PPFD to DLI: Turning Light Intensity Into a Daily Dose
How PPFD, photoperiod and DLI relate, why DLI predicts growth better than intensity alone, and target ranges for common crops.
Intensity versus daily dose
PPFD measures instantaneous light intensity, the number of photosynthetic photons landing on a square meter each second, in umol/m2/s. On its own it does not tell you how much light a plant gets over a day, because that also depends on how long the light is on. DLI, the Daily Light Integral, rolls intensity and duration into one number: the total moles of photons per square meter across the whole photoperiod. Two very different setups, a bright light for a short day and a gentle light for a long day, can deliver the same DLI and drive similar growth.
Working the formula
The conversion is direct: DLI = PPFD x hours x 3600 / 1,000,000. The 3600 turns hours into seconds, and dividing by one million converts micromoles to moles. A canopy sitting at 400 umol/m2/s for a 16-hour day receives 400 x 16 x 3600 / 1,000,000, which is 23.04 mol/m2/day. Drop the intensity to 200 umol/m2/s over 12 hours and the dose falls to 8.64 mol/m2/day. Because the relationship is linear, you can trade intensity for hours as long as the product stays where you want it.
Target ranges by crop type
Different crops thrive at different daily doses. Leafy greens, herbs and shade-tolerant houseplants do well from roughly 6 to 12 mol/m2/day. Most fruiting and flowering crops, including tomatoes, peppers and many ornamentals, want something in the 12 to 30 range, with the upper end pushing heavier yields. Sun-loving, high-light crops such as cannabis in flower or field crops often target above 30 and can use 35 to 45 or more. These are guidelines, not hard limits, since temperature, CO2 and nutrient supply all shift the ideal light level.
Measuring and averaging honestly
A DLI is only as good as the PPFD behind it. Fixtures are brightest directly below and dimmer toward the edges, so a single center reading overstates what the whole canopy sees. Take PPFD at several points across the growing area, average them, and use that figure. If you dim the light or change hanging height partway through the day, the simple single-PPFD model no longer applies, and you would need to sum each segment. For steady setups, one careful average captures the daily dose well.