Why one daily number is better than “bright” or “low” light
Plant tags say “bright, indirect light,” which means different things in a Florida sunroom and a Seattle apartment in December. DLI replaces that with a measurement. It counts every bit of usable light that lands on a leaf from morning to night, the way a rain gauge counts every drop in a day.
That’s why a weak light left on for a long time can deliver the same DLI as a strong light used for a few hours. It’s also why indoor plants often struggle in winter: days are short and window light is weak.
The two numbers behind DLI
PPFD: how strong the light is
PPFD (photosynthetic photon flux density) measures how much plant-usable light reaches a surface each second, in micromoles per square meter per second (µmol/m²/s). Grow light makers usually publish a PPFD chart showing the value at different distances below the light.
Hours: how long the light lasts
For a grow light, this is the time on your timer. For a window, it’s the hours of useful daylight that reach the plant.
The formula
DLI = PPFD × hours × 0.0036
Iowa State University Extension gives the same formula. The 0.0036 turns seconds into hours (× 3,600) and micromoles into moles (÷ 1,000,000). DLI is measured in moles per square meter per day (mol/m²/day).
Example: a grow light gives 200 µmol/m²/s at the leaves and runs 12 hours a day. 200 × 12 × 0.0036 = 8.6 mol/m²/day. That suits medium-light houseplants (6–10), but it’s short of what basil wants (12–17).
What counts as low, medium and high
| Light level | DLI (mol/m²/day) | Typical plants |
|---|---|---|
| Low | 3–6 | Foliage houseplants |
| Medium | 6–10 | Foliage and flowering houseplants, cuttings, seedlings (upper end) |
| High | 12–16 | Flowering houseplants, succulents, herbs and leafy vegetables, seedlings (lower end) |
| Very high | 18–30 | Herbs, fruits and vegetables grown for fruit |
These ranges come from Iowa State University Extension. For comparison, Purdue University notes that outdoor DLI ranges from about 5 to 60 depending on season and location, and greenhouses seldom exceed 25. See targets for specific plants.
How to raise your plant’s DLI
- Leave the light on longer, up to 16 hours a day. Most indoor plants do well with 12–14.
- Move the light closer. PPFD rises quickly as distance shrinks; check the maker’s chart so you don’t scorch leaves.
- Use a stronger light, or add a second one for wide shelves.
- Move the plant closer to a south- or west-facing window in the Northern Hemisphere.
Calculate your plant’s DLI now
Sources
- Iowa State University Extension: How to determine how much supplemental light to provide indoor plants
- Iowa State University Extension: Important considerations for providing supplemental light to indoor plants
- Torres & Lopez, Purdue University Extension HO-238: Measuring daily light integral in a greenhouse