Record volume measurements over a period of time. For example, you might record the liquid volume in a container each day for several weeks in order to compare weekly averages.
Add your measurements and divide by the number of measurements in the set. For example, consider the following information for Week One in the previous example: 4 mL, 5 mL, 2 mL, 1 mL, 6 mL, 4 mL and 3 mL. Add the measurements to get 25. Divide by 7 since there are seven measurements to get an average of 3.57 mL.
Compare your averages over time to look for the lowest measurement. This will be your minimum average volume. For example, consider the following averages for four weeks: 3.57 mL, 2.89 mL, 5.02 mL and 3.45 mL. Since 2.89 mL is the lowest average figure, this would be the minimum average volume for the month.