Lumens to Watts Calculator
Enter lumens and source efficacy to estimate optical watts. Optionally enter measured electrical input watts to calculate lumens per electrical watt.
Estimated optical power: W
Fixture luminous efficacy: lm/W
Formula: optical W = lm ÷ assumed lm/W; fixture lm/W = lm ÷ electrical W.
Optical watts are an estimate; fixture lm/W uses measured electrical input and does not mean optical watts.
A QUICK WALKTHROUGH
How to use this tool
- Enter luminous flux in lumens (lm).
- Enter the assumed luminous efficacy in lumens per watt (lm/W) to estimate optical watts.
- Optionally enter measured electrical input power to calculate fixture efficacy as lumens per electrical watt.
Efficacy is required
There is no universal direct conversion from lumens to watts. Lumens measure photopic luminous flux, while watts measure radiant or electrical power. The result depends on the light source spectrum and the chosen luminous efficacy.
Formula and scope
Estimated optical watts = lumens ÷ entered source efficacy (lm/W). If measured electrical input watts are also entered, fixture efficacy = lumens ÷ electrical input watts (lm/W). These are distinct quantities: the first estimates radiant optical power using your efficacy assumption; the second describes visible luminous flux per electrical watt.
Local calculation
Values are calculated in this browser and are not uploaded or stored.
GOOD TO KNOW
Common questions
Can I convert lumens to watts without another value?
No. Enter a source-specific luminous efficacy in lm/W; different lamps and LEDs can produce different lumens per watt.
Does the result equal the electrical input rating?
Not necessarily. The result uses your luminous-efficacy assumption and represents estimated optical power. Driver, ballast, and fixture losses may make electrical input higher.
What efficacy should I use?
Use a measured or manufacturer-provided value for the exact source. If you are only planning, document the assumed lm/W because the estimate changes directly with it.