This document describes a spectrophotometric method for determining the chlorophyll-a content as a measure of the amount of phytoplankton for all types of water. Assuming a maximum sample volume of 2 l, chlorophyll-a content values of 5 ¼g/l or more can be determined. The determination limit can be calculated by each lab individually and it can be as low as 0,5 ¼g/l using 2 l of sample (or even more) and a 50 mm cuvette.