Light intensity alone doesn't tell you much — a bright light for two hours delivers less total light than a moderate one running for twelve. Daily Light Integral (DLI) is the number that actually matters: the total amount of usable light your plants receive over a full day, and it's one of the biggest levers you have over yield indoors.
"Growers obsess over PPFD and forget it's a snapshot, not the whole picture. DLI is the number that actually predicts your harvest." — Andrew Woodward
What DLI actually measures
DLI is expressed in moles of light per square meter per day (mol/m²/day). It's calculated from your light's PPFD (photosynthetic photon flux density, measured in µmol/m²/s) and how many hours a day it runs:
DLI = PPFD × photoperiod (hours) × 3600 ÷ 1,000,000
So a light delivering 600 µmol/m²/s for a 16-hour photoperiod works out to: 600 × 16 × 3600 ÷ 1,000,000 ≈ 34.6 mol/m²/day.
DLI targets by crop
| Crop | Target DLI (mol/m²/day) |
|---|---|
| Seedlings & propagation | 6–10 |
| Microgreens | 8–14 |
| Lettuce & leafy greens | 12–17 |
| Herbs (basil, mint, cilantro, parsley) | 14–20 |
| Strawberries | 12–18 |
| Peppers | 18–25 |
| Cucumbers | 18–28 |
| Tomatoes | 20–30+ |
How to use this chart
- Start with your photoperiod, not just your light's wattage. Two setups with identical fixtures can deliver very different DLI just by running 14 vs. 18 hours a day.
- Raise DLI gradually. Plants adapt to light intensity over days, not instantly — a sudden jump from 15 to 30 mol/m²/day can stress or bleach foliage that wasn't acclimated.
- More isn't always better. Once a crop hits its saturation point, additional light stops translating into additional growth and just adds heat and electricity cost.
- Fruiting crops want more light than leafy ones. Tomatoes, peppers, and cucumbers are putting energy into fruit development, not just leaf area, so they can use — and need — meaningfully more DLI than lettuce or herbs.
The bottom line
If you only track one lighting number, make it DLI. It ties your fixture's output and your photoperiod together into the one figure that actually correlates with growth, and it's what lets you compare two completely different lighting setups on equal footing.
Building out your lighting? Aero-Gro Academy's Mastering Light module walks through sizing a light to your DLI targets room by room.
Tools for Serious Growers
Everything mentioned in this article — and more — available in the Aero-Gro store.