In this paper, we present two methods for calibration of acceleration sensors that are inexpensive, in-situ, require minimum user interaction and are targeted to a broad set of acceleration sensor applications and devices. We overcome the necessity of orthogonal axes alignment by extending existing calibration methods with a non-orthogonal axes model. Our non-orthogonal method can furthermore be used to enable automatic calibration for 1- or 2-axes accelerometers or realize a simultaneous mass-calibration of sensors with minimum effort. The influence of noise to the presented calibration methods is analysed.