Here is the formula for calculating DLI with a quantum meter.

 μmol m-2s-1 x (,3600 x photoperiod) / 1,000,000 = DLI

 -          μmol m-2s-1 is a reading, or averaged readings, from the quantum meter

-          3,600 is the number of seconds in an hour

-          Photoperiod is the period (in hours) of light exposure per 24 hours

-          1,000,000 is the number of μmols per mole

 For example, 38.1 μmol m-2s-1 x 3600 seconds/hour x 12 hour photoperiod / 1000000 μmoles/mol = 1.6 mol. m-2d-1 (or 1.6 moles/day).

The more scheduled readings a user takes with their quantum meter throughout the day, the more representative the data becomes.  You can substitute an averaged reading (per day) for the single reading in the above equation. 

The photoperiod in the equation serves to estimate values for missed measurements, and can be removed from the equation in a scenario where readings are consistently taken throughout the day (such as item 3501 Greenhouse Growth Tracker).

Formula taken from Jim Faust? Light Management I (of III) article found in Spectrums articles section (#A050).