Daily Light Integral Calculator (DLI from PPFD & hours)

Understanding the daily light integral (DLI) calculator — concept and principle

The daily light integral (DLI) is the total amount of photosynthetic light a plant receives in a day. Growers use it to decide how long to run grow lights, how bright they must be and whether a crop is getting enough light. This calculator combines lamp light with daylight and compares the total with a target.

100 µmol/m²/s for 14 hours adds 100 × 14 × 3600 ÷ 10⁶ = 5.04 mol; with 6 from daylight the total is 11.

How it is calculated

DLI from lamps = PPFD × hours × 3,600 s ÷ 1,000,000 = PPFD × hours × 0.0036 mol/m²/day.
Total DLI = lamp DLI + daylight DLI.
Lamp hours needed = (target − daylight DLI) ÷ (PPFD × 0.0036).
PPFD needed in the set hours = (target − daylight DLI) ÷ (hours × 0.0036).
Average PPFD over 24 h = total DLI × 10⁶ ÷ 86,400.

The principle

PPFD (photosynthetic photon flux density) measures how many photons in the 400–700 nm band hit each square metre per second. Plants respond to the total photons received, so multiplying the rate by time gives a daily dose, just as a rain gauge sums rainfall. Crops have characteristic light needs; Purdue Extension groups them as low-light (3–6), medium (6–12), high (12–18) and very high (above 18 mol/m²/day).

Worked example

Default inputs: lamp PPFD 100 µmol/m²/s, 14 hours, daylight DLI 6, target 12.

Lamp DLI = 100 × 14 × 0.0036 = 5.04 mol/m²/day.
Total DLI = 5.04 + 6 = 11.04, shown as 11 — a medium-light crop level (6–12).
Hours needed for 12 = (12 − 6) ÷ (100 × 0.0036) ≈ 16.7 hours.
Or keep 14 hours and raise PPFD to (12 − 6) ÷ (14 × 0.0036) ≈ 119 µmol/m²/s.
Average PPFD over 24 h = 11.04 × 10⁶ ÷ 86,400 ≈ 128 µmol/m²/s.

Each point is the calculator re-run with only “Daylight DLI already received (mol/m²/day)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Daylight DLI already received (mol/m²/day)	Total DLI (mol/m²/day)
1.5	6.5
3	8
4.5	9.5
6	11
7.5	12.5
9	14
12	17

How to read the result

Where does your result fall?

Below the crop's need, plants grow slowly and may stretch; above it, light costs more without extra benefit and some crops suffer. Some plants are sensitive to day length, so adding hours can change flowering; increasing intensity may be better for them. Measure PPFD at plant height, since it falls with distance from the lamp.

Common mistakes

Using lux or watts instead of PPFD.
Measuring PPFD at the lamp instead of at the canopy.
Forgetting that greenhouse glass reduces outdoor daylight.
Ignoring photoperiod effects when extending lighting hours.

Keep in mind: Light needs vary by crop and growth stage. Use crop-specific guidance and measured light levels.

Frequently asked questions

What is a good DLI for vegetables?

Many fruiting vegetables need high DLI (often 15 or more); leafy greens usually need less. Check crop-specific recommendations.

How do I convert PPFD to DLI?

Multiply PPFD by hours of light and by 0.0036.

Is DLI the same as lux?

No. Lux measures brightness to the human eye; DLI counts photosynthetic photons.

Sources: Faust, J. E., Holcombe, V., Rajapakse, N. C. & Layne, D. R. (2005). The effect of daily light integral on bedding plant growth and flowering. HortScience, 40(3), 645–649. · Torres, A. P. & Lopez, R. G., Measuring Daily Light Integral in a Greenhouse. Purdue Extension HO-238-W

Checked 9 Nov 2026. Torres A.P. & Lopez R.G., “Measuring Daily Light Integral in a Greenhouse”, Purdue Extension HO-238-W (DLI = average PPFD × 0.0864; crop groups 3–6, 6–12, 12–18, >18 mol/m²/day); Ebba J., “Supplemental Lighting Run Time Worksheet”, UNH Cooperative Extension, January 2020 (lamp hours = additional DLI ÷ (lamp PPFD × 0.0036)).

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