Bathing in a bathtub filled with water is a custom common in Japan. Since comfort is important in bathing, comfortable bathtub design is desired. In the previous study, the algorithm to estimate reaction forces from bathtub was constructed. However, in order to evaluate the comfort of the bathtub in simulation completely, it was necessary to know bathing posture without experiments. The objective of this study was to construct the algorithm to estimate bathing posture. This algorithm determines the bathing posture that minimizes weighted summation of i) sum of joint torques of hip, knee, and ankle, ii) passive elastic joint torques of toes, and iii) inverse sign of the buoyancy. In the calculation, gravity, buoyancy, passive elastic joint torque and estimated reaction forces by algorithm constructed in previous study were considered. In order to examine this algorithm, a bathing experiment to obtain the data of bathing posture for 10 subjects and 4 bathtub shapes for each subject was conducted. It was found that the estimated bathing posture were consistent well with the measured bathing posture within 5.2% error.