PPFD & DLI Calculator

PPFD (Photosynthetic Photon Flux Density) is the number of light photons in the 400–700 nm range hitting a square metre of canopy each second. It's the single best measure of how much usable light your plants are actually receiving. Watts and lumens are misleading because they don't tell you how much of that energy is in the wavelengths plants use.

Tomatoes and peppers want roughly 400–700 µmol/m²/s during the day. Below 400 you'll get leggy growth and few fruit; above 700 you risk leaf bleaching unless you have CO₂ supplementation.

PPF (µmol/s) is the total photon output of a light, regardless of where the photons land. PPFD (µmol/m²/s) is how many of them actually hit a given square metre. Manufacturers usually publish PPF; what your plants experience is PPFD.

DLI (Daily Light Integral) is the total amount of usable light a plant receives over 24 hours, in mol/m²/day. It's PPFD × hours-on × 0.0036. Most leafy greens want a DLI of 12–17; fruiting crops want 20–30.

It depends on the plant. As a rule of thumb for modern LEDs at ~2.5 µmol/J efficacy: leafy greens 20–30 W/ft², flowering plants 40–60 W/ft². The calculator above does the conversion properly using your actual tent area and target PPFD.

Microgreens want a DLI of about 6–12 mol/m²/day. Lettuce and most leafy greens do well at 12–17. Tomatoes and peppers want 20–30. Cannabis-style high-light flowering crops push 35–45. Below the lower end, plants stretch and yields drop; above the upper end, you stop seeing extra growth without supplemental CO₂.

You can't measure it precisely without a quantum sensor (the cheapest decent ones are about USD $200). The next-best option is a smartphone app like Photone with the diffuser cap — accurate to within 10–15% on white-spectrum LEDs. For broad-spectrum or red-heavy lights, smartphone-based measurements drift further off. The calculator above estimates expected PPFD from the manufacturer's published PPF, which is usually within 10% of measured reality if the light is hung at the recommended height.

Three reasons. First, published PPF is measured in a controlled lab, often at peak input voltage; real-world output runs 5–15% lower. Second, PPF is the total photon output regardless of where the photons land — your plants only get the fraction that hits the canopy, which depends on hang height, reflector design, and tent walls. Third, manufacturers sometimes publish theoretical maximums rather than steady-state numbers. Always cross-check claimed specs against grower-community measurements before buying.