PPFD tells you how much light is hitting your canopy at any given moment. DLI tells you how much total light the plant receives over the entire day — and that's the number that actually matters for growth, yield, and quality. A plant under 400 PPFD for 18 hours gets a very different DLI than the same light for 12 hours, and that difference affects everything from node spacing to bud density.
What DLI is
DLI (Daily Light Integral) is the total number of photosynthetically active photons (PAR) delivered to a square meter of canopy per day. It's measured in mol/m²/d (moles of photons per square meter per day).
Think of PPFD as the speed of a car and DLI as the total distance traveled. PPFD is instantaneous intensity. DLI is the accumulated total over the full photoperiod.
The formula
DLI = PPFD × hours × 3600 / 1,000,000
Where:
- PPFD is in µmol/m²/s
- Hours is the photoperiod length
- 3600 converts hours to seconds
- 1,000,000 converts µmol to mol
Quick shortcut: PPFD × hours × 0.0036 = DLI
Example calculations
| PPFD (µmol/m²/s) | Photoperiod | DLI (mol/m²/d) |
|---|---|---|
| 200 | 18 hours | 12.96 |
| 400 | 18 hours | 25.92 |
| 600 | 18 hours | 38.88 |
| 800 | 18 hours | 51.84 |
| 600 | 12 hours | 25.92 |
| 800 | 12 hours | 34.56 |
| 1000 | 12 hours | 43.20 |
| 1200 | 12 hours | 51.84 |
Note that 800 PPFD × 18h and 1200 PPFD × 12h produce the same DLI (51.84). The plant receives the same total light — but the higher instantaneous intensity in the 12h scenario has different implications for heat stress, water use, and CO2 demand.
DLI targets by growth stage
| Stage | DLI target | PPFD range | Photoperiod |
|---|---|---|---|
| Clones/seedlings | 12–18 mol/m²/d | 100–300 µmol/m²/s | 18–24h |
| Early veg | 20–30 mol/m²/d | 300–500 µmol/m²/s | 18h |
| Late veg | 30–45 mol/m²/d | 500–700 µmol/m²/s | 18h |
| Flower (no CO2) | 35–45 mol/m²/d | 600–800 µmol/m²/s | 12h |
| Flower (with CO2) | 45–65 mol/m²/d | 800–1200 µmol/m²/s | 12h |
| Autoflower | 40–55 mol/m²/d | 600–800 µmol/m²/s | 18–20h |