Aim: Bayley Scales of Infant and Toddler Development (Bayley-III) determines scaled scores and converts these into composite scores. It was shown that applying of the German and the US manual leads to different results. This study aims to systematically analyze the differences between the US and German Bayley-III version and to develop conversion equations. Method: This simulation study generated a dataset of pairs of US and German Bayley III composite scores (cognitive: n=4,416, language: n=240,000, motor: n=314,000) by converting the same number of achievable tasks for 48 age groups. Bland-Altman-plots and regression analysis was performed to develop conversion equations for all age groups. Results: German and US Bayley-III scores demonstrate distinct slope and interception for cognitive, language and motor composite scores. Lower developmental performance lead to higher composite scores with US norms compared to German norms (up to 15 points). These differences varied between age groups. With newly developed conversion equations the results can be converted (R²>0.98). Interpretation: This study confirms systematic differences between US and German Bayley test results due to different reference cohorts. Our data considers the full age range and adds conversion equations. These findings need to be acknowledged when comparing Bayley Scores internationally.