What is the difference between PPFD and DLI?

PPFD (Photosynthetic Photon Flux Density) is the instantaneous light intensity hitting the canopy, measured in micromoles per square metre per second. DLI (Daily Light Integral) is the total amount of light delivered over a whole day, in moles per square metre per day. DLI accounts for how long the light is on; two setups with the same PPFD give very different DLIs if one runs 12 hours and the other 18.

What is the formula for DLI?

DLI = PPFD × photoperiod (hours) × 3600 ÷ 1,000,000. The 3600 converts hours to seconds because PPFD is per second, and dividing by a million converts micromoles to moles. For example, 400 umol/m²/s for 16 hours gives 400 × 16 × 3600 / 1e6 = 23.04 mol/m²/day.

What DLI does my plant need?

Leafy greens and herbs thrive around 12-17 mol/m²/day; seedlings and low-light foliage want 6-12; fruiting crops like tomatoes and peppers need 20-30 or more; cannabis in flower wants 35-45. Too little DLI gives leggy, weak growth; too much can bleach leaves or waste energy. The calculator flags whether your setup lands in the chosen crop's range.

How do I measure PPFD at home?

A dedicated PAR/quantum meter is most accurate. Smartphone lux apps are a rough proxy but inaccurate because they are weighted for human vision, not photosynthesis, and conversions vary by light spectrum. Many quality grow lights publish a PPFD map showing intensity at various hanging heights — read PPFD at your canopy distance from that.

Can I trade PPFD for a longer photoperiod?

Partly. DLI is the product of intensity and duration, so a dimmer light running longer can match a brighter one running shorter — useful for leafy greens. But fruiting and flowering plants often need a specific photoperiod to trigger flowering, and some need a dark period, so you cannot simply run the light 24 hours. Match both the DLI and the photoperiod your crop requires.

Grow Light PPFD & DLI Calculator

The Grow Light PPFD & DLI Calculator converts the light intensity at your plant canopy into the Daily Light Integral — the single most useful number for judging whether an indoor grow light is strong enough for a given crop. It is built for indoor gardeners, hydroponic growers and anyone dialling in an LED setup.

How it works

Plants do not care about a single instant of brightness; they care about the total light delivered over a day. PPFD measures the instant, and the photoperiod sets the duration, so the DLI is their product with unit conversions:

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

The 3600 converts hours into seconds (because PPFD is a per-second flux), and dividing by 1,000,000 converts micromoles to moles. The calculator then compares the result against the recommended DLI range for your chosen crop and colour-codes it as low, on-target or high.

Typical DLI targets:

Seedlings / low-light foliage  — 6-12 mol/m²/day
Leafy greens & herbs           — 12-17
Fruiting crops (tomato, pepper)— 20-30
High-light flowering           — 30-45

Worked example and tips

An LED panel delivering 400 umol/m²/s at the canopy, run 16 hours a day:

DLI = 400 × 16 × 3600 / 1,000,000 = 23.04 mol/m²/day

That comfortably suits leafy greens and herbs and approaches the lower end for fruiting crops. To push tomatoes harder you could either raise PPFD (lower the light or increase power) or extend the photoperiod.

Tips:

Measure PPFD at the canopy, not at the light. Intensity falls off sharply with distance, so the figure at the leaf tops is what matters.
DLI trades intensity for time — a dimmer light running longer can match a brighter one — but respect each crop’s required photoperiod and dark period, especially for flowering.
Avoid over-lighting. Excess DLI bleaches leaves and wastes electricity; matching the target range is more efficient than maxing out.
Smartphone lux apps are unreliable for PPFD because they weight light for human vision. Use a PAR meter or the manufacturer’s PPFD map.