What is DLI?
The light a plant gets in a day, counted in photons: PPFD is how many photosynthetic photons land on a square meter each second, and the daily light integral multiplies that by the seconds the light is on. It's the rain gauge to PPFD's rainfall rate, and it's the number crop research is written in — because a plant doesn't care whether it got 400 µmol for 18 hours or 600 for 12; it got 26 moles either way (mostly — see photoperiod below).
500 µmol/m²/s for 18 hours: 500 × 18 × 0.0036 = 32.4 mol. The 0.0036 is the seconds in an hour over the micromoles in a mole. Purdue's greenhouse version uses 0.0864 — that's the same constant for a full 24-hour day of averaged readings.
DLI by PPFD and photoperiod
| PPFD | 12 h | 16 h | 18 h | 20 h | 24 h |
|---|---|---|---|---|---|
| 200 µmol | 8.6 | 11.5 | 13.0 | 14.4 | 17.3 |
| 300 µmol | 13.0 | 17.3 | 19.4 | 21.6 | 25.9 |
| 400 µmol | 17.3 | 23.0 | 25.9 | 28.8 | 34.6 |
| 600 µmol | 25.9 | 34.6 | 38.9 | 43.2 | 51.8 |
| 800 µmol | 34.6 | 46.1 | 51.8 | 57.6 | 69.1 |
| 1,000 µmol | 43.2 | 57.6 | 64.8 | 72.0 | 86.4 |
| 1,200 µmol | 51.8 | 69.1 | 77.8 | 86.4 | 103.7 |
mol/m²/day. Shaded cells are past 50 mol — where a crop without CO₂ enrichment stops turning extra photons into growth. PPFD is the average over the canopy, not the number under the center of the light.
How much light do plants need?
Purdue's classes: low-light crops 3–6 mol, medium 6–12, high 12–18, very high above 18 — with the note that most floriculture crops finish best at 10–12 or more, and that a greenhouse rarely sees above 25 even in summer. The targets the calculator checks against, and the PPFD that hits the middle of each at common photoperiods:
| Crop / stage | Target | PPFD for the middle |
|---|---|---|
| Seedlings & cuttings (propagation) | 4–11 | 174 @ 12 h · 130 @ 16 h · 116 @ 18 h |
| Low-light houseplants (ferns, pothos, peace lily) | 3–6 | 104 @ 12 h · 78 @ 16 h · 69 @ 18 h |
| Most bedding plants & herbs in pots | 6–12 | 208 @ 12 h · 156 @ 16 h · 139 @ 18 h |
| Lettuce & leafy greens | 12–17 | 336 @ 12 h · 252 @ 16 h · 224 @ 18 h |
| Sun bedding plants, basil, most herbs | 12–18 | 347 @ 12 h · 260 @ 16 h · 231 @ 18 h |
| Tomato, pepper, cucumber | 18–30 | 556 @ 12 h · 417 @ 16 h · 370 @ 18 h |
| Cannabis — seedlings & clones | 10–20 | 347 @ 12 h · 260 @ 16 h · 231 @ 18 h |
| Cannabis — vegetative | 25–40 | 502 @ 18 h |
| Cannabis — flowering | 35–50 | 984 @ 12 h |
mol/m²/day and µmol/m²/s. Vegetable and floriculture rows from Purdue HO-238-W (Faust's crop bars) and Cornell CEA; the cannabis rows are grower convention and are labeled that way in the tool. Photoperiod-sensitive stages have their hours fixed — only intensity can move them.
1,000 µmol over a 12-hour flower day is 43.2 mol — the middle of the 35–50 band, which at 12 hours runs 810–1,157 µmol. That's where the number comes from, and there it's right. Run the same light for 18 hours in veg and it's 64.8 mol, past the 50 where a crop without CO₂ stops responding; over a tomato's 16-hour day it's 57.6, nearly double the top of its 18–30 (which wants 313–521 µmol). The commercial rooms that push 1,200–1,500 µmol run 1,200–1,500 ppm CO₂, where the ceiling moves to 60-plus. In a tent breathing room air, the DLI number is how you know when to stop buying light — and when to dim the one you have.
