What this calculator does
Daily light integral is the total number of photosynthetically useful photons landing on a square metre over a day. Instantaneous light readings in µmol/m²/s say nothing on their own, because a bright four-hour day and a dim sixteen-hour day can deliver the same total.
DLI is what plants respond to. Most crops have a target range: lettuce and many leafy greens do well around 12 to 17 mol/m²/day, fruiting crops such as tomatoes want 20 to 30, and low-light foliage plants manage on 4 to 6. Matching supply to that range is the practical reason for calculating it.
The formula
PPFD in micromoles per square metre per second is multiplied by 3,600 seconds to give micromoles per hour, multiplied by the photoperiod in hours, then divided by a million to convert micromoles to moles. The result is moles of photosynthetically active photons per square metre per day.
| Term | Meaning |
|---|---|
| PPFD | Photosynthetic photon flux density: the instantaneous rate of useful photons arriving, in µmol/m²/s. |
| DLI | Daily light integral: the total over a day, in mol/m²/day. |
| Photoperiod | Hours of light per day. |
| PAR | Photosynthetically active radiation, wavelengths from 400 to 700 nm, which is what PPFD counts. |
The inputs explained
| Field | What to enter |
|---|---|
| Photosynthetic photon flux density (PPFD) (µmol/m²/s) | Photosynthetic photon flux density in µmol/m²/s, measured at canopy height with a quantum sensor. |
| Photoperiod (hours/day) | Hours of light per day. For natural light this is effective daylight hours rather than sunrise to sunset, since early and late light contributes little. |
When to use it
Setting up grow lights
Fixture output and run time trade off against each other, and DLI is the figure that tells you whether a given combination reaches the target.
Deciding whether a location has enough light
Measuring PPFD at a windowsill or in a greenhouse and converting to DLI gives a direct comparison against crop requirements.
Reducing energy cost
The same DLI from lower intensity over longer hours often costs less to run, and the calculation shows which combinations are equivalent.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What DLI does each light level give?
A range of PPFD values over a fixed twelve-hour photoperiod.
| PPFD | Daily light integral | Photon flux over the day | Photoperiod used |
|---|---|---|---|
| 200 µmol/m²/s | 8.64 mol/m²/day | 8,640,000 µmol/m² | 12.0 h |
| 400 µmol/m²/s | 17.28 mol/m²/day | 17,280,000 µmol/m² | 12.0 h |
| 600 µmol/m²/s | 25.92 mol/m²/day | 25,920,000 µmol/m² | 12.0 h |
| 900 µmol/m²/s | 38.88 mol/m²/day | 38,880,000 µmol/m² | 12.0 h |
Questions
What DLI do plants need?
It varies by crop. Low-light foliage plants manage on 4 to 6 mol/m²/day, lettuce and herbs do well at 12 to 17, and fruiting crops such as tomatoes and peppers want 20 to 30. Below the range growth is slow and leggy; well above it, light stops being the limiting factor.
How do I measure PPFD?
With a quantum sensor or PAR meter held at canopy height. Phone light meter apps measure lux, which weights wavelengths by human eye sensitivity rather than by photosynthetic usefulness, and converting lux to PPFD requires a factor specific to the light source.
Can I trade intensity for hours?
Largely yes, within limits. The same DLI from lower intensity over longer hours produces broadly similar growth for many crops, and is often cheaper to run. The limits are that photoperiod itself controls flowering in day-length-sensitive species, and very low intensity over very long hours is not equivalent.
What DLI does natural sunlight provide?
Enormously variable: from around 5 to 10 mol/m²/day in winter at high latitudes to 50 or more on a clear midsummer day. Greenhouse supplementary lighting is generally sized to top up winter light to a target rather than to supply the whole requirement.
For the humidity side of growing conditions, see the vapor pressure deficit calculator. For heat accumulation over a season, see the growing degree days calculator.