DLI (daily light integral) is the total PAR light delivered to a surface over 24 hours, in moles of photons per square metre per day (mol/m²/day). It joins intensity and duration into one number, which matters because plants respond to both. Growth tracks DLI more closely than it tracks PPFD alone.
In practice
A canopy at 900 µmol/m²/s for 12 hours receives 900 × 12 × 0.0036 ≈ 39 mol/m²/day. The same canopy in veg at 600 µmol/m²/s for 18 hours gets about 39 mol/m²/day too. Outdoors, clear summer days at mid-latitudes reach roughly 50–60 mol/m²/day.
Rough indoor targets: seedlings and clones 6–15, veg 25–40, flower 35–55 mol/m²/day (higher only with CO2, good heat removal and strong genetics).
DLI is why flower intensity matters so much: the photoperiod is fixed near 12 hours, so the only way to add light is to raise PPFD. It is also why sunrise and sunset ramps add little to the daily total: low PPFD for an hour contributes little to DLI.
Units and formula
- DLI (mol/m²/day) = PPFD (µmol/m²/s) × hours × 3,600 ÷ 1,000,000
- Shortcut: DLI = PPFD × hours × 0.0036
- Required PPFD = DLI ÷ (hours × 0.0036)
Don't confuse with
- PPFD: an instantaneous rate, not a daily total.
- Photoperiod: hours of light only, with no intensity.
