Grow Light PPFD, DLI & Wattage Calculator

How it works

DLI (mol/m²/day) = PPFD × hours × 3,600 ÷ 1,000,000; PPFD needed = DLI × 10⁶ ÷ (hours × 3,600); watts = PPFD × area ÷ efficacy (µmol/J) ÷ delivery fraction; kWh = W × h ÷ 1,000

Plants count photons, not watts or lux: photosynthetic photon flux density is the number of photons in the 400–700 nm band hitting a square metre each second, and the daily light integral sums them over the photoperiod into moles per square metre per day, the single number that best predicts growth. Leafy greens are content with 12–17 mol, fruiting crops need 22–30 and cannabis flowers ask for 40 or more. A grow light's photon efficacy in micromoles per joule converts the required photon flux into electrical watts — modern LEDs at 2.5–3 µmol/J need half the power of HPS for the same DLI — and because much of a fixture's output misses the canopy without reflective walls, a delivery factor is applied. In greenhouses the natural DLI is measured and supplemental light makes up the shortfall.

Worked example

A lettuce canopy of 2 m² under 300 µmol/m²/s for 16 hours receives a DLI of 17.3 mol — at the top of the 12–17 range. To supply the midpoint of 14.5 mol from scratch you need 252 µmol/m²/s, or about 250 W of 2.5 µmol/J LED at 80% delivery (125 W/m²), costing around $0.60 a day at $0.15/kWh.

Frequently asked questions