Skilled workers are crucial to the success of software development. The current practice in research and industry for assessing programming skills is mostly to use proxy variables of skill, such as education, experience, and multiple-choice knowledge tests. There is as yet no valid and efficient way to measure programming skill. The aim of this research is to develop a valid instrument that measures programming skill by inferring skill directly from the performance on programming tasks. Over two days, 65 professional developers from eight countries solved 19 Java programming tasks. Based on the developers' performance, the Rasch measurement model was used to construct the instrument. The instrument was found to have satisfactory (internal) psychometric properties and correlated with external variables in compliance with theoretical expectations. Such an instrument has many implications for practice, for example, in job recruitment and project allocation.