How this calculator works
Lumens are a measure of visible light and watts are a measure of electrical power, so converting between them needs the lamp efficiency, called luminous efficacy and measured in lumens per watt. An incandescent filament wastes most of its power as heat and produces about 13 lumens per watt, while an LED produces roughly six times as much light for the same power. That is why a 9 W LED can replace a 60 W incandescent: both give about 800 lumens. Choose a lamp type to use a typical efficacy, or enter the figure from the product datasheet for an exact conversion.
Formula
watts = lumens / efficacy in lumens per watt. lumens = watts x efficacy.
Worked example
A lamp producing 800 lumens at 80 lumens per watt draws 800 / 80 = 10 W. The same 800 lumens from an incandescent lamp at 13 lumens per watt needs 61.5 W. Going the other way, a 55 W compact fluorescent at 55 lumens per watt gives about 3,025 lumens.
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Frequently asked questions
Why do LED labels show both lumens and a wattage equivalent?
The lumens are the real output and the wattage equivalent is a shopping aid that compares it with the old incandescent bulb it replaces. A 60 W equivalent LED is simply one that produces about 800 lumens.
Is the efficacy figure on the label the same as this one?
Usually close. The Lighting Facts label gives lumens and watts, and dividing one by the other gives the efficacy. Fixture losses can make the delivered figure lower than the lamp alone.
Does a higher efficacy always mean better light?
No. Colour rendering, colour temperature, flicker and beam control matter too. A highly efficient lamp with poor colour rendering can make a room look dull.
Can I use this for a whole fixture?
Yes, as long as you use the lumens the fixture actually delivers and the watts it actually draws, including the driver. For a fixture with several lamps, add the lumens and the watts.