Lighting Lumen Efficacy Calculator
Solve for luminous efficacy, luminous flux or power, then plan a room with the lumen method. Compare lamp types, estimate running cost, and see charts, a fixture-layout diagram and real lamp examples.
Efficacy, not brightness
η tells you how much light you get per watt. A higher η means the same lumens for less power.
The lumen method
Fixtures needed = (lux × area) ÷ (lumens per fixture × UF × MF). UF covers light lost to walls and fittings, MF covers dirt and ageing.
The 683 lm/W ceiling
No source can exceed 683 lm/W, the value for pure green light at 555 nm. White light sources are far below that.
Source vs system
Lamp efficacy ignores driver, optics and fitting losses. System efficacy of a luminaire is usually lower than the bare lamp.
How to Use This Calculator
Luminous efficacy links three quantities: luminous flux Φ (lumens), electrical power P (watts) and their ratio η. Choose which one to solve, enter the other two, then read the result.
1. Choose what to solve for
Pick η (from lumens and watts), Φ (light output from η and power) or P (power needed for a given light output and efficacy).
2. Enter your values
Type a number, tap a preset (450 to 20,000 lm, 5 to 100 W, efficacy 15 to 150), or drag the slider under each field.
3. Pick the units
Flux can be in lm or klm, power in W or kW, illuminance in lux or foot-candles, area in m² or ft². Values convert when you change a unit.
4. Calculate and read the result
Press Calculate & show result. It scrolls to the result card with the answer, the step-by-step math and the percentage of the 683 lm/W theoretical maximum.
5. Compare and check the charts
See where your lamp sits against incandescent, CFL, fluorescent, HID and LED ranges, and how power falls as efficacy rises.
6. Plan a room and cost, or load a real lamp
Advanced is on by default: enter lux, area, UF, MF, hours and price for the fixture count and running cost. Use Basic for a simpler view, or load a lamp from the tables below.
What This Tool Offers
Built around the standard efficacy formula η = Φ ÷ P, measured in lumens per watt (lm/W).
Solve in any direction
Find efficacy, light output or power. The formula rearranges for you and shows each step.
Unit conversion
lm and klm, W and kW, lux and foot-candles, m² and ft², so you never convert by hand.
Type, preset or drag
Every main input has quick presets and a draggable slider for fast "what if" exploration.
Charts and lamp comparison
An efficacy scale, range bars for common lamp types, and a power-versus-efficacy curve with your lamp marked.
Room planning
The lumen method gives the number of fixtures, total power, power density and a drawn layout of your room.
Cost and real examples
Annual energy cost and savings against another lamp, plus typical incandescent, CFL, fluorescent, HID and LED examples.
Typical lamp examples
Representative power and light output (illustrative, varies by model). Efficacy is computed from the numbers. Click Load to try one.
| Lamp | Power (W) | Flux (lm) | η (lm/W) | % of 683 max |
|---|
Efficacy by lamp type
| Lamp type | Typical η (lm/W) |
|---|
Ranges are typical values. The best modern LED products exceed these, and individual models vary.
Indicative illuminance targets
Typical design targets. Always check the standard that applies to you (for example EN 12464-1 or IES recommendations). Click Use to set the target.
| Space | Typical lux |
|---|
Unit reference
| Conversion | Value |
|---|---|
| 1 lux | 1 lm per m² |
| 1 foot-candle | 10.764 lux |
| 1 ft² | 0.092903 m² |
| 1 klm | 1,000 lm |
| Theoretical maximum | 683 lm/W at 555 nm |
Validation and sources
683 lm/W: SI definition of the candela at 540 THz (about 555 nm)
Lumen method: N = (E · A) / (Φ · UF · MF), standard lighting design practice
Every result is cross-checked by recomputing η = Φ / P
Lamp figures and lux targets on this page are indicative. Use the manufacturer's data sheet and your local lighting standard for design work.
Formulas
Luminous efficiency = η / 683 × 100 %
Total lumens needed = E · A / (UF · MF) · N = ceil(total / Φ)
Power density = N · P / A · Achieved lux = N · Φ · UF · MF / A
Annual kWh = P / 1000 · hours · Same lumens at η₂: P₂ = Φ / η₂
Results are first-order estimates for comparison and planning.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| η | Luminous efficacy (efficiency of converting electrical power to visible light) | lm/W |
| Φ | Luminous flux (total visible light output) | lm |
| P | Electrical power input | W |
What it means
Lumen efficacy is the ratio of luminous flux (lumens) to electrical power (watts), expressed in lm/W. It measures how efficiently a light source produces visible light. Higher efficacy means more light per watt. LED lamps have high efficacy (often 80‑100 lm/W) compared to incandescent (10‑15 lm/W) and fluorescent (50‑80 lm/W). Example: A 10W LED producing 800 lumens has efficacy = 800/10 = 80 lm/W. This metric is used to compare different lighting technologies and to select energy‑efficient lighting. Efficacy is a key parameter in lighting design and energy codes.
Worked example
Lumen Efficacy – Practical Example
Real‑World| Parameter | Value |
|---|---|
| Luminous flux | 800 lm |
| Power input | 10 W |
| Formula | η = Luminous Flux / Power |
Common mistakes
Watch for unit mismatches (W vs kW, single- vs three-phase) and remember to include power factor or efficiency where the formula requires it.Applications
Lighting lumen efficacy η = Luminous Flux(lm) / Power(W) measures how efficiently a lamp produces visible light. Higher efficacy means more lumens per watt. Engineers use it to select energy‑efficient lighting, to design lighting systems, and to meet sustainability goals. This is key for lighting design and energy conservation.
- Lighting design and fixture selection
- Energy‑efficient lighting upgrades (LED, CFL)
- Building energy code compliance
- Cost savings analysis for lighting
- Educational understanding of light output
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