The daily light integral calculator above converts a light intensity and a day length into the single figure horticulturists use to describe how much light a crop actually receives: DLI, in moles of photosynthetically active photons per square metre per day. It also compares that total against a reference target you select or type in, and works out what photoperiod or what intensity would be needed to reach it.
Arb Digital publishes this as a unit-conversion and planning tool. The target ranges it offers are quoted from a university extension publication and are presented as what that source reports, not as instructions for your crop. Growing decisions belong with your own extension service or horticultural adviser, who can see your structure, your latitude and your season.
What This Daily Light Integral Calculator Does
Enter the photosynthetic photon flux density at canopy level and the number of hours the lights run. The tool integrates one over the other to give the day's total photon delivery, adds any ambient daylight contribution you supply, and reports the result in mol/m²/day.
Because the relationship is a simple product, the two solve-back figures are often more useful than the total. The tool reports how many hours you would need at the current intensity to hit the target, and what intensity you would need at the current photoperiod. Those are the two levers a grower actually has, and seeing both quantified makes the trade-off between running longer and running brighter explicit.
The ambient field exists because greenhouse lighting is supplemental. Sunlight already delivers a DLI that varies by latitude, season and glazing transmission, and supplemental lamps are there to top it up. Entering zero treats the calculation as a sole-source indoor room, where the lamps deliver everything.
How to Use It
- Measure PPFD at canopy height with a quantum sensor, taking several readings across the area and averaging them.
- Enter the photoperiod your controller actually runs, not the setting you intended.
- Add the ambient DLI if you are in a greenhouse. Leave it at zero for a closed room.
- Choose a reference target from the extension figures, or switch to custom and enter one from a source appropriate to your crop.
- Read the two solve-back figures to see whether the shortfall is easier to close with hours or with intensity.
The Formula and How It's Calculated
PPFD is measured in micromoles of photons per square metre per second. DLI is measured in moles per square metre per day. Converting between them is a matter of multiplying by the seconds in the photoperiod and dividing by a million to get from micromoles to moles:
DLI = PPFD × 3,600 × hours ÷ 1,000,000
Which collapses to the shortcut every grower memorises: DLI = PPFD × hours × 0.0036.
Work the default. A canopy PPFD of 250 µmol/m²/s over a 16 hour photoperiod gives 250 × 16 × 0.0036 = 14.4 mol/m²/day. Against a 12.5 target that is a surplus of 1.9. Solving back, 12.5 ÷ (250 × 0.0036) = 13.9 hours would reach the target at the same intensity, or 12.5 ÷ (16 × 0.0036) = 217 µmol/m²/s would reach it over the same 16 hours.
What the Extension Sources Report
Purdue University Extension's vegetable programme, writing about managing daily light integral to improve vegetable transplant quality, defines DLI as the total photosynthetically active radiation a plant receives over a 24 hour period, expressed in mol·m⁻²·d⁻¹. It reports transplant-quality ranges of roughly 10 to 15 mol·m⁻²·d⁻¹ for lettuce, cucumber, squash and cabbage, and 15 to 20 for eggplant, pepper and tomato.
The measurement side is covered in Purdue Extension's bulletin Commercial Greenhouse Production: Measuring Daily Light Integral in a Greenhouse, by Roberto Lopez. Two things are worth taking from those sources rather than from this page. First, the ranges are crop and stage specific, and a transplant figure is not a production figure. Second, the useful DLI is the one measured under your own glazing, screens and structure, which can be a long way below the DLI outdoors on the same day.
There is a practical reason the two solve-back figures are shown separately rather than as a single recommendation. Most growing spaces are constrained on one axis and not the other. A propagation bench with fixed fixtures can only move hours; a room already running twenty hours can only move intensity. Presenting both lets you read off the one you can actually change and ignore the one you cannot, instead of being handed an answer that assumes a flexibility the structure does not have.
Why DLI Beats Intensity Alone
A PPFD reading describes one instant. Plants do not respond to instants; they accumulate photons and convert them over a day, which is why two rooms with the same instantaneous intensity can produce very different crops if one runs for twelve hours and the other for eighteen. DLI captures both dimensions in one number, and it is the number that correlates with biomass accumulation.
It also makes greenhouse and indoor systems comparable. A greenhouse in winter at high latitude may collect only a fraction of the DLI the same structure receives in June, and that gap is invisible in an intensity reading taken at noon on a bright day. Expressing everything as a daily total lets you see the seasonal shortfall and size the supplemental lighting against it.
