Most growers think about light in terms of wattage or PPFD. But the metric that actually predicts plant growth is DLI — Daily Light Integral. Understanding DLI is the difference between guessing and knowing exactly how much light your cannabis plants need.
What is DLI?
Daily Light Integral (DLI) measures the total amount of photosynthetically active radiation (PAR) your plants receive in a 24-hour period. It's measured in mol/m²/d (moles of photons per square meter per day).
Think of it like this: PPFD tells you how hard it's raining. DLI tells you how much total rain fell that day. A plant doesn't care if it gets intense light for 6 hours or moderate light for 18 hours — what matters is the total light it receives.
The DLI Formula
DLI = PPFD × Hours × 0.0036
Example: 500 µmol/m²/s × 12 hours × 0.0036 = 21.6 mol/m²/d
Why DLI Matters More Than PPFD Alone
Two growers can have the same light but get wildly different results. A Spider Farmer G1500 putting out 400 µmol/m²/s for 18 hours delivers a DLI of 25.9 — perfect for vegetative growth. That same light at 12 hours? Just 17.3 DLI — barely enough.
This is especially important when transitioning photoperiod plants to flower. You're cutting from 18 hours to 12, which means you need to increase PPFD intensity by about 50% to maintain the same DLI.
DLI Targets by Growth Stage
DLI targets vary noticeably between autoflowers and photoperiods because they run on different light schedules. Autoflowers stay on 20 hours from seed to harvest, so they can hit higher DLI at lower intensity. Photoperiods drop to 12 hours in flower, which forces PPFD up to compensate. Both tables below match the per-week charts further down this page exactly.
Autoflower (20h light cycle)
| Growth Stage | DLI Target | PPFD Range | Light Hours |
|---|---|---|---|
| Seedling (week 0–1) | 12–16 mol/m²/d | 170–225 µmol/m²/s | 20h |
| Vegetative (week 2–4) | 20–40 mol/m²/d | 280–560 µmol/m²/s | 20h |
| Flowering (week 5–11) | 45 mol/m²/d | ~625 µmol/m²/s | 20h |
| Late flower (week 12+) | 32–40 mol/m²/d | 445–560 µmol/m²/s | 20h |
Photoperiod (18h veg → 12h flower)
| Growth Stage | DLI Target | PPFD Range | Light Hours |
|---|---|---|---|
| Seedling (week 0–1) | 12–16 mol/m²/d | 185–250 µmol/m²/s | 18h |
| Vegetative (week 2–6) | 20–38 mol/m²/d | 310–590 µmol/m²/s | 18h |
| Flowering (week 7–13) | 38–40 mol/m²/d | 880–925 µmol/m²/s | 12h |
| Late flower (week 14+) | 32–36 mol/m²/d | 740–833 µmol/m²/s | 12h |
Autoflower DLI Schedule
Autoflowers are unique because they don't depend on light cycle changes to trigger flowering. This means you can run 20 hours of light throughout the entire grow, achieving higher DLI at lower PPFD intensities — less stress on the plant, more total light.
At 20 hours, you only need 625 µmol/m²/s to hit 45 DLI. With 12 hours, you'd need 1,042 µmol/m²/s for the same DLI — that's nearly double the intensity, more heat, more electricity, and more stress.
Daily Light Integral — Autoflowering Cannabis
Light schedule: 20 hours/day
DLI ramps up through veg, peaks around 45 mol/m²/d in mid-flower, then tapers in the final weeks as the plant focuses on bulking and ripening rather than new growth.
Photoperiod DLI Schedule
Photoperiod plants need the light cycle change (18/6 to 12/12) to trigger flowering. This drop in light hours means you must compensate with higher PPFD to maintain adequate DLI during flower.
Daily Light Integral — Photoperiod Cannabis
Light schedule: 18h veg → 12h flower
Notice the DLI targets during flowering — you need strong lights to maintain 40+ DLI in just 12 hours. This is where premium lights like the Spider Farmer SE5000 (480W) earn their price tag.
Practical DLI Calculator
Here are quick references for common light schedules:
Seedling (18h)
200 µmol/m²/s × 18h
13.0 DLI
Early Veg (18h)
400 µmol/m²/s × 18h
25.9 DLI
Late Veg (18h)
600 µmol/m²/s × 18h
38.9 DLI
Auto Flower (20h)
500 µmol/m²/s × 20h
36.0 DLI
Photo Flower (12h)
700 µmol/m²/s × 12h
30.2 DLI
Photo Peak Flower (12h)
925 µmol/m²/s × 12h
40.0 DLI
How to Measure Your DLI
You need a PAR meter or a smartphone app (like Photone or Korona) to measure PPFD at canopy level. Take readings at multiple points across your grow space — light intensity drops significantly at the edges.
Once you know your average PPFD, multiply by your light hours and 0.0036 to get your DLI. If it's below target, you have three options:
Increase light intensity
Turn up the dimmer or lower the light (watch for heat stress)
Increase light hours
Only for autoflowers or during veg (not for photoperiod flowering)
Upgrade your light
A more powerful LED like the Mars Hydro TS1000 or Spider Farmer G1500 can make a big difference
Key Takeaways
DLI = PPFD × hours × 0.0036. Total daily light matters more than intensity alone.
Seedlings need 12–16 DLI; veg ramps from 20 to 38–40; flower peaks at 40–45. Mature plants taper down to 32–40.
Autoflowers benefit from 20h schedules — higher DLI at lower intensity means less stress.
When switching photoperiods to 12/12, increase PPFD by ~50% to maintain DLI.
Measure at canopy level with a PAR meter or smartphone app — light intensity drops at the edges of your space.
For a complete walkthrough of every growth stage, check our Complete Growing Guide. If you need help choosing the right light for your DLI targets, see our Equipment Recommendations.
Growing and want to track your progress? Start a Grow Diary and use our AI Plant Health Tool to check on your plants along the way.