Grow Light PPFD Calculator
Calculate PPFD (light intensity) for your grow space and find the right light wattage for cannabis, vegetables, or herbs at any growth stage.
What is PPFD?
PPFD stands for Photosynthetic Photon Flux Density — it measures the number of photons in the PAR (Photosynthetically Active Radiation, 400–700nm wavelength) range that hit a surface per second, expressed in μmol/m²/s (micromoles per square meter per second).
Think of it as “how intense is the light for plants” — similar to lux for human eyes, but calibrated to what plants actually use for photosynthesis.
Target PPFD by growth stage:
| Stage | Target PPFD | Daily Light Integral (DLI) |
|---|---|---|
| Seedlings / clones | 100–300 μmol/m²/s | 6–12 mol/m²/day |
| Vegetative | 400–600 μmol/m²/s | 20–30 mol/m²/day |
| Early flower | 600–900 μmol/m²/s | 30–40 mol/m²/day |
| Peak flower / fruiting | 900–1,500 μmol/m²/s | 40–60 mol/m²/day |
Estimated PPFD formula:
PPFD (μmol/m²/s) ≈ (Wattage × Efficacy) ÷ Coverage area (m²)
Where efficacy = photon output per watt. Modern LED grow lights:
- Budget LED: 1.5–2.0 μmol/J
- Mid-range LED: 2.0–2.5 μmol/J
- High-end LED: 2.5–3.0 μmol/J
- HPS (1000W): ~1.7 μmol/J (reference)
Worked example:
480W LED grow light (efficacy 2.5 μmol/J) over a 4ft × 4ft canopy:
- Coverage area = 4 × 4 × 0.0929 = 1.486 m² (converting sq ft to m²)
- PPFD ≈ (480 × 2.5) ÷ 1.486 = 807 μmol/m²/s
This falls in the early flowering range — appropriate for most fruiting plants.
Daily Light Integral (DLI):
DLI = PPFD × hours per day × 3600 ÷ 1,000,000
At 807 μmol/m²/s for 18 hours/day:
- DLI = 807 × 18 × 3600 ÷ 1,000,000 = 52.3 mol/m²/day — peak flowering range.
Heat and CO2 considerations:
Above 1,000 μmol/m²/s, plants can use supplemental CO2 (1,000–1,500 ppm) to increase photosynthesis further. Without elevated CO2, most plants hit a light saturation point around 1,000–1,200 μmol/m²/s where extra light produces heat without additional growth benefit.