This document specifies a laboratory method for the determination of chlorophyll a content in algae. The method was initially tested and evaluated on the microalgae species Nannochloropsis sp. and a heat treated algal product tomato soup with Nannochloropsis sp. supplement, and the macro algae species Ulva sp., Furcellaria lumbricalis, and Saccharina latissima. During an Interlaboratory Trial the method was tested on the microalgae species Nannochloropsis sp. and the macro algae species Saccharina latissima. The microalgae species Nannochloropsis sp. and Phaodactlylum sp. and the macro algae species Ulva sp. and Saccharina latissima were tested in a Round Robin test. This document is only validated for chlorophyll a, but it can be used for other chlorophylls as well.