Grow Light Calculator: PAR to Lux and DLI
Two free calculators for anyone growing under lights. Convert a lux reading into PPFD (or a PPFD target into the lux to aim for), and work out your daily light integral (DLI) from PPFD and hours. Each one tells you which growth stage your number fits, using the targets from our grow light distance guide and light schedule guide.
40,000 lux is about 769 PPFD
That is in the 600 to 900 band for veg and flower.
Red and blue only ("blurple") LEDs: do not use lux at all. A lux meter barely sees their light, so any conversion is meaningless; use a quantum meter or the maker's PPFD map.
600 PPFD for 18 hours is 38.9 mol/m²/day
DLI = PPFD x hours x 0.0036. It is the day's total light, in moles per square meter per day. Twelve hours at 900 PPFD is the same dose as 18 hours at 600.
Light targets by stage
| Stage | PPFD | Lux, white LED (roughly) | DLI |
|---|---|---|---|
| Clones, until rooted | 50 to 150 | 3,000 to 10,000 | 3 to 10 at 18/6 |
| Seedlings, weeks 1 and 2 | 200 to 400 | 13,000 to 27,000 | 13 to 26 at 18/6 |
| Seedlings, weeks 3 and 4 | 400 to 600 | 27,000 to 40,000 | 26 to 39 at 18/6 |
| Vegetative growth | 600 to 900 | 40,000 to 60,000 | 39 to 58 at 18/6 |
| Flowering | 600 to 900, more if the tops stay cool | 40,000 to 60,000 | 26 to 39 at 12/12 |
The reasons behind each number, and how to hang a light to hit them evenly, are in our grow light distance guide. One thing worth knowing before you move a light to change these numbers: in a reflective grow tent, raising or lowering a light barely changes the average. It changes how evenly the light lands. Set the amount with the dimmer.
Where the conversion factors come from
Lux measures light the way a human eye sees it, which is mostly green; plants use red and blue too. So the number of lux in one unit of PPFD depends on the light's color mix, and every light type needs its own factor.
| Light source | Lux per 1 PPFD | Source |
|---|---|---|
| Sunlight | 54 | Apogee, after Thimijan and Heins1 |
| Cool white fluorescent | 74 | Apogee, after Thimijan and Heins1 |
| High pressure sodium | 82 | Apogee, after Thimijan and Heins1 |
| Double-ended HPS | 77 | Apogee1 |
| Metal halide | 71 | Apogee, after Thimijan and Heins1 |
| Ceramic metal halide, 4200 K and 3100 K | 65 and 59 | Apogee1 |
| White LED | 67 (about 15 PPFD per 1,000 lux) | Sharakshane2, a preprint |
| White LED with added deep red | 52 | Our estimate for a light getting a fifth to a third of its output from 660 nm red; no published factor exists |
Treat any lux conversion as a good estimate, not a measurement. If you can borrow a quantum PAR meter once, read the same spot with both and you have a correction for your own light. Our guide covers meters, phone apps and lux meters.
- Apogee Instruments. Conversion: PPFD to lux. Factors after Thimijan, R. W., & Heins, R. D. (1983). Photometric, radiometric, and quantum light units of measure: a review of procedures for interconversion. HortScience, 18(6), 818-822. https://doi.org/10.21273/HORTSCI.18.6.818
- Sharakshane, A. (2018). An easy estimate of the PFDD for a plant illuminated with white LEDs: 1000 lx = 15 µmol/s/m². bioRxiv (preprint, not peer-reviewed). https://doi.org/10.1101/289280