Lux to DLI converter: lux, foot-candles, PPFD and DLI explained

Enter a light reading in any unit. The converter shows it in the others, and what kind of houseplant that much light can grow.

A constant reading held for the hours you entered. Real daylight rises and falls through the day, so a single midday reading overstates the daily total; for a window, use the Window Light Planner.

The four units, in plain terms

Lux and foot-candles: brightness as people see it

Lux measures light weighted by the sensitivity of the human eye, which peaks in green. One foot-candle is one lumen per square foot, which is 10.764 lux. Most cheap light meters and phone apps read in lux or foot-candles. They are fine for comparing two spots in the same room, but they are not the plant's view of light: plants use red and blue light that the eye barely counts.

PPFD: light as a plant sees it, right now

Photosynthetic photon flux density counts the photons between 400 and 700 nm (photosynthetically active radiation, PAR) arriving on a square meter each second, in micromoles: µmol/m²/s. Full sun near solar noon is around 1,800 (Thimijan and Heins, 1983). Converting from lux needs a factor for the light's spectrum, which is why the converter asks for the source: about 54 lux of daylight make one µmol/m²/s (Thimijan and Heins, 1983), while white LEDs give about 15 µmol/m²/s per 1,000 lux.

DLI: the day's total

Daily light integral adds PPFD up over a whole day: PPFD × hours × 0.0036 gives mol/m²/day. It is the most useful single number for plants because a dim spot lit for 14 hours can match a brighter one lit for 6. Greenhouse growers plan with it; the planner estimates it for a spot near your window.

How much light houseplants need

Plant light category	Keeps growing from	PPFD needed over 12 h	Holds on down to
Low light	3 mol/m²/day	69 µmol/m²/s	0.5 mol/m²/day
Medium light	6 mol/m²/day	139 µmol/m²/s	1.1 mol/m²/day
Bright indirect	10 mol/m²/day	231 µmol/m²/s	2.2 mol/m²/day
Direct sun	12 mol/m²/day	278 µmol/m²/s	10.8 mol/m²/day

Growth levels from Iowa State Extension's DLI classes for indoor plants; minimums from Illinois Extension's foot-candle guide. See how they are set.

Questions

How do I convert lux to PPFD?

Multiply lux by a factor that depends on the light source: about 0.0185 for sunlight, about 0.015 for white LEDs and 0.0135 for cool-white fluorescent tubes. The result is in µmol/m²/s.

How do I get DLI from PPFD?

Multiply PPFD (µmol/m²/s) by the hours the light is on and by 0.0036. For example, 200 µmol/m²/s for 12 hours gives 8.6 mol/m²/day.

What is bright indirect light in numbers?

On this site, bright indirect light means a daily light integral of about 10 mol/m²/day, the top of Iowa State Extension's medium class for indoor plants (6 to 10).

Can my phone measure light for plants?

Phone light-meter apps measure lux with the camera or ambient sensor. They are useful for comparing spots, but they are not calibrated instruments, so treat the numbers as rough.

Lux, foot-candles, PPFD and DLI