Total content is used mostly for balances. In order to calculate the areas of different depths, the contours are cut out of a topographical map and weighed. The formula is derived from the substance concentration (taken as a linear function of the depth), times the volume of a truncated cone. Content of one layer: [Mathematical expression] fu Square root of upper area f1 Square root of lower area cu Concentration at upper depth c1 Concentration at lower depth Δz Difference of depths (i.e. thickness of layer) Sum of contents of all layers gives total content of the lake.