How this instrument works
PPFD (photosynthetic photon flux density) measures light intensity at an instant — how many photosynthetically active photons hit a square meter each second. That number alone is incomplete: a bright light running two hours and a dimmer light running twelve can deliver wildly different total light to a plant despite either one being 'brighter' at any given moment. Daily Light Integral solves that by accumulating PPFD over the full photoperiod, giving growers a single number for how much usable light a crop actually received over a day.
The math is a straightforward unit conversion. PPFD is measured in micromoles of photons per square meter per second; multiplying by the photoperiod's seconds (hours x 3,600) gives total micromoles per square meter for the day, and dividing by 1,000,000 converts micromoles to moles, the unit DLI is conventionally reported in: mol/m²/day. Both 3,600 and 1,000,000 are exact unit-conversion constants, not fitted or approximated values.
DLI matters because PPFD and photoperiod are interchangeable levers that reach the same destination differently: a grower can hit a DLI target with a bright light for fewer hours or a dimmer light for more hours, and crops generally respond to the accumulated total more than to the instantaneous intensity. Different crop categories have published target DLI ranges — leafy greens typically want roughly 12-17 mol/m²/day, fruiting crops like tomatoes want 20-30, and this instrument lets a grower check where a given PPFD-and-photoperiod combination actually lands.
- Enter your light meter's reading, or your fixture's rated output, into PPFD — photosynthetic photon flux density (µmol/m²/s).
- Enter the number of hours the light runs (or the natural day length) into Photoperiod (hours of light per day) — up to 24.
- Read Daily Light Integral (mol/m²/day) directly beneath the inputs; it recalculates as either field changes.
- To hit a target DLI, hold one field steady and adjust the other until the result lands in your crop's recommended range.
- Photoperiod cannot exceed 24 hours — the instrument will flag an entry above that as invalid.
Worked example — 400 µmol/m²/s over a 12-hour photoperiod
Enter 400 into PPFD — photosynthetic photon flux density (µmol/m²/s) and 12 into Photoperiod (hours of light per day), typical readings for a greenhouse tomato crop under supplemental lighting. Daily Light Integral (mol/m²/day) reads 17.28: (400 x 12 x 3,600) / 1,000,000 = 17,280,000 / 1,000,000 = 17.28 mol/m²/day.
That figure sits squarely inside the widely cited 12-25 mol/m²/day range recommended for fruiting greenhouse crops, meaning this combination of intensity and duration is delivering an adequate — though not maximal — daily light budget. A grower short of the target could raise PPFD, extend the photoperiod, or do a bit of both to close the gap.
Questions
What is the difference between PPFD and DLI?
PPFD is an instantaneous rate — how intense the light is right now, in micromoles of photons per square meter per second. DLI is that rate accumulated over an entire photoperiod, giving total photons delivered per day in moles per square meter. PPFD tells you how bright the light is; DLI tells you how much light the plant actually received by the end of the day.
Can I raise DLI by increasing photoperiod instead of intensity?
Yes — PPFD and photoperiod are interchangeable in the formula, so a dimmer light run longer can match a brighter light run for fewer hours at the same DLI. Some research even suggests extending photoperiod at a lower PPFD can produce better growth outcomes than raising PPFD at a fixed photoperiod, though the ideal balance varies by crop and lighting system.
What DLI range should I be targeting for my crop?
It depends heavily on the crop: leafy greens like lettuce typically do well around 12-17 mol/m²/day, while fruiting crops such as tomatoes and peppers often want 20-30 mol/m²/day or more to maximize yield. These are commonly cited horticultural benchmarks rather than hard limits, and local light availability, temperature and CO2 levels all interact with how a crop responds to a given DLI.
Why does the formula use 3,600 and 1,000,000 specifically?
Both are exact unit-conversion constants, not agronomic estimates. PPFD is measured per second, so multiplying by 3,600 (seconds in an hour) times the photoperiod's hours converts the rate to a per-day photon count in micromoles; dividing by 1,000,000 converts micromoles to moles, the standard unit DLI is reported in. Neither number depends on the crop or the light source.
Does this calculator account for light lost through a greenhouse roof or canopy?
No — it computes DLI directly from whatever PPFD value you enter, so if that reading comes from a light meter placed at canopy height, transmission losses through glazing are already baked into the number. If you instead enter a fixture's rated output at the source, the result won't reflect losses from distance, shading or greenhouse covering, so measuring PPFD at plant level gives the most accurate DLI.
Can photoperiod be more than 24 hours?
No — a day only has 24 hours, so the instrument caps photoperiod at that value and flags anything higher as an invalid entry. Some supplemental-lighting setups do run close to 24 hours for certain propagation stages, but the physical daily limit still applies to the DLI calculation.