DLI Calculator

What a DLI calculator measures

PPFD is a snapshot: the light landing on a square metre each second. The daily light integral (DLI) is the whole day’s total, in moles. Crop targets are given as that total, so DLI is the figure to compare.

Getting from one to the other is a unit change. There are 3,600 seconds in an hour and a million micromoles in a mole, so µmol to DLI means PPFD × hours × 0.0036. The same DLI can come from strong light over a short day or weaker light over a long one, which is why the calculator can solve for either.

Crops differ widely. Houseplants in low light want 3–6 mol/m²/day and bright-light ones 6–12; on a 12-hour day that is 69–139 and 139–278 µmol/m²/s, against 694–926 for flowering cannabis.

PPFD to DLI: a worked example

A canopy average of 600 µmol/m²/s for 18 hours gives 38.9 mol/m²/day, inside the 30–40 cannabis band for veg. The same light for 12 hours gives 25.9, below it. The bar under the result shows where the day falls: below, inside or above the crop’s band.

For flower, switch to “PPFD for a DLI”: a 12-hour day needs 694–926 µmol/m²/s for the same band. Or use “Hours for a DLI”: at 600 µmol/m²/s, 35 mol/m²/day takes 16.2 hours. At 18 hours, it needs 540 µmol/m²/s.

Cannabis DLI, autoflowers and CO₂

For cannabis (weed), growers are traditionally advised 30–40 mol/m²/day. One indoor LED study found yield still rising up to about 78 on 12-hour days, so the band is a start, not a ceiling. On a 12-hour day, 78 means about 1,806 µmol/m²/s, past the 1,500 where the calculator adds a CO₂ note.

Autoflowers flower by age and are commonly grown under 18–20 hours of light. For the same 30–40 band that is 463–617 µmol/m²/s at 18 hours and 417–556 at 20 hours.

For cannabis stages, the calculator also gives the CO₂ to make full use of the light: at least 800 ppm, or 1 ppm per µmol/m²/s. Plan the gas with the CO₂ calculator for grow rooms.

How to act on the result, and common mistakes

• Below target in veg: add hours or raise PPFD. At 600 µmol/m²/s, 30–40 mol/m²/day takes 13.9–18.5 hours.
• Below target in flower: keep the 12-hour day and raise PPFD by dimming up, lowering the light or adding a fixture.
• Above target: dim the light or, outside flower, shorten the day. In flower, a little above 40 is not always a fault: the calculator notes the study that measured yield still rising.
• Using the reading under the centre of the light. DLI needs the canopy average. Measure several points, or work it out from your fixtures with the LED grow light calculator.
• Too much light for young plants. Clones and seedlings want 4–12 mol/m²/day, only 62–185 µmol/m²/s over 18 hours.
• Changing the photoperiod: the “Same DLI over 12 h / 18 h” line shows the swap. 38.9 mol/m²/day is 900 µmol/m²/s over 12 hours or 600 over 18.

Lux to DLI without a PAR meter

A light meter reads lux, which is weighted to human eyes. The calculator converts it with a factor for each light: sunlight 0.0185, white LED 0.015, metal halide 0.0141, fluorescent 0.0135 and HPS 0.0122.

So 40,000 lux under a white LED is about 600 µmol/m²/s, or 38.9 mol/m²/day over 18 hours. The same reading under HPS is about 488 µmol/m²/s and 31.6 mol/m²/day. Treat these as estimates, good to about 20% at best. A meter in foot-candles works too: the calculator converts at 10.76 lux per foot-candle, and “Use This PPFD” copies the estimate into the main field.