Grow Light Calculator

Daily light integral from PPFD and hours, with the extra time needed to hit a target.

What it does

A grow light rating tells you how bright a fixture is at one moment. Plants care about the total amount of usable light they collect across the day, and that quantity is the daily light integral. Two schedules can share the same DLI, one short and intense, the other long and gentle.

The starting unit is PPFD, the photosynthetic photon flux density at the canopy in micromoles per square meter per second. Multiply the rate by the number of seconds the light runs, then divide by one million to reach moles. A full hour contributes 3,600 seconds, so one hour at 100 PPFD adds 0.36 mol per square meter.

This tool converts PPFD and photoperiod into DLI, then compares the result with a target you choose. It also reports how many extra hours the same fixture would need to reach that target, and the total light delivered across the canopy area. That last number matters when you plan several plants under one fixture.

Targets depend on the crop. Leafy greens often finish near 12 to 17 mol/m2/day, tomatoes and other fruiting crops commonly run 20 to 30, and some high light crops go higher with matching carbon dioxide and nutrients. The table shows how PPFD and hours combine for common targets.

The calculation assumes the PPFD value is uniform over the canopy and constant through the photoperiod. Real fixtures fade toward the edges, dim with heat, and lose output with age. A cheap quantum sensor reading at the center can overstate what the outer plants receive.

Daily light integral is only one input to growth. Temperature, carbon dioxide, water, and nutrition set how much of that light the plant can actually use. Raising the DLI without adjusting the rest can bleach leaves instead of filling fruit.

How to use

1. Choose Metric or Imperial for the canopy area.
2. Enter the growing area under the fixture.
3. Enter the PPFD measured or rated at canopy height.
4. Enter how many hours the light runs each day.
5. Enter the DLI target for the crop and growth stage.
6. Read the current DLI, the extra hours needed, and the total moles per day.

Formula

DLI

Daily light integral in moles per square meter per day.

PPFD

Photosynthetic photon flux density in umol/m2/s at the canopy.

hours

Photoperiod in hours per day.

area

Canopy area in square meters or square feet.

The 0.0036 factor combines the unit conversions: 3,600 seconds per hour divided by 1,000,000 micromoles per mole. It works the same in metric and imperial because PPFD is always metric. The area only scales the total light; it does not change the DLI, which is already per square meter.

Worked example

Reference table

Limits

• Assumes one PPFD value across the whole canopy; edge plants usually receive less.
• Ignores sunlight, which can supply a large part of the DLI in a greenhouse.
• Rated fixture output is not the same as measured PPFD at the leaf after distance and reflector losses.
• Does not model light saturation, photoperiod responses, or crop-specific needs.

Common mistakes

• Multiplying PPFD by hours without the 0.0036 conversion.
• Using the fixture wattage as if it were PPFD.
• Measuring PPFD at the fixture instead of at the top of the canopy.
• Running 24 hours of light and expecting more growth; many crops need a dark period.
• Ignoring the DLI drop as plants grow taller and the light sits farther away.

Try it yourself

Quick checks. Answers sit tucked under each question.

Questions

Sources

• Virginia Cooperative Extension: calculating and using daily light integral
• Cornell University greenhouse lighting factsheet

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