Estimate the electrical power needed for a given light output, find lumens from actual watts, or calculate luminous efficacy from a product label. This relationship depends on the lamp or fixture rating you supply.
How to use this calculator
For Lumens to watts, enter the required light output and the product efficacy in lm/W. For Watts to lumens, enter actual electrical power and efficacy. If a package gives only lumens and watts, use Find efficacy first. The default 100 lm/W is an example you can replace; it is not a universal LED specification.
Brightness is not the same as power
Lumens measure luminous flux, or visible light output. Watts measure electrical power. Luminous efficacy connects them by expressing how many lumens a source produces for each watt it uses. Products with equal light output can draw different amounts of power.
Use actual electrical watts, not a marketing statement such as “60 W equivalent.” A luminaire rating can include losses in the fixture and driver, while an individual light-source rating may use a different boundary. For a useful comparison, keep the measurement boundary consistent. Color, beam distribution, and room geometry also affect the lighting experience and are not determined by this formula.
Examples
An 800-lumen lamp
At an assumed 100 lm/W, producing 800 lm requires 8 W. At 80 lm/W, the same light output requires 10 W. Both are calculations from assumptions, not product recommendations.
Reading a product label
A lamp labeled 1,100 lm and 11 W has an efficacy of 100 lm/W. Enter those values in Find efficacy to use the actual specification.
Formula
watts = lumens / luminous efficacy
lumens = watts × luminous efficacy
luminous efficacy (lm/W) = lumens / actual watts
Precision and interpretation
There is no single exact conversion from lumens to watts across all lamps. Use a rated or measured efficacy for the product and operating conditions. The comparison table holds your efficacy constant. It does not model dimming, aging, temperature, or the illumination level on a work surface. For electricity cost, use the linked Electricity Calculator after finding actual power.
Definitions and further reading
Unit and quantity reference provides the underlying definitions. The examples above are worked calculations for this tool.