The one constant that turns PPFD into a daily total
DLI is the running total of photosynthetic photons a surface collects in a day. You get it by multiplying the instantaneous intensity (PPFD) by the hours the light is on, then scaling from micromoles per second to moles per day. That scaling factor is a fixed 0.0036.
The 0.0036 comes from two conversions: 3,600 seconds in an hour, and one million micromoles in a mole (3600 ÷ 1,000,000 = 0.0036). DLI is in mol/m²/day, PPFD in µmol/m²/s.
PPFD and photoperiod to DLI
Reading down a column shows the trade-off: dimmer light over longer hours can match brighter light over fewer.
| PPFD (µmol/m²/s) | 12 h | 14 h | 16 h | 18 h |
|---|---|---|---|---|
| 100 | 4.3 | 5.0 | 5.8 | 6.5 |
| 200 | 8.6 | 10.1 | 11.5 | 13.0 |
| 300 | 13.0 | 15.1 | 17.3 | 19.4 |
| 400 | 17.3 | 20.2 | 23.0 | 25.9 |
| 600 | 25.9 | 30.2 | 34.6 | 38.9 |
Values are DLI in mol/m²/day. Above roughly 20 most crops are near light saturation, so extra light adds little growth.
What DLI different plants want
- Low-light plants (ferns, shade foliage). 5–10 mol/m²/day. Below about 5 growth stalls.
- Leafy greens and herbs (lettuce). 6–14 mol/m²/day. The usual target for vertical farms and LED racks.
- Fruiting crops (peppers, strawberries). 15–22 mol/m²/day. Tomatoes want more than 20 and often need supplemental light.
- Too much is possible. Above about 25, especially on young plants, extra light causes photoinhibition rather than more growth.
Common questions
What is the difference between PPFD and DLI?
PPFD is the light intensity at one instant, in micromoles per square metre per second, like the brightness of a bulb. DLI is the total light a plant receives over a full day, in moles per square metre per day. DLI equals PPFD times hours times 0.0036.
How do I hit a target DLI if I choose the hours?
Rearrange the formula: required PPFD equals DLI divided by (hours times 0.0036). To reach a DLI of 12 over 14 hours you need about 238 micromoles per square metre per second.
Can a lux meter or a phone app measure DLI?
No. A lux meter reads brightness as the human eye sees it, not the photon count plants use for photosynthesis. You need a PPFD (PAR) meter for an accurate reading.
Can I use low light for many hours instead of bright light for few?
Up to a point. The same DLI can be reached either way, but most plants still need some hours of darkness for normal metabolism, so keep the photoperiod at or below about 20 hours.
