Grow Light Calculator

PPFD (Photosynthetic Photon Flux Density) measures the number of photons of photosynthetically active radiation (PAR, 400 to 700 nm) hitting a surface per second, in units of micromoles per square meter per second (umol/m2/s). It is the most relevant measure of light intensity for plant growth. Low-light plants need 50 to 150 umol/m2/s; high-light plants like tomatoes need 400 to 1,000 umol/m2/s.

DLI (Daily Light Integral) is the total amount of PAR delivered per day, in units of moles per square meter per day (mol/m2/day). DLI = PPFD (umol/m2/s) x photoperiod (hours) x 3,600 / 1,000,000. For example, 400 umol/m2/s for 16 hours gives DLI = 400 x 16 x 3,600 / 1,000,000 = 23.04 mol/m2/day. Most vegetables need 20 to 30 mol/m2/day.

Distance depends on the light's intensity and beam angle. Most LED grow light manufacturers provide a PPFD map showing intensity at different distances and positions. As a starting guide for full-spectrum LEDs: seedlings and low-light plants at 24 to 36 inches, vegetative growth at 18 to 24 inches, fruiting and flowering at 12 to 18 inches. Always check your specific fixture's documentation.

Modern high-efficiency LED grow lights typically deliver adequate PPFD at 30 to 50 watts per square foot for fruiting crops and 20 to 30 watts for leafy greens. Older HPS fixtures require 40 to 60 watts per square foot. These are rough guides; actual performance depends on efficacy (umol/J). A high-quality LED at 2.5 umol/J outperforms a budget fixture at 1.5 umol/J even at the same wattage.

Most vegetables need 14 to 18 hours of light per day to reach their DLI target. Seedlings do well at 16 hours. Fruiting plants benefit from 16 to 18 hours. Short-day plants like chrysanthemums flower when daylength drops below 12 hours. Always provide a dark period: plants need darkness for respiration and certain physiological processes.