What is DLI and why does it matter for plants?

Daily Light Integral is the total amount of usable light a plant receives in a day, measured in moles of photons per square metre. It is the single best predictor of plant growth and flowering, because plants respond to total daily light, not just peak brightness.

How do PPFD and DLI relate?

PPFD is the instantaneous photon flux in micromoles per square metre per second. DLI is PPFD integrated over the photoperiod. The conversion is DLI equals PPFD times seconds of light divided by one million, since a mole is a million micromoles.

What PPFD should I use for outdoor sun?

Full, unobstructed midday summer sun is around 2000 micromoles per square metre per second. Bright overcast is roughly 400 to 800, and dappled shade under a tree might average 200 to 500. Because outdoor light varies through the day, the figure you enter is an effective average.

What DLI do different plants need?

As a rough guide, full-sun vegetables and fruiting crops want 20 to 40 mol per day, leafy greens and many perennials are happy at 10 to 20, and true shade plants tolerate 5 to 10. Below about 5 mol most plants struggle to do more than survive.

Does this replace a quantum meter?

No. A proper quantum or PAR meter logged through the day gives a real DLI. This tool estimates DLI from your sun hours and an assumed PPFD, which is ideal for quickly judging a planting site but less precise than a measured integral.

Solar PPFD & DLI Outdoor Garden Calculator

A planting site’s brightness matters less than how much total light it delivers over a day. This calculator converts your sun hours and an estimated photon flux into a Daily Light Integral, the figure that actually drives plant growth, and tells you what will thrive there.

How it works

DLI is simply photon flux summed over the lit part of the day. Because a mole is a million micromoles, the conversion from PPFD and sun hours is:

DLI (mol/m²/day) = PPFD (µmol/m²/s) × hours × 3600 / 1,000,000

For example, 6 hours of full sun at an effective 1500 micromoles:

1500 × 6 × 3600 / 1,000,000 = 32.4 mol/m²/day

The result is matched against common plant categories: roughly 20 to 40 mol for full-sun crops, 10 to 20 for part shade, 5 to 10 for shade, and under 5 for deep shade where most plants only survive.

Tips and notes

Outdoor light is never constant, so treat your PPFD as an effective average for the lit hours rather than a peak. Direct summer sun peaks near 2000 micromoles but spends much of the day lower, so 1200 to 1600 is a realistic working average for an open site. Hazy skies, walls, and tree canopies all cut the figure sharply — a north-facing border might only manage a few hundred micromoles for a few hours, landing it firmly in shade-plant territory.