How Do I Calculate DLI From PPFD?
DLI = PPFD × photoperiod hours × 0.0036 — multiply your PPFD reading (µmol/m²/s) by your daily light hours and then by 0.0036 to get Daily Light Integral in mol/m²/day, the number that actually predicts plant growth, because the same PPFD run for 12 hours delivers roughly double the DLI of 6 hours.
The one thing to know: DLI (Daily Light Integral, mol/m²/day) is PPFD accumulated over your total light hours per day — it's the number that actually predicts plant growth, because the same PPFD for 12 hours delivers roughly double the DLI of 6 hours. Two setups with identical PPFD but different photoperiods give a plant very different total light. Always calculate DLI (PPFD × photoperiod hours × 0.0036) rather than judging a light by PPFD alone.
Worked example
Say your PPFD-at-distance chart (or PAR meter) reads 300 µmol/m²/s at your canopy height, and your timer runs the light 12 hours a day: 300 × 12 × 0.0036 = 12.96 mol/m²/day. Cut the photoperiod to 6 hours and the identical fixture at the identical height delivers half that — about 6.5 mol/m²/day. Nothing about the light changed; the DLI did. That's the whole argument for calculating it.
Getting an honest PPFD number first
The formula is only as good as its input. PPFD falls as you raise a fixture and rises as you lower it, so use the chart row (or your own meter reading) at your real mounting height — see What PPFD Do My Plants Need? for the stage-by-stage bands, and why "equivalent watts" tells you nothing.
Two lights that publish the chart you need
Every link below goes to a live tagged Amazon search, never a fixed listing.
HLG (Horticulture Lighting Group) — QB288 V2
Why it's here: Ships with an honest, independently-echoed PPFD-at-distance chart rather than a lumens or watt-equivalence number, letting you set mounting height to hit a target PPFD directly.
Best for: Growers who want to design their canopy around a specific PPFD/DLI target instead of guessing from a watt rating
Watch out: Higher upfront cost than quantum-board clones, and the bare-board kits require you to source your own driver and know your wiring before you can safely run it.
See current options on AmazonMars Hydro — FC-E4800 (Diablo)
Why it's here: One of the few budget-tier lights that publishes an actual PPFD-at-height chart instead of only a lumens or 'equivalent watts' figure.
Best for: A 3x3-4x4ft tent grower who wants a manufacturer PPFD map to plan hanging height around
Watch out: The published PPFD chart is the manufacturer's own bench test, not a third-party PAR map — treat it as a starting point and verify light levels with your own meter once installed.
See current options on AmazonThis page is general equipment-spec reference information, not plant-health or growing advice for any specific crop or property.
Frequently Asked Questions
Multiply your PPFD reading by your daily light hours, then by 0.0036: DLI = PPFD × photoperiod hours × 0.0036. For example, the same PPFD run for 12 hours delivers roughly double the DLI of 6 hours — which is why DLI, not a single PPFD snapshot, is the number that actually predicts plant growth.
DLI (Daily Light Integral, mol/m²/day) is PPFD accumulated over your total light hours per day. Two setups with identical PPFD but different photoperiods give a plant very different total light, so judging a light by PPFD alone misses half the picture. Always calculate DLI rather than comparing PPFD snapshots.
From the manufacturer's PPFD-at-distance chart, or from your own PAR/quantum meter reading at your actual mounting height. PPFD changes with distance from the fixture, so use the chart row (or meter reading) that matches your real canopy height — not the single headline number on the listing, and never an 'equivalent watts' figure, which measures nothing about photons reaching the plant.
Among our current picks, the HLG QB288 V2 ships with a PPFD-at-distance chart that independent measurements have echoed, and the Mars Hydro FC-E4800 is one of the few budget-tier lights that publishes an actual PPFD-at-height chart at all — though the FC-E4800's chart is the manufacturer's own bench test, so treat it as a starting point and verify with a meter once installed.