The present disclosure provides systems, methods, and apparatuses for estimating tension in post-tensioned rods, such as anchor rods that support structural steel to concrete. A method of estimating tension in a target post-tension rod may include selecting a plurality of post-tensioned rods and modeling tension for the plurality of post-tensioned rods as a function of frequency differences between a first and a second modal frequency and cantilever length of each rod. The method may further include measuring the length of a cantilever of the target post-tensioned rod, determining first and second modal frequencies of the target post-tensioned rod, and determining a frequency difference between the first and second modal frequencies. The method may further include estimating tension in the target post-tensioned rod based on at least the model.