What Is Daily Light Integral?
Daily Light Integral (DLI) measures the total amount of photosynthetically active radiation (PAR) that lands on a square metre of growing area over one full day. It is expressed in moles of photons per square metre per day (mol/m2/day). Unlike an instantaneous light reading, DLI accounts for both how bright the light is and how long it shines, which is what actually drives plant growth, since photosynthesis responds to the total light received over the day rather than the peak intensity at any one moment.
The DLI Formula
DLI is calculated from PPFD (photosynthetic photon flux density, in micromol/m2/s) and the photoperiod (hours of light per day):
DLI (mol/m2/day) = PPFD (umol/m2/s) x photoperiod (hours) x 3600 / 1,000,000
The 3,600 converts hours into seconds (since PPFD is a per-second measurement), and dividing by 1,000,000 converts micromoles into moles. For example, a PPFD of 400 umol/m2/s over a 12 hour photoperiod gives a DLI of 400 x 12 x 3600 / 1,000,000 = 17.28 mol/m2/day.
Typical DLI Targets by Crop
| Crop or Plant Type | Target DLI (mol/m2/day) |
|---|
| Low-light houseplants, leafy greens (lettuce, spinach) | 10 - 15 |
| Herbs and seedlings | 12 - 20 |
| Fruiting vegetables (tomatoes, cucumbers) | 20 - 30 |
| Peppers and high-light fruiting crops | 25 - 35 |
| High-light flowering crops | 35 - 45 |
| Outdoor full sun, NZ summer (approx) | 40 - 55 |
| Outdoor overcast, NZ winter (approx) | 5 - 10 |
Why DLI Matters for Growers
Two setups can have very different PPFD readings but deliver the same DLI. A grow light at 800 umol/m2/s for 6 hours gives the same DLI as 400 umol/m2/s for 12 hours (17.28 mol/m2/day either way). This means growers can trade off light intensity against photoperiod to hit a target DLI, which is useful when a fixture cannot be dimmed, when electricity costs matter, or when heat build-up from strong lights is a concern. Measuring DLI rather than just PPFD is the more reliable way to diagnose whether a crop is light-limited, especially in a New Zealand greenhouse over autumn and winter when both light intensity and day length drop.
How to Measure PPFD
PPFD is measured with a quantum PAR meter placed at canopy height, pointed straight up (or at the leaf surface angle for a more precise reading). Many LED grow light manufacturers also publish a PPFD map for their fixture at various mounting heights, which can be used instead of a meter if you know your light's height above the canopy. Sunlight PPFD varies through the day and with cloud cover, so outdoor DLI is usually estimated from long-term solar radiation data rather than a single reading.
Related Calculators
Sources: Standard DLI formula (PPFD x photoperiod x 3600 / 1,000,000) as used in horticultural lighting science. Crop DLI ranges are widely published guidance figures used by commercial greenhouse growers and grow light manufacturers.
This calculator provides an indicative DLI based on the PPFD and photoperiod you enter. Actual DLI from natural sunlight varies with cloud cover, season, latitude, and time of day, and PPFD from grow lights varies with fixture height, spread, and dimming. Use a PAR meter for the most accurate on-site reading.
