Daily Light Integral Calculator
Work out the daily light integral your plants receive from light intensity and photoperiod, or the intensity you need to reach a target DLI.
🌱 What is the Daily Light Integral?
Daily light integral, usually shortened to DLI, is the total quantity of photosynthetically active photons that land on one square metre of growing area over a complete day. It is measured in moles of photons per square metre per day. Where a single PPFD reading tells you how bright the light is at one instant, DLI tells you how much light the plant actually banked over the whole day, which is what drives photosynthesis, dry matter accumulation and yield.
The measure earns its keep in a long list of situations. Greenhouse growers use it to decide when supplemental lighting must switch on, because a cloudy winter week can halve the natural daily light integral and stall a crop. Vertical farm operators size their fixtures and set photoperiods against a crop-specific DLI target, then trade intensity against run hours to chase the cheapest electricity. Houseplant owners use it to work out why a fiddle-leaf fig two metres from a north window is dropping leaves. Cannabis and tomato growers use it to push a crop toward the upper end of its useful range without wasting energy past the point of diminishing return. Researchers report it because it makes light treatments comparable between experiments run in different seasons.
The most common mistake is treating a lux reading or a wattage figure as a light measurement for plants. Lux is weighted by the human eye's sensitivity curve, which peaks in green and falls away in exactly the deep red and blue bands that photosynthesis relies on, so a magenta LED panel can read dim on a lux meter while delivering plenty of usable photons. Fixture wattage is worse still, because it describes electrical input rather than photon output. The second common mistake is assuming light and time are freely interchangeable. They are not: almost every crop needs an uninterrupted dark period, and short-day species will not flower without one.
This calculator works in three directions. Enter a PPFD reading and a photoperiod to get the daily light integral. Enter a lux reading with the matching light source and it converts to PPFD first. Or enter the DLI you want and it tells you the intensity needed to reach it in the hours you have available. A crop table shows how the result compares against 12 common DLI targets, and the chart plots iso-DLI curves so you can see how much intensity you save by extending the photoperiod.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Leafy Green Rack Under LED
300 µmol/m²/s at canopy, lights on 16 hours a day
Example 2 — Dim Propagation Bench
200 µmol/m²/s at canopy, lights on 12 hours a day
Example 3 — Lux Meter Reading in a Greenhouse
20,000 lux of daylight, 10 usable hours of light
Example 4 — Sizing a Fixture for Tomatoes
Target DLI of 22 mol/m²/day over a 14 hour photoperiod
❓ Frequently Asked Questions
🔗 Related Calculators
What is daily light integral (DLI)?
Daily light integral is the total number of photosynthetically active photons landing on one square metre over a full day, expressed in moles per square metre per day. It combines light intensity and how long the light is on, so it is the single best measure of how much usable light a plant actually receives.
What is the formula for daily light integral?
DLI = PPFD x photoperiod x 3600 / 1,000,000, which simplifies to DLI = PPFD x hours x 0.0036. At 300 micromoles per square metre per second for 16 hours, DLI = 300 x 16 x 0.0036 = 17.28 mol per square metre per day.
What DLI does lettuce need?
Lettuce and most leafy greens perform well between 12 and 17 mol per square metre per day. Below about 10 the leaves grow thin and stretched, and above roughly 20 you gain little extra yield while raising the risk of tipburn and bolting.
What DLI do tomatoes need?
Greenhouse tomatoes want roughly 22 to 30 mol per square metre per day for good fruit set and sugar accumulation. That is why indoor tomato growing needs serious lighting: a bright indoor windowsill delivers only about 4 to 6 mol per square metre per day.
How do I convert lux to PPFD?
Divide the lux reading by a source-specific factor: 54 for sunlight, 74 for cool white fluorescent, 75 for white LED, 82 for high-pressure sodium, 71 for metal halide, 50 for incandescent and about 34 for magenta red and blue LED. A 20,000 lux daylight reading gives roughly 370 micromoles per square metre per second.
Why is lux a poor measure of plant light?
Lux is weighted by the human eye's sensitivity curve, which peaks in green and drops off sharply in the deep red and blue that drive photosynthesis. A magenta LED fixture can look dim to a lux meter while delivering plenty of usable photons, so a single lux figure can mislead by a factor of two or more.
Can I hit a high DLI by running lights for 24 hours?
Only for a few species. Lettuce and some herbs tolerate continuous light reasonably well, but most crops develop leaf chlorosis, oedema or physiological disorders without a dark period, and short-day plants will not flower at all. Most growers cap the photoperiod at 16 to 18 hours.
What is PPFD and how is it different from DLI?
PPFD, photosynthetic photon flux density, is an instantaneous rate in micromoles per square metre per second, like a speedometer reading. DLI is the accumulated total over the day, like the distance travelled. Two setups can share a PPFD and still deliver very different daily light integrals.
What PPFD do I need for a target DLI of 22 over 14 hours?
Rearranging the formula gives PPFD = DLI / (hours x 0.0036) = 22 / (14 x 0.0036) = 436.5 micromoles per square metre per second at canopy level. Stretching the same target over 18 hours would drop the requirement to about 340.
What is a typical outdoor DLI?
In a temperate mid-latitude location, a clear summer day delivers roughly 40 to 60 mol per square metre per day in full sun, dropping to 5 to 15 in midwinter. Tropical full sun can exceed 60. Shade under a tree canopy can cut those figures by 80 to 95 percent.
Does the calculator account for light lost between the fixture and the canopy?
No. It uses whatever PPFD or lux figure you enter, so take the reading at canopy height with the plants in place. Intensity falls off roughly with the square of distance, and reflective walls, glazing and canopy shading all change what the leaves actually get.
How accurate is the lux conversion for LED grow lights?
For broad-spectrum white LEDs the divisor of 75 is usually within about 10 percent. For magenta red and blue fixtures the divisor of 34 is a rough average across a very wide spread of spectra, and errors of 30 percent or more are common. Use a quantum PAR meter when accuracy matters.