How much light is a day’s worth?
PPFD is how hard it’s raining light right now. DLI is how much fell all day. Plants care about the second one.
600 µmol/m²/s
From a PAR meter, or a phone app if you take it with a grain of salt.
18 h
Daily light integral
38.9 mol/m²/day
Late veg into flower
- Same light at 12 hours
- 25.9 mol
- What the plants get after the flip if you change nothing.
- To keep 39 mol at 12 hours
- 900 PPFD
- How bright it would have to be.
The math. PPFD × hours × 3,600 seconds, divided by a million. That’s moles of light per square meter per day.
Why the flip matters. Going from 18 hours to 12 cuts the day’s light by a third unless the light gets turned up. That’s why the second pair of numbers is there.
The bands. Commonly quoted ranges, and growers argue about them. Watch the plants before you trust a chart, including this one.
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