Where the Simple Product Stops Being Enough
The formula assumes the intensity is constant across the photoperiod, which is true of a lamp on a timer and completely untrue of sunlight. A real greenhouse day rises and falls, so the correct ambient figure comes from an integrating sensor logging across the whole day, not from a single spot reading multiplied by daylight hours. Using the latter routinely overstates the solar contribution.
Uniformity is the other assumption. A single reading directly under a fixture can be double the reading between fixtures, and averaging several points across the canopy gives a far more honest input. It is also worth remembering that DLI counts photons in the 400 to 700 nanometre band only; it says nothing about spectrum, and two fixtures delivering an identical DLI with different spectral distributions will not necessarily produce the same plant.
One more assumption is buried in the ambient field. Adding a solar DLI to a lamp DLI treats the two as interchangeable moles, which is reasonable for photosynthesis but ignores the fact that they arrive at different times of day. Supplemental lighting run at dawn and dusk fills the shoulders of the natural curve; run at midday it stacks on top of a period that may already be near saturation, delivering the same total on paper and less benefit in the crop. The number is a budget, not a schedule.
The Trade-off Between Hours and Intensity
Because DLI is a product, a shortfall can be closed either way, and the two routes have different costs and different biological consequences. Extending the photoperiod is usually the cheaper option per mole delivered, since the fixtures are already installed and only the running hours increase. But photoperiod is also a signal: day-length-sensitive crops use it to decide when to flower, so extending the day to chase a DLI target can trigger or delay flowering in species where that matters.
Raising intensity avoids that problem but costs capital, and there is an upper limit beyond which additional photons produce diminishing returns as photosynthesis saturates. Neither route is universally right. What the calculator does is put numbers on both so the decision is made with the arithmetic visible, and then the horticultural judgement about which one suits the crop belongs to someone who knows the crop.
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Browse All Free Tools Contact Arb DigitalCommon Mistakes to Avoid
- Using a single PPFD reading under a fixture as the canopy average, which can overstate DLI substantially.
- Estimating ambient DLI from a spot reading × daylight hours. Sunlight is not constant, so only an integrating logger gets it right.
- Applying a transplant DLI target to a production crop, or one crop's figure to another.
- Confusing lux or foot-candles with PPFD. Those are weighted for human vision, not for photosynthesis.
- Chasing a DLI target by extending the day in a crop whose flowering responds to photoperiod.
Related Free Tools From Arb Digital
For lighting measured in human-vision units, the lumen to lux calculator and the luminous flux converter handle the photometric side, and the room lighting lumens calculator sizes ordinary interior lighting. On the physics of a photon there is the photon energy calculator. For running costs, see the electricity bill calculator. On the growing side there is the crop yield calculator and the bulb planting calculator. Browse the full free online tools hub for more.
Frequently Asked Questions
Multiply PPFD in micromoles per square metre per second by 3,600 seconds and by the number of hours, then divide by one million. The shortcut is PPFD multiplied by hours multiplied by 0.0036.
Purdue Extension reports transplant-quality ranges of about 10 to 15 mol per square metre per day for lettuce, cucumber, squash and cabbage, and 15 to 20 for eggplant, pepper and tomato. Those are the figures that source gives for transplants specifically.
Moles of photosynthetically active photons per square metre per day, written mol per square metre per day. It counts photons in the 400 to 700 nanometre waveband and says nothing about their spectral distribution.
Not reliably. Lux is weighted for the sensitivity of the human eye, which peaks in green, while photosynthesis responds across the whole 400 to 700 nanometre band. Any lux-to-PPFD conversion depends on the exact spectrum of the light source.
The arithmetic treats them identically, since DLI is their product. Extending hours is usually cheaper because the fixtures already exist, but photoperiod also signals flowering in day-length-sensitive crops, so the choice is horticultural rather than purely mathematical.
With a quantum sensor logging continuously across the whole day, since sunlight varies from dawn to dusk. Multiplying a midday spot reading by the hours of daylight consistently overstates the solar contribution.
No. It counts photons between 400 and 700 nanometres and treats them all equally. Two fixtures delivering the same DLI with different spectral distributions can produce noticeably different plants, so spectrum has to be considered separately.
This calculator performs a unit conversion and reports target ranges quoted from the university extension publications linked above. It is not horticultural advice. Crop, stage, structure, latitude and season all affect light requirements, and your own extension service or adviser is the authority for your growing situation.
