Using compliant materials leads to continuum robots undergoing large deformations. Their nonlinear behavior motivates the use of model-based controllers. They require state estimation as an essential step to be deployed. Available sensors are usually realized by introducing rigid bodies to the soft robot or inserting soft sensors made of materials different from the robot itself. Both approaches result in changes in the system’s dynamics. Optical measurements are problematic, especially in confined spaces. This can be avoided when the sensor is located at the robot's base. This paper studies the state estimation of a pneumatically actuated soft robot using the measured forces and torques at its base. For the first time, this is done using an unscented Kalman filter without restraining the dynamics to a planar or quasi-static motion while applying it to a real system. Real-time capability is achieved with our implementation. The state estimation is tested in a Cosserat rod simulation and on the physical system. The position is estimated with an accuracy of three to five millimeters for a 130 millimeter long pneumatic robot.