Measurement of development is often less precise than that of height and weight. Developmental scores are typically based on passing one or more developmental markers, but do not have interval scale so calculating differences between scores can be nonsensical. Age-specic standardized scores are sometimes used, but fail to have a common metric that allows comparison of developmental scores across age. The goal of this study is to develop a quantitative developmental score (D-score) with improved measurement characteristics. The basic assumption of the D-score is the existence of a common continuous scale for the development. Scores of 2151 children between 0 and 2 years on a Dutch developmental instrument were analysed. Application of the Rasch Model resulted in excellent reliability and satisfactory fit. This indicates that the new quantitative D-score succeeds in representing outcomes of the instrument on a common interval scale. Age-conditional reference values for the D-score were derived by means of the LMS method. The definition of the D-scores is not specific to age, so the D-score of a measured person can be compared to the D-score of another person of a different age. Difference scores between sessions can be used to evaluate developmental velocity on the individual level. To our knowledge this is the first developmental scale for children with such properties.