Background: Elongation of the Achilles tendon after rupture is a frequent complication. The Achilles Tendon Length Measure (ATLM) and the Achilles Tendon Resting Angle (ATRA) are indirect length measures using the resting angle of the ankle. Copenhagen Achilles Length Measure (CALM) is a direct ultrasound measure. The purpose of this study was to examine the construct validity of ATLM and ATRA using CALM as gold standard. Methods: As the three measurements measure slightly different constructs and have different scales a direct comparison was not possible. Instead a mixed linear regression model was performed investigating the three models: CALM-ATRA, CALMATLM and ATRA-ATLM. Results: 130 patients were available for analysis. All three regression models demonstrated a statistically significant (p < 0.01) linear relationship and acceptable certainty of the estimates. Conclusion: ATRA and ATLM were found to have acceptable construct validity when using CALM as gold standard for assessing tendon elongation after an Achilles tendon rupture.