Calculate your grow light's PPFD and Daily Light Integral — and find out if your plants are getting enough light to thrive.
| Plant / Stage | PPFD Target | DLI Target | Photoperiod |
|---|---|---|---|
| 🌱 Seedling / Clone | 100–300 | 6–12 mol/m²/d | 18 hrs |
| 🌿 Vegetative | 400–600 | 20–35 mol/m²/d | 18 hrs |
| 🌸 Flower / Fruiting | 600–900 | 25–40 mol/m²/d | 12 hrs |
| 🍅 Tomatoes / Peppers | 400–700 | 22–35 mol/m²/d | 16 hrs |
| 🥬 Lettuce / Greens | 150–300 | 12–17 mol/m²/d | 16 hrs |
| 🌿 Herbs / Microgreens | 150–350 | 10–20 mol/m²/d | 16 hrs |
PPFD (Photosynthetic Photon Flux Density) measures the number of photons in the 400–700nm range hitting one square metre of plant canopy every second, expressed in µmol/m²/s. Lumens measure light brightness as perceived by the human eye — not useful for plants. Watts measure energy consumption, not light output. PPFD is the only measurement that tells you how much usable light your plants actually receive. Always ask for a manufacturer's PPFD map before buying a grow light.
DLI (Daily Light Integral) is the total amount of photosynthetically active light delivered to your plants over a full day — it's PPFD multiplied by your photoperiod in seconds, divided by one million. DLI lets you compare lighting setups regardless of photoperiod length. A plant needs a minimum DLI to grow well, and you can hit that target with either a bright light on a short schedule or a dimmer light running longer. Vegetative plants typically need 20–35 mol/m²/day; flowering plants need 25–40 mol/m²/day.
The most accurate way is a dedicated quantum PAR meter (like the Apogee MQ-500) placed at canopy height. A cheaper option is the Photone app on a modern iPhone — it uses your camera sensor to estimate PPFD and is reasonably accurate for LED lights. Manufacturer PPFD maps are useful as a starting point but represent ideal centre-point measurements — real-world edge readings are typically 30–50% lower. Never rely on wattage or lumen ratings to estimate your actual PPFD.
Tomatoes and peppers are high-light crops that thrive at 400–700 µmol/m²/s with a DLI of 22–35 mol/m²/day. Run them on a 16-hour photoperiod indoors. In Canada, supplemental grow lighting for tomatoes and peppers is almost essential from October through March when natural light is insufficient even on sunny days. A 300W LED in a 3×3 tent will handle 2–4 plants at productive PPFD levels year-round.
Yes — light burn is real and can permanently damage your plants. Symptoms include bleached or yellowing upper leaves closest to the light, upward leaf curl, and stunted growth at shoot tips. Most plants max out their photosynthetic capacity at around 1,000–1,500 µmol/m²/s — beyond that you're wasting electricity and risking damage. If your PPFD is too high, raise the light, reduce your dimmer, or shorten your photoperiod before increasing CO₂ supplementation (which raises the ceiling).
Dial in every part of your grow.