Light meters and grow lights
Lux to PPFD converter
Light meters and phone apps read in lux or foot-candles, which measure light the way our eyes see it. Plants respond to PPFD. Pick the light source to get an approximate conversion either way.
PPFD, approximately
225 µmol/m²/s
High light (1,000 fc or more): what high-light plants need.
Low Medium High light (fc)
- Lux
- 15,000
- Foot-candles
- 1,394
- Factor used
- 66.7 lux per µmol/m²/s
| Hours | mol/m²/day |
|---|---|
| 12 h | 9.7 |
| 14 h | 11.3 |
| 16 h | 13.0 |
| 18 h | 14.6 |
Why the light source matters
Lux is weighted by the human eye, which is most sensitive to green-yellow light near 555 nm. PPFD counts every photon from 400 to 700 nm equally, including the red and blue that plants use well. So the same lux reading means a different PPFD under the sun, a fluorescent tube or a sodium lamp: sunlight works out at about 54 lux per µmol/m²/s, high-pressure sodium at 82. For white LEDs we use 1,000 lux ≈ 15 µmol/m²/s, from an analysis of many white LED spectra.
A foot-candle is one lumen per square foot, which is 10.764 lux. The gauge shows Missouri Extension's houseplant light levels in foot-candles: low 50–250, medium 250–1,000 and high from 1,000.
Measure at the top of the plants with the sensor held level and clean, as Purdue's guide advises. Virginia Tech notes that a phone app can give a home grower useful information, but a quantum (PAR) sensor reads PPFD directly.
Sources
- Apogee Instruments, Conversion – PPFD to Lux: lux per µmol/m²/s for sunlight (54), cool white fluorescent (74), high-pressure sodium (82), double-ended HPS (77), metal halide (71) and ceramic metal halide (65 at 4200 K, 59 at 3100 K), with the definitions of lux and the foot-candle. Its reverse table (0.0185 and so on) agrees within 0.5%.
- Thimijan RW, Heins RD. Photometric, radiometric, and quantum light units of measure: a review of procedures for interconversion. HortScience 1983;18(6):818–822. doi:10.21273/HORTSCI.18.6.818. The source of the per-lamp factors.
- Sharakshane A. An easy estimate of the PFDD for a plant illuminated with white LEDs: 1000 lx = 15 µmol/s/m². bioRxiv 289280, 2018. A preprint, not peer reviewed: the white LED factor.
- Stallknecht E. Calculating and Using Daily Light Integral (DLI). Virginia Cooperative Extension SPES-720NP, 2025: foot-candles per µmol/m²/s (sunlight 5.01, HPS 7.62, metal halide 6.60, cool white fluorescent 6.87), quantum sensors and phone apps.
- Torres AP, Lopez RG. Measuring Daily Light Integral in a Greenhouse. Purdue Extension HO-238-W: keeping light sensors level and clean.
- Trinklein DH. Lighting Indoor Houseplants. University of Missouri Extension G6515: low, medium and high light in foot-candles.
Questions
What about purple (red and blue) LED grow lights?
No single factor works for them. Lux follows the eye, which sees deep red and blue poorly, so a lux reading says little about these lights, and none of the sources gives a factor. Use the maker's PPFD chart or a PAR meter.
Is the result exact?
No. Each factor is a typical figure for that kind of light; a particular lamp's colour temperature shifts the real value. The two ceramic metal halide lamps, at 65 and 59 lux per µmol/m²/s, show how much colour temperature alone can change it.
How do I turn PPFD into a daily total?
Multiply it by the hours the light is on and by 3,600, then divide by 1,000,000. The table under the result does this for 12 to 18 hours, and the DLI calculator compares the total with your plants' target band.