While the slump test is the most commonly used method for evaluating the workability of concrete, tests can be imprecise and cause waste and delays. This article describes an instrumented probe that can be mounted inside a truck mixer to evaluate concrete workability for mixtures with slump values ranging from 30 to 280 mm (1.25 to 11 in.). It is shown that an empirically developed calibration relationship provides good correlation with slump data as well as slump flow data obtained using mixers and mixtures in numerous countries.