Daily Light Integral (DLI) Calculator

What is Daily Light Integral (DLI)?

The Daily Light Integral (DLI) describes the total number of photosynthetically active photons (PAR, 400-700nm wavelength) that are delivered to a specific area over a 24-hour period. It is expressed in moles of photons per square meter per day (mol/m²/day).

DLI is one of the most important environmental parameters in horticulture and plant science because it integrates both light intensity and duration into a single meaningful number. A plant receiving moderate light all day may receive the same DLI as one receiving intense light for a shorter period.

Think of it like this: PPFD is like the speed of water flowing from a faucet, while DLI is the total volume of water collected in a bucket over the entire day.

How to Calculate DLI

The DLI formula is straightforward:

Where:

• PPFD is in µmol/m²/s (micromoles of photons per square meter per second)
• Photoperiod is in hours
• 3600 converts hours to seconds
• 1,000,000 converts µmol to mol

Example Calculation

A greenhouse receiving 500 µmol/m²/s for 12 hours:

What is PPFD?

PPFD (Photosynthetic Photon Flux Density) measures the number of photosynthetically active photons that fall on a given surface area per second. It is measured in µmol/m²/s (micromoles per square meter per second).

PPFD is different from PPF (Photosynthetic Photon Flux), which measures the total number of photons emitted by a light source regardless of direction. PPFD is what actually reaches the plant surface.

Typical PPFD values:

• Full sunlight at noon: 1,500–2,200 µmol/m²/s
• Overcast day: 200–500 µmol/m²/s
• Shade: 50–200 µmol/m²/s
• Indoor grow light: 200–1,000 µmol/m²/s (varies by light)
• Office lighting: 5–20 µmol/m²/s

Understanding DLI Values

DLI Categories

• Very Low (1–5 mol/m²/day): Deep shade plants only — ferns, some orchids, pothos
• Low (5–10 mol/m²/day): Shade-tolerant plants — African violets, peace lily, lettuce seedlings
• Moderate (10–20 mol/m²/day): Many foliage plants, herbs, lettuce, spinach, microgreens
• High (20–30 mol/m²/day): Most flowering plants, tomatoes, peppers, cucumbers
• Very High (30–50+ mol/m²/day): Full sun crops — corn, cotton, sunflowers, cannabis

DLI Requirements for Common Plants

Effects of Low DLI

When plants don't receive enough light:

• Slow or stunted growth
• Thin, elongated stems (etiolation)
• Pale or yellow leaves
• Reduced flowering
• Poor fruit quality (in tomatoes: bland, watery)
• Weak root development
• Increased susceptibility to disease
• Reduced photosynthesis rate

DLI in Greenhouses vs. Outdoors

Outdoor DLI varies significantly by:

• Latitude: Equatorial regions receive 40–65 mol/m²/day; northern areas may get 5–25 mol/m²/day in winter
• Season: Summer DLI can be 3–4x winter DLI at temperate latitudes
• Cloud cover: Overcast days reduce DLI by 50–80%
• Day length: Longer days naturally increase DLI

Greenhouse DLI is typically 40–70% of outdoor DLI due to glazing transmission losses.

Supplemental Lighting

When natural DLI is insufficient, grow lights can supplement:

• Calculate the DLI deficit: Target DLI − Natural DLI = Supplemental DLI needed
• Determine supplemental PPFD: PPFD = Supplemental DLI / (Supplemental hours × 0.0036)
• Choose appropriate fixtures to deliver that PPFD at canopy level

PPFD vs Lux

Lux measures visible light as perceived by the human eye. PPFD measures photons useful for photosynthesis. The conversion factor depends on the light source:

• Sunlight: 1 µmol/m²/s ≈ 54 lux
• High-pressure sodium: 1 µmol/m²/s ≈ 71 lux
• LED (white): 1 µmol/m²/s ≈ 63 lux
• Fluorescent: 1 µmol/m²/s ≈ 74 lux

Frequently Asked Questions