How Do You Calculate Daily Light Integral (DLI)?
Daily light integral equals PPFD multiplied by the photoperiod in hours, multiplied by 3,600 seconds per hour, divided by 1,000,000. In short, DLI (mol/m²/day) = PPFD (µmol/m²/s) × hours × 0.0036.
The factor 0.0036 converts micromoles per second into moles per day: an hour holds 3,600 seconds and a mole holds 1,000,000 micromoles. A PPFD of 300 µmol/m²/s for 16 hours gives 300 × 16 × 0.0036 = 17.28 mol/m²/day, and 100 µmol/m²/s for 10 hours gives 3.6 mol/m²/day.
DLI captures both intensity and duration in one number, which makes it the most useful single measure of how much light a plant actually gets. Sunlight changes through the day, so a natural-light DLI comes from averaging PPFD readings or logging them with a sensor. The DLI calculator sits alongside the other free plant and garden calculators on PlantoScan.
What Is the Difference Between PAR, PPFD and DLI?
PAR is the light plants use for photosynthesis, the 400–700 nm waveband. PPFD is the instantaneous amount of PAR landing on a surface, in µmol/m²/s. DLI is the total PAR received over one day, in mol/m²/day.
PPFD is a snapshot, like speed; DLI is the total, like distance traveled. A quantum meter, also called a PAR meter, measures PPFD directly by counting photons in the 400–700 nm range. Grow-light manufacturers often publish a PPFD map showing readings at a set hanging height, with the highest values under the center of the fixture and lower values toward the edges.
Outdoor full sun at midday in summer reaches roughly 2,000 µmol/m²/s, and outdoor DLI in the United States ranges from under 10 mol/m²/day in northern winters to 50 or more in summer. Indoors, window glass, screens and distance from the window cut that light sharply, so a houseplant a few feet from a window often receives a small fraction of the outdoor DLI.
How Much Light Do Plants Need?
Low-light houseplants need a DLI of about 2 to 6 mol/m²/day, medium-light houseplants 6 to 12, high-light houseplants and herbs 10 to 20, and fruiting vegetables such as tomatoes and peppers 20 to 30 or more.
| Plant group | Examples | DLI (mol/m²/d) | PPFD over 12 h | PPFD over 16 h |
|---|---|---|---|---|
| Low-light houseplants | Snake plant, ZZ plant, pothos, peace lily | 2–6 | 46–139 | 35–104 |
| Medium-light houseplants | Philodendron, dracaena, peperomia | 6–12 | 139–278 | 104–208 |
| Seedlings and transplants | Vegetable and flower starts | 6–12 | 139–278 | 104–208 |
| High-light houseplants and herbs | Succulents, cacti, citrus, basil | 10–20 | 231–463 | 174–347 |
| Leafy greens | Lettuce, spinach, kale | 12–17 | 278–394 | 208–295 |
| Fruiting vegetables | Tomatoes, peppers, cucumbers | 20–30+ | 463–694+ | 347–521+ |
Generally cited DLI ranges; individual species and cultivars vary. PPFD in µmol/m²/s = DLI ÷ (hours × 0.0036).
The ranges overlap because light needs vary within each group and with temperature, humidity and growth stage. Shade-tolerant species such as those in the list of low-light houseplants for dim rooms survive near the bottom of the scale but grow faster with more light. Sun-loving houseplants that need bright light stretch, lose color and stop flowering below about 10 mol/m²/day.
How Do You Calculate the PPFD a Grow Light Needs?
Required PPFD equals the target DLI divided by the photoperiod in hours times 0.0036. A DLI of 12 mol/m²/day over a 14-hour photoperiod needs 12 ÷ (14 × 0.0036) = 238 µmol/m²/s at the canopy.
Longer photoperiods lower the PPFD a fixture must deliver: the same 12 mol/m²/day needs 278 µmol/m²/s over 12 hours but only 208 µmol/m²/s over 16 hours. Running a smaller grow light for more hours is often the cheaper way to reach a target DLI, as long as the plant is not photoperiod-sensitive.
Short-day plants such as poinsettias and chrysanthemums flower only when nights are long enough, and long-day plants such as spinach bolt as days lengthen. Keep photoperiods at or below 16 to 18 hours for most plants, and give seedlings and freshly moved plants a week or two at lower PPFD before raising the light to the full target.
Can You Convert Lux to PPFD?
Lux converts to PPFD only approximately, because lux measures brightness as the human eye sees it and PPFD counts photosynthetic photons. For sunlight, 1 lux equals roughly 0.0185 µmol/m²/s, so 10,000 lux is about 185 µmol/m²/s.
The conversion factor depends on the light's spectrum. Sunlight contains a broad mix of wavelengths, while white LEDs, fluorescent tubes and high-pressure sodium lamps concentrate their output differently, so their lux-to-PPFD factors are typically lower than the sunlight value, and purple red-and-blue LEDs read misleadingly low on a lux meter. The calculator's lux mode applies the sunlight factor and labels the result as approximate.
A lux meter or phone light-meter app is still useful for comparing spots in the same room under the same light source, such as finding the brightest windowsill. Match those readings to each species' light requirement in the individual plant care guides, and use a quantum meter when setting up grow lights.
Frequently Asked Questions
- What is a good DLI for houseplants?
- Low-light houseplants such as snake plant and ZZ plant grow with a DLI of about 2 to 6 mol/m²/day. Most foliage houseplants do well at 6 to 12, while succulents, cacti and citrus need 10 to 20 or more to stay compact and flower.
- Is a longer photoperiod the same as brighter light?
- A longer photoperiod raises DLI the same way brighter light does, so 150 µmol/m²/s for 16 hours equals 200 µmol/m²/s for 12 hours. Day length also controls flowering in photoperiod-sensitive plants, and most plants need a nightly dark period.
- Can plants get too much light?
- Plants get too much light when the DLI or PPFD exceeds what their leaves can use, causing bleached, yellowed or crispy patches. Shade-adapted houseplants and seedlings are the most sensitive, especially when moved abruptly into direct sun or under a strong grow light.
- How far should a grow light be from plants?
- Grow light distance depends on the fixture's output, because PPFD drops quickly as the light moves away from the canopy. Use the manufacturer's PPFD map or a quantum meter, and set the height that delivers the PPFD this calculator gives for your target DLI.