Case 1
- Brightness wanted (lm)
- 800lm
- Luminous efficacy (lm/W)
- 100lm/W
8.00W
Open with these valuesLumens to Watts Calculator
8.00W
Result: 8.00 WDivide the lumens by the lamp's efficacy in lumens per watt. 800 lumens at 100 lm/W is 8 W from an LED; the same 800 lumens at 15 lm/W is 53 W from a filament bulb. The efficacy is a property of the lamp, so enter the one on its packaging rather than a fixed number.
Worked examples
Case 2
- Brightness wanted (lm)
- 800lm
- Luminous efficacy (lm/W)
- 15lm/W
53.33W
Open with these valuesCase 3
- Brightness wanted (lm)
- 450lm
- Luminous efficacy (lm/W)
- 80lm/W
5.63W
Open with these valuesHow it's calculated
watts = lumens ÷ efficacy in lm/W
- StepDecide how bright the lamp has to be, in lumens.
- StepEnter the efficacy of the technology — about 15 lm/W for filament, 100 for LED.
- ResultDivide; the result is the power the lamp will draw.
Reference table
| Lumens, efficacy | Lamp type | Watts |
|---|---|---|
| 800, 15 | Filament incandescent | 53.33 |
| 800, 60 | Compact fluorescent | 13.33 |
| 800, 100 | Ordinary LED | 8.00 |
| 450, 80 | Small LED candle lamp | 5.63 |
| 1000, 120 | High-efficacy LED | 8.33 |
Questions
How do I convert lumens to watts?
Divide the luminous flux in lumens by the lamp's efficacy in lm/W. An 800-lumen LED at 100 lm/W needs just 8 W — far less than the 60 W a filament bulb once used for the same brightness.
What is luminous efficacy?
It measures how efficiently a light source turns electrical power into visible light, in lumens per watt. A higher value means more light for the same energy. Typical ranges are 10–17 lm/W for incandescent, 15–25 for halogen, 45–75 for compact fluorescent and 80–120 for LED.
Why can I no longer compare lamps by wattage alone?
Wattage measures energy use, not brightness. It was a reliable proxy only while every lamp had a similar efficacy. A 10 W LED now matches or beats a 60 W filament bulb, so lumens are the correct unit for comparing brightness.
What efficacy should I enter for an LED?
Most consumer LED lamps sit between 80 and 120 lm/W, and 100 lm/W is a safe middle. High-efficiency fixtures and bare chips reach 150–200 lm/W. The exact figure is the rated lumens divided by the rated watts on the packaging.
Is there an upper limit?
Yes. No source can exceed 683 lm/W, the value for monochromatic green light at 555 nm where the eye is most sensitive. White light needs other wavelengths as well, so a real white lamp stays far below that ceiling.
Sources and last check
Information, not professional advice.