Daily Light Integral Calculator
Convert average PPFD and daily light duration into the total photosynthetically active light delivered to one square meter each day.
Light inputs
Enter a nonnegative average canopy-level PPFD using a period as the decimal separator.
Use a value from 0 to 24 hours. Fractions such as 12.5 are accepted.
Daily light result
Photoperiod comparison at the current PPFD
| Light schedule | Hours/day | PPFD (µmol/m²/s) | DLI (mol/m²/day) |
|---|
The comparison holds PPFD constant and changes only the photoperiod. It is a planning aid, not a crop-specific lighting prescription.
How to use this Daily Light Integral calculator
What this calculator does. This tool estimates daily light integral, or DLI: the total number of photosynthetically active photons arriving on one square meter during a day. It combines an average photosynthetic photon flux density (PPFD) with the number of light hours. It is useful for comparing lighting schedules, checking whether an indoor fixture schedule delivers the intended quantity of light, and translating a canopy PPFD reading into a daily total. It does not determine the ideal DLI for a particular species, growth stage, cultivar, temperature, carbon-dioxide level, or production system.
When to use it. Use it when checking a grow-light timer, comparing two PPFD-and-duration combinations, estimating the daily contribution of supplemental lighting, or documenting a repeatable lighting set point. For background on how DLI integrates PPFD over time, see Virginia Cooperative Extension's guide to calculating and using daily light integral.
How to calculate. The calculator opens with a ready-to-use demonstration: 400 µmol/m²/s for 12 hours, producing 17.28 mol/m²/day. The example workbook is immediately available.
- Replace Photon flux density (PPFD) with the average PPFD measured or specified at plant-canopy height.
- Replace Light hours per day with the daily photoperiod. Results update as you type.
- Read Daily light integral (DLI), then review Lighting seconds, Daily photons, the formula line, and the photoperiod comparison table.
- Select Download Excel to save the current typed inputs and results. Reset clears the demonstration values and disables export until a complete valid pair is entered again.
Input guide. Photon flux density (PPFD) is required, measured in µmol/m²/s, and accepts a nonnegative decimal written with a period; 400 is a realistic example. Increasing PPFD raises DLI in direct proportion. Do not enter fixture wattage, lumens, lux, or a comma-decimal such as 1,5. PPFD should represent the relevant canopy area, not an isolated hotspot. Light hours per day is also required, accepts decimals from 0 through 24, and uses hours/day; 12 is the startup example. Longer duration raises DLI in direct proportion. A common mistake is entering clock time such as 1200 rather than a duration such as 12.
Output guide. Daily light integral (DLI) is the primary estimate in mol/m²/day. Zero means either zero PPFD or zero lighting time; higher values indicate more photons delivered per day, not automatically better plant performance. Lighting seconds converts the entered hours into seconds. Daily photons reports the intermediate total in micromoles per square meter before conversion to moles. The formula line shows the exact substitution. The summary pills repeat PPFD, photoperiod, and DLI from the same model. In the comparison table, Light schedule labels the current duration and standard 8-, 12-, and 16-hour alternatives; Hours/day, PPFD, and DLI show the corresponding inputs and calculated output.
Worked example. With PPFD equal to 400 µmol/m²/s and a 12-hour photoperiod, lighting time is 12 × 3,600 = 43,200 seconds. Multiplying 400 by 43,200 gives 17,280,000 µmol/m²/day. Dividing by 1,000,000 converts micromoles to moles, so DLI equals 17.28 mol/m²/day. The first-open result, formula, table's 12-hour row, and Excel workbook all use these same values.
How the DLI formula works
The constant-light formula is DLI = PPFD × hours × 3,600 ÷ 1,000,000. The factor 3,600 converts hours to seconds, matching the “per second” part of PPFD. Dividing by one million converts micromoles to moles. Apogee Instruments provides a practical explanation of converting instantaneous PPFD to daily light integral.
This shortcut assumes the entered PPFD is a representative average during the photoperiod. Sunlight and dimmed fixtures vary over time, so a series of PPFD readings integrated across the day is more accurate than one instantaneous reading. DLI describes photon quantity in the photosynthetically active range; it does not capture spectral distribution, leaf angle, canopy uniformity, or plant response.
Measurement and interpretation tips
Measure PPFD near the plant canopy and sample multiple positions when light distribution is uneven. The National Institute of Standards and Technology explains the distinction between radiometric and photometric quantities in its optical radiation measurement resources. For plant lighting, use a quantum sensor or a calibrated device intended for PPFD rather than substituting lux without a spectrum-specific conversion.
Two schedules can produce the same DLI: for example, doubling PPFD while halving duration preserves the mathematical total. Plants may nevertheless respond differently because photoperiod, peak intensity, temperature, and dark-period biology also matter. Use DLI as one planning variable and compare the result with crop-specific guidance from a qualified horticultural source.