In the ISO16840-1 standard, “representation” of seated posture was defined, but the procedure of “how to measure” seated posture was not. The purpose of this paper is to report our work in the development of software to measure seated posture according to the ISO16840-1 standard. We evaluated the reliability of the software by using a metal model representing human seated posture and by calculating the standard deviation of within- and between-examiner measurements. We also evaluated validity by comparing our results with a contact three-dimensional measurement device. As a result, standard deviations are almost less than 2.2 degrees. Differences between the software and the contact device were less than 4.8 degrees. In conclusion, the software is practical and suited for daily clinical use.