DLI Calculator.
DLI Calculator
Daily Light Integral. The daily dose of photosynthetically active photons your plants receive.
What is DLI?
Daily Light Integral (DLI) is the total quantity of photosynthetically active photons that strike a square meter over a 24-hour period, expressed in moles per square meter per day (mol/m²/day). It is the daily dose of usable light for plants.
DLI matters because plants don't accumulate biomass based on instantaneous brightness — they accumulate it based on total photons available across the day. Short photoperiods at high intensity and longer photoperiods at lower intensity can produce the same DLI and roughly the same daily growth potential.
Like VPD, DLI is one of the few metrics that compresses several variables into a single decision-grade number. Where VPD is the dominant environmental number, DLI is the dominant lighting number.
The math
Given PPFD (Photosynthetic Photon Flux Density, in μmol/m²/s) and photoperiod (hours of lights-on per day):
DLI (mol/m²/day) = PPFD × hours × 3600 / 1,000,000
Going the other direction — solving for required PPFD given a target DLI and chosen photoperiod:
required PPFD (μmol/m²/s) = DLI × 1,000,000 / (hours × 3600)
To cover an area, multiply by area:
total PPF needed = required PPFD × area_m²
And since fixtures don't deliver uniform PPFD across the canopy, expect efficiency losses of 20–35% from fixture spec PPF to canopy PPFD depending on hang height, reflectivity, and overlap.
DLI targets by crop
| Crop | Veg DLI | Flower / fruit DLI | Notes |
|---|---|---|---|
| Lettuce / leafy greens | 12–17 | — | High DLI causes tip-burn |
| Spinach | 12–17 | — | Bolts at high DLI + high temp |
| Kale | 15–20 | — | — |
| Basil | 15–25 | — | — |
| Microgreens | 6–12 | — | Short cycle |
| Tomato (indeterminate) | 22–30 | 25–35 | — |
| Tomato (determinate) | 18–25 | 22–30 | — |
| Cucumber | 20–30 | 25–30 | — |
| Pepper (bell) | 18–30 | 25–32 | — |
| Strawberry | 17–22 | 20–27 | — |
| Cannabis (clone/seedling) | 8–12 | — | Low light during root establishment |
| Cannabis (early veg) | 18–25 | — | — |
| Cannabis (mid/late veg) | 25–35 | — | — |
| Cannabis (flower wk 1-4) | — | 35–50 | Building biomass + flower set |
| Cannabis (flower wk 5-8) | — | 45–65 | Highest demand of common crops |
| Cannabis (flower wk 9+) | — | 35–50 | Reduce as ripening progresses |
| Mushroom (oyster) | 0.5–2 | 0.5–2 | Indirect; low light is enough |
| Hops | 25–40 | 30–45 | Long-day flowering trigger |
Energy and cost reality
Hitting cannabis flower DLI of 50 mol/m²/day at 12/12 photoperiod requires 1,160 μmol/m²/s at canopy, sustained for 12 hours — the upper edge of what high-PPE LEDs deliver. Hitting 65 mol/m²/day requires 1,500 μmol/m²/s, which most fixtures can only sustain in narrow zones near their center.
The practical limit is rarely the fixture — it's heat. More photons mean more energy converted to plant biology and (the rest) to heat. Removing that heat through HVAC dominates the energy budget. A lighting decision is also a cooling decision.
This is why high-DLI cannabis facilities are also CO₂-enriched: at high DLI, plants become CO₂-limited, and enrichment unlocks the additional growth the photons could otherwise drive.
Greenhouse and outdoor reality
For greenhouse and outdoor operations, DLI varies massively by location, season, and weather. A cloudy March day in Vermont might deliver 8 mol/m²/day; a clear July day in Phoenix might deliver 60+ mol/m²/day.
Greenhouse operators use supplemental lighting to fill the gap to a target DLI:
required supplemental DLI = target DLI − measured natural DLI
Supplemental lighting controlled by PAR sensor feedback can dim or brighten in real time as cloud cover changes. This is more energy-efficient than running fixed-output supplementals during all photoperiod hours.
References
DLI math is standard plant physiology. Per-cultivar DLI targets synthesized from peer-reviewed agronomy literature, university-extension publications (UF/IFAS, Cornell CEA, Michigan State, Wageningen UR), and grower community practice.
Free under CC BY 4.0. Cite as "OAT DLI Calculator (openagriculturetechnology.com)".