Watts to Lumens Calculator
Convert watts to lumens with the lamp's luminous efficacy. Shows how much light a bulb of a given wattage actually produces.
How to use it
- Choose the lamp type to set its luminous efficacy, or pick the closest match.
- Enter the remaining values — the result and the working update instantly.
- Check the extra tiles for the same figure in other units.
This calculator works out watts to lumens from power and lamp type. It shows the substituted formula underneath every result, so you can check the arithmetic or copy the working straight into a report.
The formula
Φv(lm) = P(W) × η(lm/W)
Lumens, lux, candela — the difference
These three get used interchangeably and mean quite different things. Lumens measure the total light a source emits in every direction. Candela measures intensity in one particular direction, which is why a torch and a bare bulb can share a lumen figure and look nothing alike. Lux measures how much light lands on a surface — one lumen spread over one square metre.
That last point is why the same bulb gives plenty of light in a small room and not enough in a large one: the lumens have not changed, but the area they are spread over has.
Luminous efficacy
Efficacy is how many lumens a lamp produces per watt of electricity, and it is what makes wattage a poor guide to brightness. An old incandescent bulb manages about 15 lm/W; a modern LED reaches 90 lm/W or better. That is the whole reason a 10 W LED replaces a 60 W filament bulb.
The presets here are typical mid-range figures for each technology. If the manufacturer publishes an efficacy for your exact model, use that instead — the spread within a category is wider than most people expect.
Everything runs in your browser
There is no server involved in the calculation. The numbers you type never leave your device, nothing is logged, and the page keeps working with no connection once it has loaded. That is not a marketing line — it is simply how the tool is built, because a calculation like this has no reason to touch a network.
Questions people ask
What is luminous efficacy?
It is how many lumens of light a lamp produces per watt of electricity. An old incandescent bulb manages about 15 lm/W; a modern LED reaches 90 lm/W or more, which is why a 10 W LED replaces a 60 W bulb.
What is the difference between lumens and lux?
Lumens measure the total light a source emits. Lux measures how much of that light lands on a surface — one lux is one lumen spread over one square metre. The same bulb gives fewer lux in a bigger room.
How accurate are the lamp presets?
They are typical mid-range figures for each lamp technology. If the manufacturer publishes an efficacy for your exact model, use that number instead for a precise result.
Last updated 17 August 2026