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Lumens per watt (lm/W) is a measure of luminous efficacy: the amount of visible-light output, in lumens, produced for each watt of electrical power input. Calculate it by dividing luminous flux by the relevant input power. A higher figure means more light output per stated watt, but it does not by itself tell you whether a product is bright enough or right for a particular space.
What does lumens per watt mean?
A lumen (lm) measures luminous flux—the visible-light output represented by the metric. A watt (W) measures power input. The ratio lm/W therefore describes light output per unit of power, rather than brightness alone.
The CIE International Lighting Vocabulary defines a light source’s luminous efficacy as being expressed in lumen per watt (lm·W⁻¹), based on CIE S 017:2020, second edition: CIE International Lighting Vocabulary, entry 17-21-089.
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Divide the luminous flux in lumens by the relevant power input in watts:
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lm/W = luminous flux (lm) ÷ input power (W)
For a luminaire—a complete lighting fixture—the U.S. Department of Energy gives the calculation as “LE = (Luminous Flux) / (Luminaire Watts Input.” In other words, use the fixture’s light output and the power input for that fixture, not an unrelated component rating. See DOE FEMP’s commercial and industrial LED luminaire guidance.
For example, a hypothetical product that outputs 1,200 lumens while drawing 10 watts has an efficacy of 120 lm/W (1,200 ÷ 10). This is an illustration of the arithmetic, not a measured product result.
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Check what the lm/W figure measures
The denominator and output need to describe the same measurement boundary. A figure may refer to a lamp or LED source, or to a complete luminaire that includes the fixture system. These are not interchangeable: the DOE distinguishes source efficacy from luminaire efficacy. When reading a specification, check whether the stated output and wattage apply to the source, lamp, or whole fixture. See DOE’s LED Basics.
ENERGY STAR’s Lamps Version 2.1 specification defines lm/W as the quotient of total luminous flux emitted by total light-source power input. That definition is specific to the lamp specification; a complete-fixture figure uses the luminaire’s output and input basis. See ENERGY STAR Lamps Program Requirements Version 2.1.
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What a higher lm/W value tells you—and what it does not
A higher lm/W value means more luminous output for each watt on the stated measurement basis. It does not establish that the light has the right brightness, color, distribution, or fit for the intended application. A highly efficacious fixture can still be the wrong choice for a room or task. The DOE recommends comparing like products and selecting lighting for the application in its luminaire purchasing guidance.
Is there a “good” lm/W value?
There is no single universal threshold in the cited guidance. Published criteria differ by product category and program, so figures should not be treated as directly comparable requirements across different kinds of lighting. For context, the DOE’s 2023 FEMP guidance lists these category-specific values for commercial and industrial luminaires:
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| Product category | Published efficacy value |
|---|---|
| 1 ft × 4 ft commercial troffers | 120 lm/W |
| 2 ft × 2 ft commercial troffers | 123 lm/W |
| 2 ft × 4 ft commercial troffers | 140 lm/W |
| Industrial low-bay luminaires producing 5,000 to less than 10,000 lm | 143 lm/W |
| Industrial high-bay luminaires producing at least 10,000 lm | 175 lm/W |
These are values in DOE FEMP guidance updated June 2023, not universal definitions of an efficient light. ENERGY STAR’s downlight criteria page, accessed October 4, 2026, reports a minimum efficacy of 82 lm/W for downlights; that criterion belongs to its downlight category and program. Check the ENERGY STAR Downlights Key Product Criteria page for current requirements, since criteria can change.
How to use lm/W when comparing lighting
- Compare products in the same category and intended use.
- Use values on the same basis: source or lamp compared with source or lamp, or complete luminaire compared with complete luminaire.
- Check that the stated lumens and watts describe the same product and measurement boundary.
- Consider whether the amount and distribution of light suit the space or task; lm/W alone cannot answer that.
For household bulbs, product specifications that state both lumen output and input wattage let you calculate the ratio using the same formula. The DOE’s light-bulb purchasing guidance covers this product category.
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