What is Daily Light Integral (DLI)?

Daily Light Integral (DLI) is the total amount of photosynthetically active radiation (PAR) a plant receives over one day, measured in moles of light per square metre per day (mol/m2/day). It combines light intensity (PPFD) with the length of time the light is shining (photoperiod), which matters more to plant growth than intensity alone, since a dim light for a long time and a bright light for a short time can deliver the same total DLI.

How do you calculate DLI from PPFD?

DLI (mol/m2/day) = PPFD (micromol/m2/s) x photoperiod (hours) x 3600 (seconds per hour) / 1,000,000 (to convert micromol to mol). For example, a PPFD of 400 micromol/m2/s over a 12 hour photoperiod gives DLI = 400 x 12 x 3600 / 1,000,000 = 17.28 mol/m2/day.

What DLI do most plants need?

Target DLI varies widely by crop. Low-light houseplants and leafy greens generally do well on 10 to 15 mol/m2/day. Most fruiting vegetables such as tomatoes and cucumbers want 20 to 30 mol/m2/day. High-light crops like cannabis in flowering and some fruiting crops can use 35 to 45 mol/m2/day or more. Outdoor full sun in New Zealand summer can reach 40 to 55 mol/m2/day, while a cloudy winter day may only provide 5 to 10 mol/m2/day, which is why supplemental lighting is common in greenhouses over autumn and winter.

Daily Light Integral (DLI) Calculator

Work out the Daily Light Integral (DLI) your plants are receiving from PPFD (light intensity) and photoperiod (hours of light per day). DLI is measured in moles of light per square metre per day (mol/m2/day) and is the standard way growers judge whether a crop is getting enough total light, whether outdoors, in a greenhouse, or under grow lights.

Enter your light meter's PPFD reading and your daily light hours below to see your DLI, how it compares to common crop targets, and how many hours you would need at that PPFD to reach a target DLI.

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Reference values DLI = PPFD x photoperiod (hours) x 3600 / 1,000,000. Crop DLI targets from standard horticultural lighting guidance.

1. Your Light Reading

PPFD (micromol/m2/s, from a PAR meter or grow light spec sheet)

umol/m2/s

Photoperiod (hours of light per day)

hrs/day

2. Target Comparison

Crop or plant type

Hours needed to reach target DLI at your PPFD (calculated)

hrs/day

Your Daily Light Integral

DLI
-
mol/m2/day

PPFD

umol/m2/s

Photoperiod

hours per day

Target DLI

mol/m2/day for selected crop

DLI at Different Photoperiods (your PPFD)

Photoperiod	DLI (mol/m2/day)	vs Target Low	vs Target High

Calculation Breakdown

PPFD-

Photoperiod-

Seconds per day of light-

Total micromoles per m2 per day-

DLI (mol/m2/day)-

Target Comparison

Selected crop range-

Your DLI-

Hours needed at your PPFD for target-

NoteIncrease PPFD or photoperiod to raise DLI

Summary: Enter your PPFD and photoperiod above.

What Is Daily Light Integral?

Daily Light Integral (DLI) measures the total amount of photosynthetically active radiation (PAR) that lands on a square metre of growing area over one full day. It is expressed in moles of photons per square metre per day (mol/m2/day). Unlike an instantaneous light reading, DLI accounts for both how bright the light is and how long it shines, which is what actually drives plant growth, since photosynthesis responds to the total light received over the day rather than the peak intensity at any one moment.

The DLI Formula

DLI is calculated from PPFD (photosynthetic photon flux density, in micromol/m2/s) and the photoperiod (hours of light per day):

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

The 3,600 converts hours into seconds (since PPFD is a per-second measurement), and dividing by 1,000,000 converts micromoles into moles. For example, a PPFD of 400 umol/m2/s over a 12 hour photoperiod gives a DLI of 400 x 12 x 3600 / 1,000,000 = 17.28 mol/m2/day.

Typical DLI Targets by Crop

Crop or Plant Type	Target DLI (mol/m2/day)
Low-light houseplants, leafy greens (lettuce, spinach)	10 - 15
Herbs and seedlings	12 - 20
Fruiting vegetables (tomatoes, cucumbers)	20 - 30
Peppers and high-light fruiting crops	25 - 35
High-light flowering crops	35 - 45
Outdoor full sun, NZ summer (approx)	40 - 55
Outdoor overcast, NZ winter (approx)	5 - 10

Why DLI Matters for Growers

Two setups can have very different PPFD readings but deliver the same DLI. A grow light at 800 umol/m2/s for 6 hours gives the same DLI as 400 umol/m2/s for 12 hours (17.28 mol/m2/day either way). This means growers can trade off light intensity against photoperiod to hit a target DLI, which is useful when a fixture cannot be dimmed, when electricity costs matter, or when heat build-up from strong lights is a concern. Measuring DLI rather than just PPFD is the more reliable way to diagnose whether a crop is light-limited, especially in a New Zealand greenhouse over autumn and winter when both light intensity and day length drop.

How to Measure PPFD

PPFD is measured with a quantum PAR meter placed at canopy height, pointed straight up (or at the leaf surface angle for a more precise reading). Many LED grow light manufacturers also publish a PPFD map for their fixture at various mounting heights, which can be used instead of a meter if you know your light's height above the canopy. Sunlight PPFD varies through the day and with cloud cover, so outdoor DLI is usually estimated from long-term solar radiation data rather than a single reading.

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Sources: Standard DLI formula (PPFD x photoperiod x 3600 / 1,000,000) as used in horticultural lighting science. Crop DLI ranges are widely published guidance figures used by commercial greenhouse growers and grow light manufacturers.

This calculator provides an indicative DLI based on the PPFD and photoperiod you enter. Actual DLI from natural sunlight varies with cloud cover, season, latitude, and time of day, and PPFD from grow lights varies with fixture height, spread, and dimming. Use a PAR meter for the most accurate on-site reading.