Measurement estimation is seen as an important part of mathematics learning, although still very little is known about children’s abilities in this respect. To make matters worse, criteria for the adequacy of estimates are arbitrarily chosen and differ in studies on this topic. If teachers have to evaluate students’ estimation performances, they need criteria, too. In this paper, we first present some of those studies and their criteria for adequacy. These criteria are evaluated and prepared for discussion by applying them to data from an interview study with 4th grade students estimating length and capacity. This empirical basis for discussion is complemented by data of expert opinions about the items and children’s results.