Plant Light Requirements Calculator
Use this worksheet to translate a measured photosynthetic photon flux density (PPFD) and a photoperiod into daily light integral (DLI). It is suited to a documented light scenario, not to identifying what a plant needs.
Inputs, units, and method
Enter measured PPFD in µmol/m²/s and photoperiod in hours per day. Photoperiod may range from 0 to 24 hours, and PPFD must be nonnegative. Record the sensor and measurement basis, sensor model or version, calibration and protocol, and an uncertainty or range for the measurement and photoperiod.
The supported relationship is:
DLI (mol/m²/day) = PPFD × photoperiod × 3,600 / 1,000,000
The factor 3,600 converts hours to seconds and 1,000,000 converts micromoles to moles. The result is displayed to two decimal places. This single-value method treats the entered PPFD as constant across the entered photoperiod; it does not integrate a changing time series.
Example: estimating a plant-light measurement
For PPFD of 300 µmol/m²/s over 12 hours:
300 × 12 × 3,600 / 1,000,000 = 12.96 mol/m²/day
The displayed result is 12.96 mol/m²/day. A PPFD of zero or a zero-hour photoperiod gives 0.00 mol/m²/day; that arithmetic endpoint does not establish why the measurement is zero.
DLI scenario reference
These rows show the direct effect of changing measured PPFD or duration. They are arithmetic scenarios, not plant target bands.
| PPFD | Photoperiod | DLI |
|---|---|---|
| 100 µmol/m²/s | 8 h/day | 2.88 mol/m²/day |
| 300 µmol/m²/s | 12 h/day | 12.96 mol/m²/day |
| 500 µmol/m²/s | 16 h/day | 28.80 mol/m²/day |
For a sensitivity check, rerun a documented lower and upper measured PPFD while holding photoperiod fixed, or vary photoperiod while holding PPFD fixed. Label the resulting span as a scenario range rather than a confidence interval.
Interpretation, boundaries, and next step
DLI summarizes the entered photon flux over the entered duration. Sensor placement, calibration, spectrum, spatial variation, and variation during the day can limit how representative one PPFD value is. No lux-to-PPFD conversion, species requirement, sufficiency category, diagnosis, or lighting recommendation is included.
Next, repeat measurements using the same documented sensor protocol at the positions or times relevant to your comparison. Keep each reading’s provenance and uncertainty with its DLI so changes can be traced to measurement, duration, or both.
Pair light arithmetic with related plant-care tools: the plant growth calculator accumulates growing degree days, the daylight hours calculator estimates photoperiod from latitude and date, and the plant watering calculator takes light as one of its watering inputs.
Sources
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USDA ARS, “Energy-Efficient Greenhouse Production of Petunia and Tagetes” — 2011 article; light-measurement and DLI passages, accessed 2026-07-09.
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HO 238 W — Purdue Extension documentation of DLI as PAR (PPFD) integrated over the day (PAR x 0.0864 factor), matching the worksheet’s PPFD x photoperiod arithmetic.