DLI Calculator
Calculate your daily light integral (DLI) — because it’s not just intensity that counts, but how long your plants get light.
Calculated with PlantClue
Why DLI is the more complete light metric
What is DLI?
DLI (daily light integral) is the total amount of photosynthetically active light a plant receives in one day — measured in mol/m²/day. It combines intensity (PPFD, photosynthetic photon flux density) and duration, so it’s directly tied to the plant’s rate of photosynthesis (German source). Two setups with the same PPFD but different light hours end up with very different DLI values — and very different results.
Intensity vs. duration
You can hit the same DLI in different ways: 500 µmol/m²/s × 20 h = 36 mol/m²/day, or 750 µmol/m²/s × 13.3 h = 36 mol/m²/day. In flower the photoperiod is fixed (12/12), so your PPFD has to be right. In veg you have more room to play with light hours.
DLI outdoors
Outdoors, DLI varies a lot with the season and your latitude. In Central Europe, July DLI on a south-facing spot reaches 40–55 mol/m²/day — ideal for flowering. By September it drops to 20–30 mol/m²/day. That’s why the outdoor harvest lines up with the natural onset of flowering.
DLI chart: PPFD & light hours at a glance
This chart shows the DLI (mol/m²/day) for common combinations of PPFD and light hours — calculated with the same formula as the calculator above (DLI = PPFD × hours × 3,600 / 1,000,000).
| Hours \ PPFD | 200 | 300 | 400 | 500 | 600 | 700 | 800 | 900 |
|---|---|---|---|---|---|---|---|---|
| 10 h | 7.2 | 10.8 | 14.4 | 18.0 | 21.6 | 25.2 | 28.8 | 32.4 |
| 12 h | 8.6 | 13.0 | 17.3 | 21.6 | 25.9 | 30.2 | 34.6 | 38.9 |
| 14 h | 10.1 | 15.1 | 20.2 | 25.2 | 30.2 | 35.3 | 40.3 | 45.4 |
| 16 h | 11.5 | 17.3 | 23.0 | 28.8 | 34.6 | 40.3 | 46.1 | 51.8 |
| 18 h | 13.0 | 19.4 | 25.9 | 32.4 | 38.9 | 45.4 | 51.8 | 58.3 |
| 20 h | 14.4 | 21.6 | 28.8 | 36.0 | 43.2 | 50.4 | 57.6 | 64.8 |
| Growth stage | DLI target range |
|---|---|
| Seedling / clone | 12–18 mol/m²/day |
| Vegetative stage | 28–40 mol/m²/day |
| Early flower | 38–50 mol/m²/day |
| Late flower | 40–55 mol/m²/day |
What can go wrong when planning DLI
- Ignoring DLI and optimizing for PPFD alone — without factoring in light hours, a PPFD reading on its own doesn’t tell you much.
- Trying to extend the photoperiod in flower — 12/12 is biologically required to trigger flowering, so DLI corrections have to come from PPFD.
- Hitting seedlings with veg-level DLI — too much light early on can slow root development and cause chlorosis.
- Treating DLI needs as the same for every stage — demand rises significantly from seedling to flower and should be adjusted actively.
DLI — what growers want to know
What is DLI and how is it calculated?
DLI (daily light integral) is the total amount of light a plant receives in one day — measured in mol/m²/day. Formula: DLI = PPFD × light hours (h) × 3,600 ÷ 1,000,000. Example: 600 µmol/m²/s × 18 h × 3,600 ÷ 1,000,000 = 38.9 mol/m²/day.
How much DLI do cannabis plants need?
Seedlings: 12–18 mol/m²/day. Vegetative stage: 28–40 mol/m²/day. Early flower: 38–50 mol/m²/day. Late flower: 40–55 mol/m²/day. Values above 60 mol/m²/day can cause light stress, especially without added CO2.
Can I reach my DLI with longer light hours instead of a stronger light?
Up to a point, yes — this is called photoperiod compensation. Cannabis in veg can easily handle 18–20 h of light. In flower the photoperiod is fixed (12/12). Also, PPFD has a saturation point — very high PPFD over many hours won’t give you a proportionally bigger yield.
What’s the difference between DLI and PPFD?
PPFD is the instantaneous intensity (how bright it is right now). DLI is the total amount of light accumulated over the whole day. You can reach the same DLI with high PPFD + short light hours or low PPFD + long light hours.
Why do DLI needs change from stage to stage?
Seedlings and clones have small, sensitive leaves — too much light does damage. In the vegetative stage the plant grows fast and can process more light. In flower the photosynthesis rate is at its highest, and so is the demand.
