DLI (Daily Light Integral)
Definition: Daily light integral (DLI) is the total amount of photosynthetic light a plant receives over a full day, measured in moles of photons per square meter per day (mol/m²/day). It combines light intensity and the number of hours the light is on, so it shows whether plants get enough total light, not just bright light.
How DLI (Daily Light Integral) works
PPFD is a snapshot of light intensity at one moment. DLI adds up those moments over the whole day. Two very different setups can deliver the same DLI: a moderate light running long hours, or a strong light running short hours.
The formula
DLI = PPFD × hours of light × 0.0036
The 0.0036 converts micromoles per second into moles per day (3,600 seconds per hour divided by 1,000,000).
| PPFD | Schedule | DLI |
|---|---|---|
| 400 | 18/6 | about 26 |
| 600 | 20/4 | about 43 |
| 800 | 12/12 | about 35 |
| 1,000 | 12/12 | about 43 |
Target ranges
- Seedlings: roughly 10 to 20 mol/m²/day
- Vegetative: roughly 25 to 40
- Flowering without added CO2: roughly 35 to 45
Clear summer days outdoors in much of the US deliver well over 50, which is part of why sun-grown plants get so big.
Why it matters
DLI explains why autoflowers on a 20/4 light schedule can thrive under a gentler fixture: the extra hours make up for lower intensity. It also explains why photoperiod plants in 12/12 need a stronger light to reach the same daily total. Planning by DLI helps you choose the right fixture and hanging height instead of guessing.