Stiffness (modulus of elasticity, MOE) of conifer trees is the most important trait for structural wood products. Finding a fast, reliable and non-destructive way to measure MOE is a priority for screening large progeny trials in tree breeding programmes. For Scots pine, time-of-flight (TOF) velocity measured on standing trees accounted for 47% of the variation to the benchmark SilviScan-based clearwood MOE (MOEs), under the assumption of constant wood density. If wood density was included, 59% of the variation was accounted for. The TOF stiffness measurements on standing trees were, however, more related to the clearwood MOEs in the outerwood, and the prediction was the most reliable at breast height compared to the stem base and the top section. Microfibril angle (MFA) had higher correlation with acoustic velocity (VEL) of standing trees than wood density, and among the early, transition and latewood density, the latewood density had the highest correlation with stiffness measurements on standing trees. VEL measured at breast height in combination with wood density was the most reliable predictor of MOE of standing trees for selection and breeding in Scots pine.