Related guides: PPFD vs. DLI guide · indoor grow-light guide
Daily light integral, or DLI, estimates how much photosynthetic light reaches a square meter of canopy during one day. It connects a point-in-time PPFD measurement with the number of hours the light runs.
The formula
For a steady light schedule, DLI = PPFD x hours per day x 3,600 / 1,000,000. PPFD is measured in micromoles per square meter per second, while DLI is reported in mol/m2/day.
For example, 250 PPFD for 16 hours gives 250 x 16 x 3,600 / 1,000,000 = 14.40 mol/m2/day. The DLI calculator performs this conversion, while the PPFD to DLI calculator provides the same calculation in a focused page.
Use ranges thoughtfully
Leafy greens often use lower daily-light targets than fruiting crops, but a target is not a guarantee. Cultivar, plant age, temperature, water supply, root health, and CO2 all affect how a plant handles light. A higher DLI can increase demand for water and nutrients; it can also expose heat or airflow problems that were hidden under a dimmer schedule.
Start with a conservative schedule, observe new growth, and change one variable at a time. A plant that is already heat-stressed or root-stressed is not improved simply by adding more photons.
For the next step, see the grow-light electricity cost guide.
Measure the canopy
Use PPFD at the plant canopy rather than fixture wattage. If the growing area is wide, take readings at several positions. A center reading can hide edge plants that receive much less light, while a single high reading can overstate what most leaves receive.
Keep a dark period appropriate for the crop and use a repeatable measurement position. If the fixture height changes, measure again. Pair the light log with room temperature, humidity, and the crop’s water use.
Understand the result
- PPFD describes intensity at a location and moment.
- DLI describes the accumulated light over the daily photoperiod.
- Wattage describes electrical input, not the photons reaching leaves.
Two schedules can have similar DLI but different intensity, heat, or uniformity. The calculator is a planning aid, not a substitute for measuring the canopy and watching the plant.
