Tools · Light
DLI calculator
for cannabis growers
Enter the PPFD at the canopy and the hours of light. Get the daily light integral, how it compares with the target for each stage, and the PPFD or hours you need to hit it.
Inputs
Growth stage
Result
DLI against PPFD
Method and sources. DLI = PPFD × hours × 3,600 ÷ 1,000,000 (mol/m²/day). Stage targets from our PPFD and DLI guide; how much light a plant can use depends on CO₂, climate and genetics (Rodriguez-Morrison et al., 2021, Frontiers in Plant Science 12:646020).
How DLI is calculated
PPFD is the light reaching the canopy at a given moment, in µmol of photons per square metre per second. The daily light integral adds it up over the photoperiod:
DLI (mol/m²/day) = PPFD × hours × 3,600 ÷ 1,000,000
That is why the same lamp gives very different doses in veg and flower: 600 µmol/m²/s for 18 hours delivers 38.9 mol/m²/day, exactly what 900 µmol/m²/s delivers in a 12-hour flower day.
DLI targets by stage
| Stage | Photoperiod | PPFD (µmol/m²/s) | DLI (mol/m²/day) |
|---|---|---|---|
| Clones and seedlings | 18 h | 150–300 | 10–20 |
| Vegetative | 18 h | 300–600 | 20–40 |
| Flower | 12 h | 800–1,000 | 35–45 |
| Flower with CO₂ enrichment | 12 h | 1,000–1,500 | 45–65 |
These are working ranges. How much light a plant can use depends on CO₂, temperature, VPD, the root zone and genetics. In an indoor trial, cannabis yield kept rising in a straight line up to about 1,800 µmol/m²/s at ambient CO₂, without a meaningful change in THC or CBD concentration (Rodriguez-Morrison et al., 2021).
Getting reliable numbers
- Measure PPFD with a quantum sensor at the top of the canopy, in several spots. The centre under a lamp can read twice what the corners do; use the average.
- Do not trust lux apps for PPFD. Lux is weighted for the human eye, and the conversion changes with the spectrum of the lamp.
- Raise light gradually and watch the top leaves. Bleaching or upward-curling tips mean the canopy is getting more than the rest of the room can support.
For the full background, read PPFD and DLI for cannabis. To make use of more light, see CO₂ enrichment and the VPD calculator.