DLI Formula (2026) Guide: PPFD Conversion & Targets

TL;DR

The DLI formula is DLI = PPFD × Photoperiod (hours) × 0.0036, where 0.0036 converts micromoles per second into moles per day. The reverse formula, PPFD = (Target DLI × 1,000,000) ÷ (Hours × 3,600), tells you the light intensity needed to hit a crop target. Leafy greens need 12 to 17 mol/m²/d, tomatoes need 20 to 35, and cannabis in flower needs 35 to 50. This guide covers both formulas, worked examples, a conversion table, crop targets, and the common mistakes growers make when running the math.

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What Is DLI?

Daily light integral (DLI) describes the total number of photosynthetically active photons delivered to one square meter over a 24-hour period. While PPFD is a snapshot of light intensity at a given moment (measured in µmol/m²/s), DLI is the cumulative total of that light over a full day. Think of PPFD as your speed on the highway and DLI as the total distance you travel.

The unit is mol/m²/d, which stands for moles of photons per square meter per day. One mole equals 6.022 × 10²³ photons. That sounds enormous, but 1 mol/m²/d is actually a very low DLI for plant growth, barely enough for shade-tolerant houseplants.

DLI matters because it is the metric that correlates most directly with plant growth outcomes. As Virginia Tech Extension notes, photosynthetically active radiation units like DLI “correlate well with plant growth” and support “better decisions about lighting strategies.” Ask ten growers how much light their plants need and you will get ten answers in ten different units. DLI is the one that actually answers the question for a full day of production.

For a deeper comparison of these two measurements, see our PPFD vs. DLI guide.

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The DLI Formula

Here is the standard DLI formula used across horticulture research and commercial growing:

DLI (mol/m²/d) = PPFD (µmol/m²/s) × Photoperiod (hours) × 0.0036

Each variable:

• PPFD is the photosynthetic photon flux density at your canopy, measured in micromoles of photons per square meter per second (µmol/m²/s).
• Photoperiod is the number of hours per day the light source is on (or, outdoors, the hours of usable sunlight).
• 0.0036 is the conversion constant that bridges the unit gap between µmol/s and mol/day.

Where Does 0.0036 Come From?

This is the part most resources gloss over. The constant 0.0036 handles two unit conversions at once:

1. There are 3,600 seconds in one hour (60 minutes × 60 seconds).
2. There are 1,000,000 micromoles in one mole (the “micro” prefix means 10⁻⁶).

Combine them: 3,600 ÷ 1,000,000 = 0.0036.

That is it. The factor converts “micromoles per second over some number of hours” into “moles per day.” You can also write the formula without the constant, showing the raw math:

DLI = PPFD × Hours × 3,600 ÷ 1,000,000

Both versions produce identical results. The 0.0036 form is simply more compact.

The 0.0864 shortcut: In sole-source indoor growing where lights run a full 24 hours, Erik Runkle of Michigan State University provides a simpler conversion: 1 µmol/m²/s = 0.0864 mol/m²/d. This constant is just 86,400 seconds in a day divided by 1,000,000. Multiply your average PPFD by 0.0864 and you have your DLI without needing to specify hours.

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Worked Example: Indoor Cannabis Flower Room

A cannabis cultivator runs their flower room at 900 µmol/m²/s on a 12/12 light schedule.

DLI = 900 × 12 × 0.0036 = 38.9 mol/m²/d

That lands squarely within the 35 to 50 mol/m²/d range that cannabis in flower requires. If the grower wanted to push toward 45 mol/m²/d, they could not simply extend the photoperiod to 14 hours because the plant would revert to vegetative growth. The only option is to raise intensity, a point we will return to in the common mistakes section.

For stage-by-stage cannabis lighting targets, our cannabis DLI guide breaks down seedling through harvest.

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The Reverse Formula: Calculating Required PPFD from a DLI Target

Commercial growers usually work backwards. They know the DLI their crop needs and must figure out how much light intensity their fixtures should deliver. The reverse DLI formula is:

PPFD (µmol/m²/s) = (Target DLI × 1,000,000) ÷ (Hours × 3,600)

MSU researcher Erik Runkle uses the constant 277.8 (which is simply 1,000,000 ÷ 3,600) to simplify this per-hour calculation. For an 18-hour photoperiod targeting 12 mol/m²/d, the required PPFD is 12 × 277.8 ÷ 18 = 185 µmol/m²/s.

Worked Example: Greenhouse Lettuce in Winter

This is where the reverse DLI formula becomes essential for real-world decision-making.

