Lumens to Watts Calculator
Convert lumens to watts with the lamp efficacy in lm/W: W = lumens / efficacy. Formula, worked example and a table. There is no single factor.
Try your own numbers
Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
Open in the calculator with these values
Formula
Worked example
| Input | Value |
|---|---|
| Luminous flux (lm) | 800 |
| Luminous efficacy (lm/W) | 80 |
- P = Φ / η = 800 / 80 = 10 W
Typical values
| Luminous flux (lm) | Electrical power (W) |
|---|---|
| 100 | 1.25 |
| 400 | 5 |
| 800 | 10 |
| 1,600 | 20 |
| 3,000 | 37.5 |
Other inputs are the same as in the worked example above.
Assumptions
- Power is the lamp or luminaire input power as stated for the efficacy
- Efficacy is taken from the data sheet
Efficacy depends on the lamp type and whether driver losses are counted. Compare lamps on the same basis, lamp-only or system.
Frequently asked questions
How do I convert Lumens to Watts?
Φ = P × η; P = Φ / η; η = Φ / P. Worked example: Luminous flux (lm) 800, luminous efficacy (lm/W) 80 gives 10 W.
What do I need to enter?
Enter luminous flux (lm), electrical power and luminous efficacy (lm/W). Leave blank the value you want to find, and the form solves for it. Change the unit of a value with its selector and the result stays the same.
Is there a fixed lumens to watts factor?
No. The light per watt, the luminous efficacy, depends on the lamp type and on whether driver losses are counted. Read it in lm/W from the data sheet and use the same basis when you compare lamps.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. Check design values against the standard and the equipment data you work to.
Related
Next steps in a design: Lux to WattsLED Retrofit Savings and PaybackLumens to Candela
Opposite direction: Watts to Lumens
Also see: Lux to Watts Watts to Lux Lumens to Millicandela Millicandela to Lumens
Formula guides: Lighting Formulas: Lux, Lumens and CandelaLED Formulas: Resistor Value and Power
All 78 conversionsLumens and Watts (luminous efficacy) in Elecmetric