | 86% |
| 150W | 22W | 2,600 | 85% |
Lumens per Watt (Efficacy)
| Technology | Lumens/Watt |
|---|---|
| Incandescent | 12–18 |
| Halogen | 16–24 |
| CFL | 50–70 |
| LED | 80–150 |
| High-pressure sodium | 80–140 |
| Metal halide | 70–110 |
Lighting Layout Calculation
Step 1: Room area = Length × Width
Step 2: Total lumens needed = Area × Target foot-candles
Step 3: Add 20% for light loss factor (dirt, aging, optics)
Step 4: Number of fixtures = Total lumens / (Fixture lumens × CU)
CU (Coefficient of Utilization) = 0.4–0.6 for typical rooms
Example
Kitchen: 12 × 14 ft = 168 sq ft. Target 40 fc.
Lumens needed: 168 × 40 = 6,720 lm. With 20% LLF: 6,720 / 0.80 = 8,400 lm.
Using 1,200 lm LED bulbs: 8,400 / 1,200 = 7 bulbs. Use 8 for uniform coverage.
Common Mistakes
- Using watts instead of lumens: LED efficiency varies. A 10W LED can be 800 or 1,200 lumens. Always specify lumens, not watts.
- Ignoring light loss factor: Dirt, aging, and optics reduce output by 20–30% over time. Design for the degraded output.
- Over-lighting: More isn't always better. Excessive light causes glare and wastes energy.
Standards Reference
- IES — Illuminating Engineering Society (lighting standards)
- ANSI/IES RP-1 — Recommended Practice for Office Lighting
- NEC Article 410 — Luminaires
- ENERGY STAR — LED lighting requirements
Frequently Asked Questions
How many lumens do I need for a room?
Multiply room area (sq ft) by target foot-candles. A 12×15 ft living room (180 sq ft) at 15 fc needs 2,700 lumens. Add 20% for light loss: 2,700 / 0.80 = 3,375 lumens. That's about 3 × 12W LED bulbs (1,200 lm each).
What is a foot-candle?
A foot-candle is the illuminance (light level) on a surface one foot from a standard candle. It equals one lumen per square foot. In metric, the equivalent is lux (1 foot-candle = 10.76 lux). Lighting codes and standards specify minimum foot-candles for safety and task performance.
How many lumens is a 60W bulb?
A 60W incandescent bulb produces about 800 lumens. A 9W LED produces the same 800 lumens — using 85% less electricity. Always compare lumens when switching from incandescent to LED, not watts.
What color temperature should I use?
2700K (warm white): bedrooms, living rooms — warm, cozy. 3500K (neutral white): kitchens, bathrooms — clean, natural. 4100K (cool white): offices, workshops — alert, productive. 5000K+ (daylight): garages, outdoor — very bright, blue-ish.
Do LED lumens decrease over time?
Yes. LED output decreases gradually (lumen depreciation). Quality LEDs maintain 70% of initial lumens at 50,000 hours (L70 rating). Cheap LEDs may drop below 70% in 10,000 hours. Always buy LEDs with L70 > 30,000 hours for longevity.