Ankle inversion and eversion are measured using a goniometer aligned to specific bony landmarks, with the person positioned either sitting or lying prone. The technique centers on finding the midpoint between the two ankle bones (malleoli), then tracking how far the foot tilts inward (inversion) or outward (eversion) relative to the lower leg. Normal inversion averages around 30 to 35 degrees, while eversion is smaller, typically 15 to 20 degrees.
Where Inversion and Eversion Happen
These movements occur primarily at the subtalar joint, where the talus (the bone that sits on top of the heel) meets the calcaneus (the heel bone itself). The two bones share curved, interlocking surfaces that allow the foot to rock side to side. Although the larger ankle joint above it handles most of the up-and-down motion of the foot, the subtalar joint provides the bulk of inversion and eversion. A small contribution also comes from the joints in the midfoot, but when you measure these motions clinically, you’re capturing what the subtalar joint produces.
During normal walking, the subtalar joint moves through roughly 15 degrees of combined inversion and eversion. Inversion turns the sole of the foot inward (toward the other foot), while eversion turns it outward. These aren’t pure side-to-side tilts. They happen across multiple planes simultaneously, which is part of what makes precise measurement tricky.
What You Need Before Measuring
A standard universal goniometer is the most common tool. It consists of two arms connected at a pivot point. You’ll also need a skin-safe marker or pencil to mark landmarks, since small errors in placement can shift your reading by several degrees. Have the person remove their shoes and socks, and make sure you can clearly see and feel the bony prominences on both sides of the ankle.
Goniometer Setup: Sitting Position
The sitting method is the more common approach for inversion and eversion. Have the person sit with their knee bent to about 90 degrees, the lower leg hanging freely or supported so the foot is off the ground. This relaxes the calf muscles and isolates subtalar motion.
Three landmarks guide your goniometer placement:
- Fulcrum (pivot point): the midpoint between the two malleoli on the front of the ankle. Feel for the bony bump on the inside (medial malleolus) and outside (lateral malleolus), then place the goniometer’s center pin halfway between them.
- Stationary arm: aligned with the midline of the front of the lower leg, using the shin bone (tibial crest) as your reference. The arm should point straight up along this line.
- Moving arm: aligned with the midline of the top of the foot, running along the second metatarsal (the long bone leading to the second toe).
With the goniometer in place and the foot in a neutral starting position, ask the person to actively tilt the sole of their foot inward as far as they can. Read the angle. Return to neutral, then ask them to tilt the sole outward for eversion. Record both values.
Goniometer Setup: Prone Position
The prone (face-down) method is preferred by some clinicians because it gives a clearer view of the heel and reduces compensatory movements from the hip or knee. The person lies on their stomach with the foot hanging off the end of the table.
The landmarks shift to the back of the ankle:
- Fulcrum: midpoint between the two malleoli on the back (posterior) aspect of the ankle.
- Stationary arm: aligned with the midline of the back of the lower leg.
- Moving arm: aligned with the midline of the back of the heel (calcaneus).
This method tracks heel position rather than forefoot position, which can give you a more direct reading of what the subtalar joint is doing. The trade-off is that the calcaneus is a rounded bone, and centering the moving arm consistently on its midline takes practice.
How Ankle Position Affects Your Reading
One important detail: whether the ankle is bent upward (dorsiflexed) or relaxed changes the measurement significantly. Research on young males found that placing the ankle in dorsiflexion reduced mean inversion from 32 degrees to 22 degrees, while eversion stayed about the same. This happens because dorsiflexion locks the talus more tightly into the ankle joint, restricting how much the subtalar joint can move. For consistent, comparable results, you should keep the ankle in the same position every time you measure, and note that position in your records.
The neutral position of the subtalar joint itself isn’t perfectly straight. On average, it sits at about 3 degrees of varus (slight inward tilt) from the midline of the shin. This means “neutral” is not the same as “foot pointing straight ahead,” which is a common source of error for beginners.
Normal Ranges to Expect
Inversion consistently has a larger range than eversion. In a relaxed, non-dorsiflexed ankle, you can expect roughly 30 to 35 degrees of inversion and 15 to 20 degrees of eversion. These numbers vary with age, activity level, and individual anatomy, so the most clinically useful comparison is usually side to side: measuring the injured ankle against the uninjured one. A difference of more than 5 degrees between sides is generally considered meaningful.
Reliability: What Affects Accuracy
Manual goniometry of the ankle has well-documented limitations. Intrarater reliability (the same person measuring repeatedly) is moderate, meaning one clinician can get reasonably consistent numbers across sessions. Interrater reliability (different people measuring the same ankle) is poor. Two different clinicians measuring the same joint can easily get readings that differ by 5 degrees or more.
The biggest sources of error are inconsistent landmark identification, variation in how firmly the foot is moved, and differences in where the examiner decides “neutral” begins. To improve your accuracy, mark your landmarks with a pen before placing the goniometer, use the same patient position each time, and if possible, have the same person perform all follow-up measurements on a given patient.
Weight-bearing measurements tend to be more reliable than non-weight-bearing ones. Studies on ankle dorsiflexion found that weight-bearing methods (like a lunge toward a wall) produced reliability coefficients of 0.93 to 0.96, compared to 0.32 to 0.72 for non-weight-bearing positions. While weight-bearing tests are more commonly applied to dorsiflexion than to inversion and eversion specifically, the principle holds: functional positions reduce measurement variability.
Smartphone Apps and Digital Tools
Several smartphone apps can substitute for a traditional goniometer with reasonable accuracy. Apps like DrGoniometer (which uses photos), Goniometer, and Clinometer have all been tested against universal goniometers and show strong correlation, with agreement coefficients above 0.90. Photo-based apps showed slightly higher agreement than accelerometer-based ones, though the difference was not statistically significant.
Digital inclinometer apps have also been validated for ankle measurements. One study found moderate to good reliability for ankle joint position sense when enough trials were averaged, specifically the middle eight of ten trials. This is more trials than the hip or knee required (three of five), reflecting the ankle’s smaller range and greater measurement difficulty. If you’re using a phone app, taking multiple readings and averaging them will give you a more trustworthy number than relying on a single measurement.
Why These Measurements Matter
People with recurrent ankle sprains show measurably impaired ability to detect inversion and eversion movements. In one study, participants with a history of repeated sprains needed significantly more joint movement before they could even perceive that their ankle was being tilted, compared to people with no sprain history. This deficit was present across slow, moderate, and fast movement speeds, suggesting the problem is a fundamental loss of joint position awareness rather than a simple reaction-time issue.
Tracking inversion and eversion range over time helps clinicians gauge recovery from sprains, monitor post-surgical progress, and identify people at risk for re-injury. A loss of eversion range, in particular, can signal stiffness in the subtalar joint that changes how forces travel up through the knee and hip during walking. Consistent measurement technique is what makes these comparisons meaningful across weeks or months of rehabilitation.