CALCULATOR
Lux to foot-candles and PPFD converter
Turn a light reading from lux, foot-candles or PPFD into the other two, for sunlight, grow lights or household bulbs, and see the daily light it adds up to.
Foot-candles, lux and PPFD
Foot-candles and lux measure the same thing, how bright light looks to people, in different units: one foot-candle is 10.764 lux. Plant care guides in the US mostly use foot-candles.
PPFD counts the light plants actually use for photosynthesis. Lux cannot be turned into PPFD with one number, because a grow light, a white LED bulb and the sun spread their light differently across the spectrum. That is why the calculator asks for the light source.
What DLI adds
DLI, the daily light integral, is the total plant-usable light a spot gets in a day. A dim spot lit for 14 hours can match a bright spot lit for 6. It is the best single number for comparing a windowsill with a grow light.
Measure your own spots
Plantburgh Light Meter reads light with your phone, shows foot-candles, lux, PPFD and DLI, and tells you which plants fit the spot.
Worked examples
A reading of 2,000 lux is about 186 foot-candles (2,000 divided by 10.764). That is medium light for most houseplants.
A spot reading 300 foot-candles is about 3,229 lux.
In sunlight, the calculator uses about 0.0185 PPFD per lux, so 10,000 lux of sun is roughly 185 PPFD. A white LED or a grow light uses a different factor, which is why the light source matters.
From PPFD to daily light
DLI, in moles of light per square meter per day, is PPFD times the hours of light times 0.0036. A spot that gets 100 PPFD for 12 hours collects about 4.3 moles a day. The same spot at 50 PPFD for 12 hours gets half that, which is why a plant can look fine in the morning and still be underlit over a whole day.
What the bands mean for houseplants
Low light for houseplants is about 50 to 250 foot-candles, medium is 250 to 1,000, and high is more than 1,000. Below about 50 foot-candles, plants will not last long.