A lettuce grower in the northern U.S. faces these conditions in December:

• Outdoor DLI: approximately 10 mol/m²/d
• Greenhouse transmission: 60% (structure and glazing reduce incoming light by roughly 40%)
• Indoor DLI from sunlight alone: 10 × 0.60 = 6 mol/m²/d
• Target DLI for lettuce: 15 mol/m²/d
• DLI deficit: 15 − 6 = 9 mol/m²/d

If supplemental lights run for 16 hours per day:

Required PPFD = (9 × 1,000,000) ÷ (16 × 3,600) = 156 µmol/m²/s

That 156 µmol/m²/s is the average canopy PPFD the supplemental LED fixtures need to deliver, not the peak reading from a single spot directly under the fixture. The distinction matters enormously and trips up many growers.

For a complete walkthrough of greenhouse deficit calculations, see our greenhouse DLI calculator guide. Running more supplemental hours also means more heat, so it is worth factoring in HVAC load from lighting during the design phase.

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Quick-Reference PPFD to DLI Conversion Table

This table applies the DLI formula at three common photoperiods. Use it for fast estimation.

Notice how the same PPFD yields dramatically different DLI values depending on photoperiod. At 400 µmol/m²/s, moving from 12 hours to 18 hours adds nearly 50% more daily light. This flexibility is the whole reason the DLI formula is more useful than PPFD alone for planning purposes.

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DLI Targets by Crop

Different crops saturate at different DLI levels. These ranges consolidate data from university extension programs and commercial production research.

One important caveat: there is no absolute DLI “requirement” for any crop. As Michigan State research notes, most plants can grow under a wide range of environmental conditions. Growth also depends on CO₂ concentration and temperature. Higher CO₂ levels shift the light saturation point upward, meaning plants can use more DLI before hitting diminishing returns. Our guide on CO₂ in controlled environments covers this interaction in detail.

For crop-specific lighting plans, we have dedicated resources for tomato DLI and PPFD and lettuce lighting requirements.

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Outdoor and Greenhouse DLI Context

The DLI formula is only as good as the PPFD value you feed it. Understanding what kind of light levels actually exist in nature and in your facility puts the math into perspective.

Outdoors, the maximum DLI is about 60 mol/m²/d on a cloudless summer day with a long photoperiod. In winter, a dark, cloudy day in the northern U.S. or Canada may deliver less than 5 mol/m²/d.

Inside a greenhouse, structure, glazing, and obstructions commonly reduce light transmission by 35% to 50%. The average greenhouse DLI in the United States usually ranges from 5 to 30 mol/m²/d depending on location, season, and structure type. This is why supplemental lighting is standard practice for year-round commercial production: sunlight alone cannot maintain consistent crop DLI targets through the winter months.

To estimate your greenhouse DLI from sunlight, measure PPFD inside and outside simultaneously, calculate your transmission percentage, and multiply the outdoor DLI by that factor. The deficit between your greenhouse DLI and your crop target is what supplemental fixtures need to make up.

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Common Mistakes When Using the DLI Formula

1. Using Peak PPFD Instead of Average Canopy PPFD

Practitioners on Reddit and grower forums report this as the most common error. Manufacturers list a peak PPFD value from the center of the light at a fixed hanging height. That number is the best-case spot reading. Using it for a DLI calculation produces an optimistically inflated result. Always use the average PPFD across your entire canopy, measured at multiple points, when plugging values into the DLI formula.

2. Assuming More DLI Always Means More Yield

This assumption only holds when plants are experiencing a deficit relative to their optimum daily light integral. Beyond that, excess light does not convert to growth. Plants have a saturation point, and pushing past it wastes energy and can cause photoinhibition or heat stress. When comparing LED and HPS efficiency, this is especially relevant because fixture choice affects how much heat accompanies each photon of light.

3. Ignoring Photoperiod Constraints for Short-Day Crops

The DLI formula treats photoperiod as a free variable, but biology does not. Cannabis in flower runs on a 12/12 schedule, and extending the photoperiod to gain more DLI would revert the plant to vegetative growth. You cannot always trade intensity for hours freely. For short-day crops, the only way to raise DLI is to raise PPFD.

4. Not Accounting for Greenhouse Transmission Losses

Running the DLI formula with outdoor weather-station data, as if your crop were sitting in an open field, will significantly overestimate the light your plants actually receive. Determine the percentage of light transmission that reaches your crops by measuring inside versus outside, then apply that factor before calculating any supplemental lighting deficit.

5. Running the Same Photoperiod for Every Crop

A 16-hour photoperiod might be perfect for basil but excessive for lettuce at the same PPFD. DLI requirements are crop-specific, and so are photoperiod tolerances. Always match the photoperiod to the crop biology, then adjust intensity to hit the DLI target.

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DLI Delivery Method: Does It Matter How You Reach the Target?

Here is a nuance most DLI formula references skip. Two growers can hit the same DLI of 20 mol/m²/d with very different strategies: one at 350 µmol/m²/s for 16 hours, another at 460 µmol/m²/s for 12 hours. Are the results the same?

