The Achilles tendon is measured using ultrasound, MRI, or physical examination depending on what you need to know: thickness, length, cross-sectional area, or whether a tear is present. A healthy tendon typically measures about 4 to 5 mm thick on ultrasound, with anything above roughly 5 to 6 mm raising concern for disease. Here’s how each measurement works and what the numbers mean.
Where the Tendon Is Measured
The Achilles tendon runs from the calf muscles down to the heel bone (calcaneus). Most clinical measurements focus on a specific zone: about 5 to 7 cm (2 to 3 inches) above where the tendon attaches to the heel. This mid-portion is where injuries and degeneration most commonly occur, and it’s the spot where imaging gives the clearest, most reproducible readings.
For length measurements, the tendon is measured from the point where the inner calf muscle (medial gastrocnemius) transitions into tendon tissue all the way down to the bottom of the heel pad. This full-length measurement matters more in surgical planning and research than in everyday diagnosis.
Ultrasound: The Most Common Method
Ultrasound is the go-to tool for measuring the Achilles tendon in most clinical settings. It’s fast, relatively inexpensive, and doesn’t involve radiation. The examiner places a small probe on the back of your ankle and slides it along the tendon in two orientations: lengthwise (longitudinal) and across the tendon (transverse).
Thickness is measured on the longitudinal view, where the examiner captures the front-to-back diameter of the tendon at that 5 to 7 cm mark above the heel. Cross-sectional area is measured on the transverse view, either by tracing the tendon’s outline manually on screen or by using an ellipse tool built into the ultrasound machine. The machine calculates the area by counting the pixels enclosed within the traced boundary.
Ultrasound reliability for Achilles tendon assessment is strong. One advanced technique called ultrasound tissue characterization has interobserver reliability scores between 0.92 and 0.95, meaning two different examiners will get nearly identical results. Another technique that measures tendon stiffness (shear wave elastography) shows similarly high reliability, with scores above 0.91.
MRI for Detailed Imaging
MRI provides the most detailed picture of the Achilles tendon and is often used when ultrasound findings are inconclusive or when surgery is being planned. Cross-sectional area on MRI is measured by tracing the tendon’s outline using a region-of-interest tool in imaging software. Clinicians typically measure at three locations along the tendon and average the results to get a representative number.
MRI excels at showing the internal structure of the tendon, including partial tears, areas of degeneration, and fluid buildup that might not be visible on ultrasound. The tradeoff is cost, time, and availability. An MRI scan takes 30 to 45 minutes and requires scheduling, while an ultrasound can be done during a regular office visit in minutes.
Physical Examination and Palpation
Not every Achilles tendon measurement requires imaging. A basic physical exam can reveal important information, especially when a tear is suspected. If you can stand on the affected leg, a clinician will run their fingers along the length of the tendon feeling for a gap in the tissue. A palpable gap indicates a torn tendon. Complete tears typically occur 2 to 6 cm above where the tendon attaches to the heel bone.
Skinfold calipers have also been used to measure tendon thickness externally, placed at the point of maximum thickness on each side. However, this approach has only about 61% accuracy and is mainly used in screening for a genetic cholesterol condition (familial hypercholesterolemia), where thickened Achilles tendons are a telltale sign. Calipers are not reliable enough for diagnosing tendon injuries or tendinopathy.
Normal Tendon Dimensions
In healthy adults, the Achilles tendon measures roughly 4.1 to 4.5 mm thick in women and 4.5 mm in men on ultrasound. Cross-sectional area at rest averages about 46 mm² in women and 54 mm² in men. These differences are significant and consistent across studies, so any measurement needs to be interpreted with your sex in mind.
During muscle contraction, the cross-sectional area shifts slightly. In men it drops to about 54 mm² (nearly unchanged), while in women it decreases to about 43 mm². This difference in how the tendon behaves under load may partly explain why injury patterns differ between men and women.
When Measurements Signal a Problem
Tendon thickness is one of the simplest indicators of disease. Research on diagnostic cutoffs suggests that a thickness above 3.7 mm at the insertion point in women, or above 4.8 mm in men, may indicate enthesopathy (inflammation where the tendon meets the bone). Other studies have used higher thresholds: 5.29 mm for both sexes, or 5.5 mm for women and 6.2 mm for men, depending on what condition is being screened for and how sensitive the test needs to be.
These numbers illustrate an important point: there’s no single universal cutoff. The threshold your clinician uses depends on what they’re looking for, whether it’s tendinopathy, an inflammatory condition, or cholesterol-related thickening.
Measuring Tendon Stiffness
Thickness alone doesn’t tell the whole story. A tendon can be normal in size but abnormally stiff or soft. Shear wave elastography, an advanced ultrasound technique, measures how fast a tiny vibration travels through the tendon tissue. Faster waves mean stiffer tissue. The result is expressed in kilopascals (kPa), a unit of pressure.
Healthy nonathletes typically have Achilles tendon stiffness around 104 to 213 kPa, while athletes tend to have stiffer tendons, in the range of 184 to 230 kPa. A tendon affected by tendinopathy often shows reduced stiffness in the damaged area, which can help pinpoint the location and severity of the problem even before the tendon visibly thickens.
Measuring Symptom Severity
Physical measurements of the tendon are only part of the picture. The VISA-A questionnaire (Victorian Institute of Sport Assessment, Achilles) is a widely used scoring tool that measures how much an Achilles tendon problem affects your daily life and activity. It scores from 0 to 100, where healthy individuals typically score 96 to 100.
Patients with Achilles tendinopathy usually score below 60, and scores rarely drop below 24 even in severe cases. A score of 90 or above is generally considered full recovery. The questionnaire covers pain during specific activities like walking, stair climbing, and hopping, giving a functional picture that imaging alone can’t provide. Many clinicians use it alongside physical measurements to track whether treatment is working over time.