The present disclosure provides a system for estimating tension in a post-tensioned rod, the system comprising an impact device for transversely-impacting a post-tensioned rod and being configured to determine a difference between a first and a second modal frequency corresponding to the vibrational response in the post-tensioned rod. Furthermore, the present disclosure provides a computing device and a non-transitory computer readable medium for determining a frequency difference between a first and a second modal frequency of a post-tensioned rod and a method for determining whether a frequency difference between a first and second modal frequency of a post-tensioned rod can be found.