In everyday life the estimation of quantities is as important as the accurate measurement. If children shall be able to estimate capacity they have to understand conversion, transitivity and unit iteration as well as to build up mental representatives, which they can use to compare and measure mentally. A case study with 35 third graders of two different primary schools in Germany is presented. All children were interviewed and took part in a paper and pencil test. The results were compared and discussed especially concerning the question if the test alone will help to identify students with special problems in dealing with capacity.