The daily light integral, or DLI, is the total amount of plant-usable light a plant receives in one day. It is measured in moles of photons per square meter per day (mol/m²/day) and combines how intense the light is with how long it is on. It is the best single number for matching a grow light to a crop.
What DLI measures
Think of light as rain and DLI as the day's rainfall total. PPFD tells you how hard it is raining at one moment. DLI adds up all the rain over the whole day. A gentle drizzle for sixteen hours can deliver the same total as a short downpour. For plants, the daily total is what drives how much they can grow. Read what PPFD is if the instantaneous number is new to you.
The DLI formula
DLI = PPFD (µmol/m²/s) × hours of light × 3,600 ÷ 1,000,000
The 3,600 converts hours to seconds, and dividing by a million converts micromoles to moles. Two worked examples:
- 200 µmol/m²/s for 16 hours = 200 × 16 × 3,600 ÷ 1,000,000 = 11.5 mol/m²/day
- 750 µmol/m²/s for 12 hours = 32.4 mol/m²/day, the example used by University of Maryland Extension
| PPFD (µmol/m²/s) | Hours of light | DLI (mol/m²/day) |
|---|---|---|
| 100 | 12 | 4.3 |
| 200 | 16 | 11.5 |
| 250 | 16 | 14.4 |
| 300 | 14 | 15.1 |
| 400 | 14 | 20.2 |
| 600 | 14 | 30.2 |
Typical DLI targets
Targets differ by crop, variety and stage, and published figures vary. Use these as starting points:
- Lettuce and leafy greens: roughly 12 to 16 mol/m²/day is a commonly cited range. In one indoor hydroponic study of iceberg lettuce, 11.5 mol/m²/day gave the best fresh weight, and a higher dose of 14.4 reduced weight and leaf area.
- Culinary herbs: often listed at about 14 to 20 mol/m²/day, although herbs such as basil can flower early if they are pushed too hard.
- Tomatoes and other fruiting plants: need much more. Figures of about 20 to 30 mol/m²/day are commonly quoted for tomatoes, and growers report that below about 20 plants grow leaves but set fruit unreliably.
- Seedlings: modest levels are enough, because small plants capture little light.
For comparison, a bright outdoor summer day can deliver about 60 mol/m²/day, and inside a greenhouse the glass and structure usually cut that substantially, so greenhouse values seldom exceed about 25. An indoor light gives you a steady dose every day instead of a swinging one.
Two ways to reach the same DLI
Because DLI is intensity times time, there are two levers:
- Raise the intensity (PPFD). Move the light closer or turn the dimmer up.
- Lengthen the photoperiod. Run the light for more hours.
Both give the same daily total, but they are not identical. Plants have a ceiling on how much instantaneous light they can use, so past that point extra intensity is wasted, and a longer day may be the better lever. Photoperiod also steers flowering in some crops, and many plants need a dark period. Read how many hours of light plants need.
Research on lettuce shows the trade-off. At a fixed daily total, a lower intensity over a longer day can match or beat a higher intensity over a shorter day, which can also cost less to deliver because the light can run at a lower setting.
How to use DLI with your grow light
- Pick a DLI target for your crop from the list above.
- Choose a photoperiod that suits the crop. For many leafy greens, 14 to 16 hours is typical.
- Work out the PPFD you need: PPFD = DLI × 1,000,000 ÷ (hours × 3,600). For 12 mol/m²/day over 16 hours that is about 208 µmol/m²/s.
- Set the height or dimmer until the canopy sits at that PPFD. See how far to hang LED grow lights from plants.
- Review as the plants grow and as you move through the stages.
Natural light plus a grow light
If plants sit near a window, daylight contributes to the DLI too, and the grow light only has to make up the difference. In winter, when days are short and the sun is low, supplemental light can lift a plant that would otherwise sit below its target. That is one reason LED grow lights are popular for herbs on a kitchen windowsill and for citrus trees brought indoors for winter.
Electricity and DLI
Delivering a higher DLI costs more electricity. A light that is efficient, meaning it produces more µmol per joule, lets you reach the same DLI with less power. See whether LED grow lights save electricity.
Worked examples by crop
Use the reverse formula to find the PPFD you need: PPFD = DLI × 1,000,000 ÷ (hours × 3,600).
| Crop | DLI target (mol/m²/day) | Hours of light | PPFD needed (µmol/m²/s) |
|---|---|---|---|
| Lettuce | 12 | 16 | About 208 |
| Basil | 16 | 12 | About 370 |
| Tomatoes (fruiting) | 25 | 14 | About 496 |
Notice how a longer day lowers the intensity you need. Lettuce reaches its target at about 208 over 16 hours, but would need about 278 over 12.
What a DLI costs in electricity
You can estimate the least electricity needed to deliver a DLI from the light's efficacy. A light with an efficacy of 2.5 µmol/J turns each joule into 2.5 micromoles of plant light, so delivering 25 mol takes 25,000,000 ÷ 2.5 = 10,000,000 joules, which is about 2.8 kWh, for every square meter of plant canopy, every day. At 18.2 cents a kWh that is roughly 50 cents per square meter per day. For lettuce at 12 mol, the figure is about 1.3 kWh, or 24 cents.
The same tomato DLI from an HPS lamp at 1.7 µmol/J needs about 4.1 kWh, or 74 cents. That is a simplified, best-case calculation, because it assumes every photon lands on a leaf, but it shows why efficacy matters most for the crops that need the most light. See do LED grow lights save electricity.
Common questions about DLI
Is DLI the same as PPFD?
No. PPFD is the intensity at one moment. DLI adds up the whole day. Two lights with the same PPFD give different DLI if they run for different hours.
Do I need a meter to use DLI?
A meter is best, but you can estimate with the maker's PPFD map and your timer hours, then confirm by watching the plants.
Does a bigger DLI always mean faster growth?
Up to a point. Each crop has a range where extra light helps and a ceiling beyond which it does not. In the lettuce study, a DLI of 14.4 did worse than 11.5.
Key takeaways
- DLI is the total plant light received in a day: PPFD × hours × 3,600 ÷ 1,000,000.
- Typical starting targets are about 12 to 16 for lettuce and greens, 14 to 20 for herbs and 20 to 30 for tomatoes.
- You can reach a target with more intensity or more hours; use the lever that suits the crop.
- More light is not always better: each crop has a ceiling.
- Efficiency decides how much electricity a given DLI costs.
Common mistakes
- Chasing the highest PPFD possible. More light only helps up to the point the plant can use it.
- Ignoring photoperiod. Intensity alone does not tell you the daily total.
- Using a phone app as if it were a lab meter.
- Forgetting that the right target changes as a plant moves from seedling to fruit.
My LED Grow Lights have a dimmer switch, so you can set both stage and target without moving the fixture. See all of our LED grow lights, or return to how plants use light.
Related reading
- How Many Hours of Light Do Plants Need?
- Why are red light and blue light important for plants?
- Back to How Plants Use Light