The formula
DLI = PPFD × hours × 0.0036. The 0.0036 converts seconds to hours and micromoles to moles. For example, 250 PPFD for 16 hours gives a DLI of 14.4.
DLI targets
| Crop | DLI | Source quality |
|---|---|---|
| Seedlings | About 6–12 | Secondary sources; figures vary |
| Leafy greens and herbs | 12 or more | Michigan State University |
| Fruiting vegetables | 15 or more | Michigan State University |
| Productive indoor tomatoes | About 20–30 | Vendor citing Virginia Tech Extension |
Where to get your PPFD figure
Use the maker's PPFD map at the hanging height you'll use, and take the average rather than the center peak. Those charts are not independently checked. A quantum sensor gives the best reading, and phone apps give a rough idea.
Longer hours or a stronger light?
You can reach the same DLI with a lower PPFD over more hours, which often lets you buy a smaller light. Some plants respond to day length, so don't simply run lights around the clock. Plants need a dark period, and 14 to 16 hours is a common choice for vegetables.