Cannabis Light Plan Calculator

It runs two separate checks and takes whichever needs more light: one based purely on covering your square footage at each fixture's rated coverage, and one based on whether that fixture count actually reaches your target PPFD once the hang-height penalty is applied. If you're hanging lights higher than the 18-inch baseline or chasing a very high PPFD target like 900+ for flowering, the PPFD-driven check often wins and pushes the fixture count above what area coverage alone would need.

The model applies roughly a 2.5% PPFD reduction for every inch you raise the fixture above an 18-inch baseline, capped so delivered PPFD never drops below 30% of the baseline value. That means going from 18 to 30 inches costs about 30% of your PPFD at the canopy, which is why the calculator may ask for more fixtures at greater hang heights even though your coverage area hasn't changed.

Each fixture type carries its own photon efficacy — µmol of usable light produced per joule of electricity — and that's what actually drives PPFD, not raw wattage. HPS 1000W is rated at only 1.7 µmol/J while LED top-tier reaches 3.0 µmol/J, so a top-tier LED array can deliver the same photon flux as HPS while drawing roughly half the electrical power, which is also why the monthly electricity cost estimate swings so much between fixture types at the same target PPFD.

PPFD is an instantaneous rate of photons hitting the canopy, while DLI totals that up over your full photoperiod to show how much light the plant actually receives in a day (calculated here as achieved PPFD × photoperiod hours × 3600 seconds, divided by one million to convert to moles). Two setups can share the same PPFD but produce very different DLI if their photoperiods differ, which is why the calculator flags 40-65 mol/m²/day as the flowering target rather than relying on PPFD alone.