How it works
DLI (mol·m⁻²·d⁻¹) = PPFD (µmol·m⁻²·s⁻¹) × photoperiod (h) × 3600 ÷ 1,000,000
Instantaneous light is measured as PPFD — the photosynthetic photon flux density, in micromoles of light per square metre per second. To get the daily dose you multiply that rate by how long the light is on. There are 3,600 seconds in an hour, and a micromole is one-millionth of a mole, so multiplying PPFD by hours and by 3600/1,000,000 converts a per-second micromole rate into moles per square metre per day. For example, 400 µmol·m⁻²·s⁻¹ for 16 hours delivers 400 × 16 × 0.0036 = 23 mol·m⁻²·d⁻¹. Purdue Extension groups crops by the DLI they need: 3–6 mol·m⁻²·d⁻¹ are low-light crops, 6–12 medium-light, 12–18 high-light, and above 18 very-high-light. In practice, seedlings and shade foliage do well on 5–10, leafy greens such as lettuce and spinach on about 12–17, and fruiting vegetables like tomatoes, peppers and cucumbers want a full-sun 20–30. Outdoor summer sun delivers 30–60 mol·m⁻²·d⁻¹, but greenhouse glazing and grow tents cut that sharply, which is why supplemental lighting and photoperiod control matter. Because a longer photoperiod and a higher intensity trade off against each other for the same DLI, you can hit a target dose with a bright light for fewer hours or a dimmer one for longer.
Sources
- Purdue Extension HO-238-W — Measuring Daily Light Integral in a Greenhouse Torres & Lopez: DLI is instantaneous PPFD integrated over the day (PAR × 0.0864 for a full day); crop light bands are 3–6 (low), 6–12 (medium), 12–18 (high) and above 18 mol·m⁻²·d⁻¹ (very-high).
FAQ
What is a good DLI for my plants?
It depends on the crop. Seedlings and shade-tolerant foliage do well on about 5–10 mol·m⁻²·d⁻¹, leafy greens and herbs such as lettuce, spinach and basil on roughly 12–17, and fruiting vegetables like tomatoes, peppers and cucumbers on a full-sun 20–30. Below about 6 you get slow, leggy growth; well above 30 most edible crops stop responding and can bleach. Match the DLI to what you are growing rather than chasing the highest number.
What is the difference between PPFD and DLI?
PPFD is an instantaneous measurement — how much photosynthetic light lands on a square metre each second, in micromoles. DLI is the accumulated total over a whole day, in moles per square metre. PPFD is like the rate of rainfall at one moment; DLI is the total depth of water in the gauge by nightfall. A modest PPFD left on for many hours can deliver the same DLI as an intense PPFD for a few hours.
How do I increase DLI without raising intensity?
Extend the photoperiod. Because DLI is PPFD multiplied by hours, running the same light for longer raises the daily dose proportionally — going from 12 to 16 hours adds a third more light at the same intensity. This is often gentler on plants than cranking up intensity, though flowering crops that respond to day length need their photoperiod held for bloom timing rather than stretched for light.
Can DLI be too high?
Yes. Once a crop reaches its light saturation point, extra light no longer boosts photosynthesis and can cause tip-burn, leaf bleaching and heat or water stress, especially without enough CO₂, water and nutrients to use it. Leafy greens are prone to tip-burn above their comfortable 12–17 range. If a fixture pushes DLI past what a crop uses, shorten the photoperiod or dim the light rather than wasting energy.
What DLI does sunlight provide?
Outdoors, a clear summer day delivers roughly 30–60 mol·m⁻²·d⁻¹, dropping to 5–20 on overcast winter days. Inside a greenhouse, glazing, framing and shading typically cut that to well under 25, and a closed grow tent starts from zero and depends entirely on the fixture. That gap is exactly why indoor growers calculate DLI and add supplemental light to reach crop targets year-round.
Estimates only. Crop DLI targets vary by species, cultivar, temperature and CO₂, and PPFD is rarely uniform across a canopy. Use the result as a planning guide for home growing and greenhouse use, confirm intensity with a quantum sensor where accuracy matters, and adjust to how your plants respond — general growing guidance, not professional advice.