Estimating PPFD from the watts on the box
If you have no meter, the fixture's efficacy — micromoles of photons per joule, on every honest spec sheet — turns wall watts into photons per second, and dividing by the canopy area (with some lost to the walls) gives a rough average PPFD. It's an estimate, and the calculator says so; a $30 lux meter and the conversion below beats it. What it's very good at is exposing the "600 W" panel that draws 100:
| Tent | 100 W | 200 W | 300 W | 450 W | 600 W |
|---|---|---|---|---|---|
| 2 × 2 ft | 526 µmol 34 / 23 mol | 1,052 µmol 68 / 45 mol | 1,578 µmol 102 / 68 mol | 2,367 µmol 153 / 102 mol | 3,157 µmol 205 / 136 mol |
| 2 × 4 ft | 263 µmol 17 / 11 mol | 526 µmol 34 / 23 mol | 789 µmol 51 / 34 mol | 1,184 µmol 77 / 51 mol | 1,578 µmol 102 / 68 mol |
| 3 × 3 ft | 234 µmol 15 / 10 mol | 468 µmol 30 / 20 mol | 701 µmol 45 / 30 mol | 1,052 µmol 68 / 45 mol | 1,403 µmol 91 / 61 mol |
| 4 × 4 ft | 132 µmol 9 / 6 mol | 263 µmol 17 / 11 mol | 395 µmol 26 / 17 mol | 592 µmol 38 / 26 mol | 789 µmol 51 / 34 mol |
| 5 × 5 ft | 84 µmol 5 / 4 mol | 168 µmol 11 / 7 mol | 253 µmol 16 / 11 mol | 379 µmol 25 / 16 mol | 505 µmol 33 / 22 mol |
Average canopy PPFD from wall watts at 2.3 µmol/J (a quality LED board), 85% of photons reaching the canopy in a reflective tent; below it, DLI at 18 h / 12 h. Real fixtures are brighter in the middle and dimmer at the edges — the average is what feeds the crop table.
| Fixture type | µmol/J | 300 W over 4 × 4 ft | DLI at 18 h | mol per $ at 15¢/kWh |
|---|---|---|---|---|
| Fluorescent / CFL | 1 | 172 µmol | 11.1 | 24 |
| Budget LED (blurple, older boards) | 1.5 | 257 µmol | 16.7 | 36 |
| HPS, 1000 W double-ended | 1.7 | 292 µmol | 18.9 | 41 |
| Quality LED (Samsung LM301-class boards) | 2.3 | 395 µmol | 25.6 | 55 |
| Top-bin commercial bar light | 2.8 | 480 µmol | 31.1 | 67 |
Efficacies are typical for the class (DLC horticultural listings and manufacturer specs); use the number on your spec sheet. Moles per dollar = 3.6 × µmol/J ÷ $/kWh — the fair comparison between fixtures, since a top-bin light delivers 1.9× the photons per kilowatt-hour of a budget panel.
With a lux meter or a phone app, divide by the source factor: about 65 lux per µmol for white LEDs, 82 for HPS, 54 for sunlight (the last two are Purdue's foot-candle factors converted). 30,000 lux under a white LED is roughly 462 µmol. Phone sensors are ±20% and blurple lights break the conversion entirely — the meter can't see the red properly.
What the light costs to run
DLI has a price. A 300 W light for 18 hours is 5.4 kWh a day — $24.30 a month at 15¢ — for the 26 mol a day it puts on a 4 × 4 canopy. A 1,000 W HPS on a 12-hour flower schedule is 12 kWh and $54 a month before the air conditioning it needs. The calculator prices your fixture at your rate; the moles-per-dollar column above is why efficacy, not watts, is the number to buy on.
Hours or intensity?
For crops that don't care about day length — greens, herbs, most vegetables in veg — the same DLI over a longer photoperiod at lower PPFD is slightly more productive, not less: lettuce trials at constant DLI grew more under 20–24 hours of gentler light than under 12 hours of harsh light, and the fixture runs cooler and cheaper. For photoperiod-sensitive crops the hours are fixed by the stage — cannabis flowers at 12, vegetates at 18 — so intensity is the only lever, which is why the crop table pins their hours. Either way, past the target the extra photons buy heat and a water bill.
Too little, too much
Under half the target: stretch — long internodes, thin stems, leaves reaching — and slow everything. Lower the light, not raise the hours, if the crop is photoperiod-sensitive; otherwise either. Over the top of the band: leaves cup or "taco", tips bleach white under LEDs, the plant drinks and feeds faster than it can use and shows what looks like a deficiency. The fix is distance or a dimmer, and the lesson is that the DLI number already told you.
Sources: Torres & Lopez, Purdue Extension HO-238-W, Measuring Daily Light Integral in a Greenhouse (formula, categories, propagation range, foot-candle factors, Faust's crop DLI table); Both et al., Cornell CEA, hydroponic lettuce DLI target; Faust & Logan, Daily Light Integral: A Research Review, HortScience 53(9); Weaver & van Iersel / Palmer & van Iersel on photoperiod vs intensity at constant DLI in lettuce; DesignLights Consortium horticultural QPL (efficacies); Apogee Instruments lux-to-PPFD conversions; grower convention for cannabis stage bands, stated as such.