DLI / Light Calculator

What is PPFD?

PPFD (Photosynthetic Photon Flux Density) measures the amount of photosynthetically active light (400–700nm wavelengths) hitting a specific area per second. It is measured in micromoles per square meter per second (μmol/m²/s). Think of it as the "intensity" of useful light reaching your canopy at any given moment.

What is DLI?

DLI (Daily Light Integral) is the total amount of photosynthetically active light received over a full 24-hour period, measured in moles per square meter per day (mol/m²/day). DLI accounts for both intensity and duration — a lower PPFD for more hours can achieve the same DLI as a higher PPFD for fewer hours.

The DLI Formula

Why Light Type Matters

Not all watts are created equal. Modern LEDs convert electricity to photosynthetically active light at roughly 2.5 μmol/J (micromoles per joule), while HPS lights achieve about 1.7 μmol/J and CFLs only about 0.8 μmol/J. This means a 400W LED produces roughly the same usable light as a 600W HPS — while running cooler and using less electricity.

Hanging Height

As you raise your light, intensity decreases following a modified inverse-square law. The "sweet spot" for most LEDs is 12–24 inches above the canopy. Too close causes light burn and bleaching. Too far reduces PPFD below useful levels. Adjust height throughout the grow — higher during seedling stage, lower during flower when plants can handle more intensity.