Light Measurement Converter
Convert light measurements — but only within the quantity that matches, because lux, lumens, candelas and nits measure genuinely different things. Luminous flux (lumens) is the total light a source emits. Luminous intensity (candelas) is light in a particular direction. Illuminance (lux, foot-candles) is light landing on a surface. Luminance (nits) is the brightness of a surface, used for screens. Converting between quantities needs extra information like area or distance, so this tool converts within each and explains the differences.
How to use this tool
- Choose the quantity you are working with: illuminance, luminous flux, intensity or luminance.
- Enter the value.
- Pick the unit you have and the unit you want; the menus update per quantity.
- Read the result and the full equivalents list.
- To go between different quantities (say lumens to lux), you also need the area or distance — those are separate calculations.
The formula
Photometric quantities describe light as the eye perceives it, and they measure different things: total output, output in a direction, light landing on a surface, and surface brightness. They are not directly interconvertible without geometry.
luminous flux: lumen (lm) — total output
luminous intensity: candela (cd) = lm per steradian
illuminance: lux (lx) = lm per m²
luminance: nit (cd/m²) — surface brightness- lumen
- Total visible light emitted
- candela
- Intensity in a direction (lm/sr)
- lux
- Illuminance, lm/m²
- nit
- Luminance, cd/m²
Converting lumens to lux needs the area lit; candela to lux needs the distance. This tool converts within each quantity, where the ratios are exact.
Worked examples
Office lighting
- Given
- 500 lux
- Result
- ≈ 46.5 foot-candles
A well-lit office is around 500 lux, which is about 46 foot-candles in US units.
Screen brightness
- Given
- 400 nits
- Result
- 400 cd/m²
A nit is exactly a candela per square metre, so a 400-nit display is 400 cd/m² — a typical bright laptop screen.
Bulb output
- Given
- 800 lumens
- Result
- 0.8 kilolumens
A 60-watt-equivalent LED emits about 800 lumens of total light; converting to kilolumens just moves the decimal.
Frequently asked questions
Lumens measure the total light a source puts out; lux measures how much of that light lands on a surface, per square metre. The same bulb gives more lux on a small area close up and less spread over a large room. Lux equals lumens divided by the lit area.
The candela measures luminous intensity — light emitted in a particular direction, per unit solid angle. A source that concentrates its light into a narrow beam has a high candela value in that direction even if its total lumen output is modest, which is how spotlights and lasers work.
A nit is one candela per square metre, a measure of luminance — the brightness of a surface as the eye sees it. Displays are rated in nits because what matters is how bright the screen surface appears; a phone might be 800 nits, an HDR TV over 1000.
Only if you know the area the light falls on, because lux = lumens ÷ area. There is no fixed lumen-to-lux ratio; spreading the same lumens over a bigger area lowers the lux. This tool converts within each quantity, where the ratios are fixed, and flags that cross-quantity conversions need geometry.
It is the imperial unit of illuminance: one lumen per square foot. One foot-candle equals about 10.76 lux, since a square metre is about 10.76 square feet. US lighting codes and older references often specify foot-candles where metric ones use lux.
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Related reading
Inches to centimetres: the complete guide
The exact factor, heights, screen sizes, and why 2.54 is no accident.
Kilograms to pounds, made simple
The exact factor, fast mental shortcuts, and the conversions you actually need.
Square feet to square metres, done right
The factor is squared — miss that and you understate area threefold.