How to use the daily light integral calculator
- Choose DLI from light level, Light level for a DLI or Hours for a DLI.
- Enter the light reading and choose PPFD, lux or foot-candles. For lux or foot-candles, set the lux per PPFD factor for your light source.
- Enter the hours of light per day or the target DLI, depending on what you are finding.
- Read the DLI in mol per square meter per day, the light level and the hours.
Formula
DLI (mol/m2/day) = PPFD (umol/m2/s) x hours of light x 3600 / 1,000,000
PPFD needed = DLI x 1,000,000 / (hours x 3600)
Hours needed = DLI x 1,000,000 / (PPFD x 3600)
PPFD from lux = lux / lux per PPFD unit; 1 foot-candle = 10.7639 lux
PPFD counts the photons that plants use, in micromoles per square meter per second. The DLI adds them up over the lit hours of a day.
A grow light and a lux reading
A steady PPFD of 600 umol/m2/s for 16 hours gives 600 x 16 x 3600 / 1,000,000 = 34.56 mol/m2/day. To reach a DLI of 40 in 16 hours the light level must be 694.44 umol/m2/s, and to reach 30 at 400 umol/m2/s the lights must run 20.833 hours.
A reading of 20,000 lux converted with 54 lux per PPFD unit is 370.37 umol/m2/s, which over 8 hours is a DLI of 10.667. A reading of 2,000 foot-candles with the same factor is 398.66 umol/m2/s.
Using a lux meter
A quantum sensor reads PPFD directly. A lux meter is cheaper but is weighted to what the human eye sees, so the lux per PPFD factor depends on the light. Daylight, white LEDs and colored grow lights all differ, which is why the factor is an input that starts at an example value. Use the figure published for your fixtures when you have one.
Other tools for growing spaces include the VPD calculator, the EC to ppm calculator, the lighting lumens calculator and the growing degree days calculator.
Averaging through the day
The formula assumes a steady light level. Sunlight rises and falls, so for daylight take readings across the day and use their average, or use a logging sensor. Clouds, seasons and shade change the total from one day to the next.
Assumptions and limits
- The light level is steady, or an average, over the lit hours, and the hours run from above 0 to 24.
- The lux per PPFD factor is an assumption you set; 54 is only a rough example for daylight.
- A foot-candle is converted to lux with 10.7639, and no other correction is applied.
- The page does not say what DLI any plant needs.
Frequently asked questions
How do I calculate DLI?
Multiply PPFD in umol/m2/s by the hours of light and by 3600, then divide by a million. 600 for 16 hours gives 34.56 mol per square meter per day.
What does DLI mean?
It is the total amount of plant-usable light that reaches a square meter in a day, counted in moles of photons.
Can I convert lux to PPFD?
Only roughly, because the conversion depends on the color of the light. Divide lux by a factor for your source; the page starts with 54 as an example and lets you change it.
How many hours of light give a target DLI?
Divide the target by the PPFD times 3600 over a million. If the answer is more than 24 hours, the light level is too low to reach the target in a day.