Free calculator

Grow light coverage & PPFD calculator

Watts don't grow plants — photons do. Wattage × efficacy gives the photon flux (PPF); spread over your canopy it becomes PPFD, and PPFD × hours becomes the daily light integral the crop actually experiences.

Fixture & coverage

Fixture wattage (actual draw)WFixture efficacyµmol/JModern LED bars 2.5–3.0, budget LEDs ~2.0–2.5, double-ended HPS ~1.7.

Coverage lengthftCoverage widthft

Photoperiodh/dayGrowth stage (target band)

Result

16 ft² (1.49 m²) canopy, 90% utilization

Average PPFDAbove target

981µmol/m²/s

Daily light integral

63.6mol/m²/day

Stage target 12–20 mol/m²/day

Photon flux (PPF)

1,620µmol/s

This assumes photons spread evenly with 90% reaching the canopy — real fixtures run hotter in the center and dimmer at the edges, and hanging height changes the map. Treat it as a shopping estimate; verify with a PAR meter or the manufacturer's PPFD chart. Above ~1000 µmol/m²/s, plants need CO₂ enrichment to use the extra light.

Every one of those watts becomes heat — size the cooling → or the power bill →