Daily Light Integral (DLI) Calculator
Enter a PPFD reading and your photoperiod to get the Daily Light Integral (DLI) in mol/m2/day. Switch to reverse mode to find the PPFD or photoperiod hours your grow-light needs to hit a target DLI. The reference table below compares your result to published targets for common crops.
Formula
Worked example
A grow-light delivering 300 umol/m2/s for 16 hours: DLI = 300 x 16 x 3600 / 1,000,000 = 17.28 mol/m2/day, which meets the upper end of the lettuce target range (12-17 mol/m2/day).
What is the Daily Light Integral?
The Daily Light Integral (DLI) is the total number of photosynthetically active photons that fall on one square metre of plant surface over a full day. It is measured in moles of photons per square metre per day (mol/m2/day) and combines two variables: intensity (PPFD in umol/m2/s) and duration (photoperiod in hours). A plant receiving a bright PPFD for only a few hours may see the same DLI as one receiving a lower PPFD for many hours. DLI is therefore the most complete single metric for matching a lighting plan to a crop requirement.
How to measure and use PPFD
PPFD (Photosynthetic Photon Flux Density) is measured with a quantum sensor or PAR meter held at canopy height. Readings vary significantly across a grow tent or greenhouse bench, so take several measurements across the footprint and average them. For supplemental greenhouse lighting, measure PPFD only when the supplemental lamp is on and the sun is at the same angle it will be during the run time. Indoor grow-lights dim over time, so re-measure every few months and adjust photoperiod to compensate for any reduction in PPFD.
Matching DLI to your crop
The reference table shows published DLI targets for common crops. Low-light crops such as seedlings and lettuce thrive at 5-17 mol/m2/day and can suffer tip-burn or bolting if DLI rises too high. Medium-light crops like basil and bedding plants generally do best at 15-25 mol/m2/day. High-light fruiting crops such as tomatoes, cucumbers and peppers need 20-30 mol/m2/day and may be light-limited below that range, reducing yield. If your DLI is lower than the target, you can increase PPFD (brighter fixture or closer mounting height), increase photoperiod (up to the crop-specific day-length limit), or both.
Supplemental greenhouse lighting
In a greenhouse, natural DLI varies by season, latitude and cloud cover. The supplemental lighting mode calculates how much DLI your grow-lights need to contribute and how many hours they must run to cover the deficit. Transmission accounts for light lost through the glazing material: clean glass transmits roughly 70-80%, polycarbonate 60-75%. For example, if outdoor DLI is 10 mol/m2/day in winter, transmission is 70%, and your lettuce target is 14 mol/m2/day, the indoor natural DLI is 7 mol/m2/day and the deficit is 7 mol/m2/day. A 150 umol/m2/s supplemental lamp would need to run about 12.9 h/day to make up the difference.
DLI targets for common crops and plants
| Crop / plant type | Light level | Target DLI (mol/m2/day) |
|---|---|---|
| Seedlings / cuttings | Low | 5 - 10 |
| Microgreens | Low-Medium | 9 - 12 |
| Lettuce | Medium | 12 - 17 |
| Spinach | Medium | 14 - 20 |
| Parsley | Medium | 10 - 15 |
| Cilantro | Medium | 15 - 20 |
| Basil | Medium-High | 15 - 25 |
| Impatiens | Low-Medium | 8 - 12 |
| Begonia / Geranium | Medium | 12 - 19 |
| Petunia | High | 20 - 25 |
| Tomato | High | 20 - 30 |
| Cucumber | High | 20 - 30 |
| Zucchini / Summer squash | High | 20 - 30 |
Based on research published by Virginia Cooperative Extension and the USDA. These are general target ranges - always consult crop-specific guidelines for commercial production.
Frequently asked questions
What is a good DLI for most crops?
It depends on the crop. Leafy greens and herbs generally perform well at 12-20 mol/m2/day. Fruiting crops such as tomatoes and cucumbers typically need 20-30 mol/m2/day for maximum yield. Seedlings and cuttings do best at 5-10 mol/m2/day to avoid stress during establishment. Always check a crop-specific guideline for commercial targets.
How do I measure PPFD without a PAR meter?
A dedicated quantum sensor or PAR meter is the most accurate tool. Some smartphone apps use the phone camera to approximate PPFD, but accuracy varies widely between devices and is generally unreliable for making lighting decisions. For indoor growing, most reputable grow-light manufacturers publish PPFD maps (photon distribution grids) for their fixtures at standard mounting heights. These maps give a reasonable starting estimate, though actual values at your canopy height may differ.
Can I just run my lights for longer instead of buying a brighter fixture?
Up to a point, yes. Many plants tolerate longer photoperiods and a lower PPFD can be offset by more hours. However, most plants need a dark period for normal flowering and metabolic rest. Long-day plants (like many lettuce varieties) may bolt if the photoperiod exceeds 16-18 hours. Fruiting crops like tomatoes flower best with a natural day-night cycle. Always check the photoperiod sensitivity of your specific cultivar before extending the run time significantly.
How does DLI change with seasons in a greenhouse?
Outdoor DLI varies enormously by latitude and season. In northern latitudes, midsummer DLI can reach 40-60 mol/m2/day, while a December day at the same location may deliver only 4-8 mol/m2/day outside. After transmission loss through glazing, winter indoor DLI may fall well below crop targets. The supplemental lighting mode in this calculator helps you determine how many hours your lamps need to run each day to compensate.
What is the difference between DLI and lux or footcandles?
Lux and footcandles measure illuminance, which is weighted by the human eye response. They count green and yellow light more heavily because those are the wavelengths humans see best. Plants absorb mostly blue and red wavelengths, so lux can be misleading for horticulture. PPFD and DLI count all photons in the 400-700 nm (photosynthetically active) range equally, making them a more accurate measure of what is actually useful to a plant. Always use mol/m2/day (DLI) or umol/m2/s (PPFD) for plant-lighting decisions.