DLI calculator — turn PPFD into daily light integral

PPFD is how bright your light is right now. DLI is how much light your plant actually received today. They are not the same thing, and the difference is what catches people out at the flip.

DLI = PPFD × hours × 0.0036 — plain unit conversion (µmol per second → mol per day), no modelling involved.

DLI targets by stage

Derived from the PPFD ranges on our light schedules guide at the photoperiod normally run for each stage — so these agree with the rest of the site by construction rather than by someone remembering to update them.

The trap at the flip

This is the part worth internalising. Veg at 400 PPFD on 18 hours delivers 25.9 mol/m²/day. Switch to 12/12 and keep the same light height, and your plant's daily total drops by a third overnight — at the exact moment it starts building flower.

To hold that same DLI through the flip you would need roughly 600 PPFD, not 400. That is why flowering PPFD targets are higher than veg ones: they are compensating for hours you just removed. Growers who flip and change nothing else are quietly running a dimmer grow.

How much light is too much?

More light does raise yield, and further than most people expect — a controlled trial found dry flower yield rose roughly 4.5× going from 120 to 1800 PPFD. But two caveats matter more than the headline. First, that ceiling assumes CO₂, temperature and feeding all keep pace; without them the plant saturates far earlier and you get light burn instead of growth. Second, the same trial found cannabinoid concentration essentially unchanged across that range — you get more grams, not stronger grams.

If your leaves are tacoing upward or bleaching pale at the top of the canopy, you are past what the rest of your environment can support. Raise the light or dim it before touching the feed.

Measuring it honestly

PPFD varies enormously across a tent — the centre under the fixture can be double the corners. A single reading is a reading of one spot, not of your grow. Take several across the canopy and work from the average, and measure at canopy height rather than at the floor.

Phone lux apps are a rough relative guide for finding hot spots within your own space, but lux is weighted to human vision rather than photosynthesis, so converting it to PPFD involves a fudge factor that depends on your light's spectrum. Useful for "is the corner darker than the middle", not for "am I at 600".

See also: light schedules by stage · VPD chart · heat stress · log your grow free

Keep reading

• Hours by stage, and when to flip
• The environment side of the same problem
• When the answer is less light, not more

Track your own grow free — the journal grades your environment against these bands →