The ruler is the standard measuring instrument used for measuring lengths. However, measuring lengths with a ruler is a challenge for children. For this reason, this study explores how children who have not received specific school instruction on its use, measure lengths with a conventional ruler. The relative object-ruler position, the strategy to justify the measurement, and the combination between them are analysed. Additionally, the possible influence of the grade and the transition from Kindergarten to Primary School is also studied. To achieve this aim, 99 children were asked to measure a cardboard strip in both a free and a directed situation. The results showed that in free measurements children tend to situate the object in the 2 hash mark of the ruler, the reading of the endpoint was identified as the most used strategy, and the combination of this strategy with lining up the object at 0 was the most commonly used in correct measurements. On the other hand, the results also showed marginal significant differences between age groups in such a way that children in the last year of kindergarten measured better than those in the first year of primary school. To conclude, the educational implications of these results are discussed.