[en] The tightness and integrity of all Swedish reactor containments depend directly and indirectly on the function of the post-tensioned system. The tendon force in containments with unbonded tendons is followed up at regular in-service inspections (ISI) to ensure that the remaining force is sufficient. At the inspections, the tendon force is measured with so-called lift-off technique, where a jack is used to lift the end anchor. The interpretation of the measuring results is not obvious. One difficulty, which affects all tendons to different extents, is the influence of friction between the tendon and duct. This influence can cause a redistribution of force along the tendons after the original tensioning. The ordinary lift-off method only measures the force at the end of the tendon, which not always represents the change of force in the rest of the tendon. A method for measuring average force along a tendon is presented in this article. In this method, the elongation of the tendon is measured when the jack force is increased to a reference level with known force distribution. Measuring results from the latest ISI in Sweden show that the end force has decreased more than the average force, especially for long tendons with high influence of friction