Lumens to Lux Calculator
With luminous flux 800 lm, area lit 4 m², lumens to lux comes to 200.00 lx — illuminance. It is reached in 4 steps, the last of which is 800 / 4, and each one is printed on the page with its numbers filled in. The formula is the one published by BIPM, not an approximation fitted to it.
Illuminance in lux from a luminous flux in lumens spread over an area, with the foot-candle equivalent and the area needed for a target level.
Formula and sources checked · How we check
Luminous flux 800 lm, Area lit 4 m²
200.00 lx
Illuminance for the example below. Editing a field recomputes the calculator below; this figure holds the answer the page was loaded with.
It is written into the HTML rather than drawn by a script, so a search engine reading this page without running JavaScript still finds an answer.
- Illuminance
800 / 4200 lx- In foot-candles
200 * 0.09290318.581 fc- Area in square feet
4 / 0.09290343.056 sq ft- Lumens needed for 300 lx over this area
300 * 41,200 lm
Worked example
An 800-lumen bulb — the usual replacement for a 60 W incandescent — spread over 4 square metres gives 200 lux, or 18.58 foot-candles. That is enough for a corridor and short of the 300 lux a desk wants, which is why one ceiling bulb over a work surface always feels dim.
How to work it out yourself
- 1.Lumens describe the lamp; lux describe the surface. The same bulb gives four times the lux over a quarter of the area, and nothing about the bulb changed.
- 2.Use the area actually being lit, not the room. A downlight over a worktop lights the worktop, and dividing its lumens by the whole floor understates what is under it.
- 3.Common targets: about 100 lux for a corridor, 300 for general office and kitchen work, 500 for reading or detailed work, 750 and up for a workshop bench.
The formula
- Illuminance
800 / 4 - In foot-candles
200 * 0.092903 - Area in square feet
4 / 0.092903 - Lumens needed for 300 lx over this area
300 * 4
Source: BIPM — the SI Brochure, on the candela, lumen and lux, NIST Special Publication 811 — conversion factors including the foot-candle
Questions people actually ask
- What is the difference between lumens, lux and candela?
- A candela is brightness in one direction. A lumen is total light output in every direction. A lux is one lumen landing on one square metre. Lamps are sold in lumens, light meters read lux, and torches are specified in candela because what matters is the beam.
- How many lumens do I need for a room?
- Multiply the area in square metres by the lux you want. A 12 m² kitchen at 300 lux needs about 3,600 lumens delivered to the work surfaces — which is four or five ordinary bulbs, not one.
- Why does a light meter read so much less than I calculated?
- Because the calculation assumes every lumen lands on the area. Real fittings throw light at walls and ceilings, lamps dim with age, and a shade can absorb a third of the output. A working allowance is to size for 1.5 times the calculated figure.
- What is a foot-candle?
- One lumen per square foot — the same idea in imperial units, still standard in US lighting specifications. One foot-candle is 10.7639104 lux, because that is how many square feet are in a square metre.
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