Vegetable seedlings need a daily light integral (DLI) of about 10–15 moles of light per square meter per day. DLI is light level times hours: a light giving 174 µmol/m²/s for 16 hours delivers 10 mol/m²/day.
Open this example in the Grow Light Planner
What DLI measures
Daily light integral (DLI) is the total amount of usable light a plant gets in one day, in moles of light per square meter per day (mol/m²/day) (Daily Light Integral Defined). Think of it like a rain gauge for light: it doesn't matter whether the light came fast or slow, only how much landed by the end of the day.
Two numbers set it:
- Light level (PPFD), in µmol/m²/s: how much light lands on the leaves each second. Light meters and spec sheets give this.
- Hours of light per day.
DLI = PPFD × hours × 0.0036
The 0.0036 just converts units: an hour has 3,600 seconds, and a mole is 1,000,000 µmol.
How much light seedlings need
| Stage | DLI (mol/m²/day) | Source |
|---|---|---|
| Young seedlings | 6–10 | Michigan State |
| Older seedlings | 10–15 | Michigan State |
| Seedlings under lights | 12 | UNH's working assumption |
The BedMath planner aims for 12 for tomatoes, peppers, and brassicas, and 10 for lettuce, greens, and onions.
Worked example: 4 trays of onions
Four 1020 trays of onions on a 48 × 24 in shelf, under 2-ft LED bars, 16 hours a day.
- Light level needed: 10 ÷ (16 × 0.0036) = 174 µmol/m²/s.
- Light delivered: 8 bars × 18 µmol/s × 0.85 ÷ 0.60 m² = 203 µmol/m²/s.
- DLI delivered: 203 × 16 × 0.0036 = 11.7 mol/m²/day, 117% of the target.
Why more than the target? Bar rows have to be no more than 6 in apart to cover the trays edge to edge, and 4 rows of 2 is the fewest that do. Light alone would need 7 bars.
Hours change DLI too
The same 8 bars (203 µmol/m²/s at the trays) give a different DLI depending on how long they run:
| Lights on | DLI (mol/m²/day) |
|---|---|
| 11 h | 8.0 |
| 12 h | 8.8 |
| 14 h | 10.2 |
| 16 h | 11.7 |
That's why a weak light can sometimes make up the difference with longer days, up to the 16-hour limit. UNH found that the hours needed vary widely from one fixture to the next (Growing Seedlings Under Lights).
Run your own numbers
The free BedMath Grow Light Planner works this out for your trays, shelves, crop, and lights, and shows every formula and source. This link opens it with the example above filled in.
Sources
Published 2026-11-03. Worked examples come straight from the planner's current math.