Michigan State University research confirms that increasing DLI by increasing PPFD or lengthening the photoperiod typically increases plant growth and yield, but the delivery method can produce different morphological outcomes. Higher intensity for shorter periods tends to produce more compact growth compared to lower intensity spread over longer hours.

Interestingly, a recent NCBI study on lettuce found that plants can tolerate multiple days of low DLI following a day with high DLI without significant growth penalties. This finding opens the door to energy-saving strategies where supplemental lighting is reduced on naturally bright days, and the DLI “budget” is managed over a multi-day window rather than a strict daily target. The practical result: reduced energy consumption and lower operating expenses for supplemental lighting.

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A Note on ePAR: The Expanding Definition of PAR

For 50 years, photosynthetically active radiation (PAR) has been defined as light in the 400 to 700 nm wavelength range. That definition is changing. Research by Dr. Bruce Bugbee and Dr. Shuyang Zhen has demonstrated that far-red photons (700 to 750 nm) contribute to photosynthesis more than previously understood.

The emerging term is ePAR (extended PAR), covering 400 to 750 nm. When measuring ePAR, the values run higher for the same light source because you are counting additional far-red photons. This means growers must keep their measurement and their crop DLI target in the same system. Mixing a PAR-based DLI target with an ePAR-based measurement will lead to overestimation.

This distinction is especially relevant for full-spectrum white LED fixtures that emit across a broad range including significant far-red output. For more on how spectrum affects plant growth alongside intensity, see our summary of light spectra impact.

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Putting the DLI Formula to Work

The DLI formula is simple math, but applying it well requires measuring accurately, understanding your crop targets, and respecting the biological constraints that the formula itself does not capture. Whether you are sizing supplemental fixtures for a greenhouse retrofit or confirming light levels in a vertical farm, these two equations (forward and reverse) are the foundation of every lighting design decision.

If you are planning a new facility or upgrading existing lighting and need help translating DLI targets into a fixture layout, talk to a Thrive lighting specialist for a free consultation.

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Frequently Asked Questions

What does the 0.0036 constant mean in the DLI formula?

The constant 0.0036 is a unit conversion factor. It accounts for two changes at once: multiplying by 3,600 to convert hours into seconds, and dividing by 1,000,000 to convert micromoles into moles. Combined, 3,600 ÷ 1,000,000 = 0.0036. Without it, the units in the DLI formula would not resolve to mol/m²/d.

How do I calculate the PPFD I need to reach a specific DLI?

Use the reverse DLI formula: PPFD = (Target DLI × 1,000,000) ÷ (Photoperiod in hours × 3,600). For example, to reach 17 mol/m²/d over 16 hours, you need (17 × 1,000,000) ÷ (16 × 3,600) = 295 µmol/m²/s average canopy PPFD.

What is a good DLI for lettuce?

Most leafy greens, including lettuce, perform well with a DLI of 12 to 17 mol/m²/d. In greenhouses during winter, reaching this range almost always requires supplemental lighting because sunlight alone may deliver only 5 to 8 mol/m²/d after transmission losses.

Can I increase DLI by running lights longer instead of brighter?

Sometimes. For long-day or day-neutral crops, extending photoperiod is a valid and often more energy-efficient way to raise DLI. But for short-day crops like cannabis in flower, extending beyond 12 hours of light would trigger a return to vegetative growth. In those cases, the only path to higher DLI is higher PPFD.

What is the difference between DLI and PPFD?

PPFD measures light intensity at a single moment in time (µmol/m²/s). DLI measures the total accumulated light over an entire day (mol/m²/d). PPFD is the speed; DLI is the distance traveled. Two grows can have the same PPFD but very different DLI values if their photoperiods differ.

What is the 0.0864 constant used for?

The constant 0.0864 is a shortcut for 24-hour sole-source environments. It equals 86,400 (seconds in a day) divided by 1,000,000 (micromoles per mole). Multiply your average 24-hour PPFD by 0.0864 to get DLI directly, without specifying photoperiod hours separately.

Does the DLI formula account for light spectrum?

No. The standard DLI formula counts all photons between 400 and 700 nm equally (the traditional PAR range). It does not weight photons by wavelength or account for far-red light beyond 700 nm. Emerging ePAR measurements extend the range to 750 nm, which produces higher readings for the same light source. Always confirm whether your meter and your target DLI are using the same spectral range.

How do I find my greenhouse’s DLI from sunlight?

Measure PPFD inside and outside the greenhouse simultaneously, then divide inside by outside to get your transmission percentage (typically 50% to 65%). Multiply the outdoor DLI for your location and time of year by this factor. The result is your greenhouse DLI from sunlight alone, and the gap between it and your crop target is the deficit your supplemental lights need to fill.