Leg length difference after hip replacement is one of the most common patient concerns, and measuring it is straightforward once you know the landmarks. The standard clinical method uses a tape measure stretched from a bony point on your pelvis to the ankle bone, comparing both sides. Differences under 10 mm (about 3/8 of an inch) are generally well tolerated, though individual perception varies widely.
The Tape Measure Method
The most widely used clinical technique measures from the anterior superior iliac spine (ASIS) to the medial malleolus. The ASIS is the bony bump at the front of each hip, the point you can feel if you press your fingers into the front of your pelvis just below your belt line. The medial malleolus is the bony knob on the inside of your ankle. Measuring the distance between these two points on each leg gives you what clinicians call the “true” or “real” leg length, because it captures the actual skeletal length rather than differences caused by pelvic tilt or muscle tightness.
To get the most reliable result, lie flat on your back on a firm surface with both legs extended and parallel. Have someone else hold the end of a flexible tape measure firmly against the ASIS on one side, then stretch it down to the tip of the inner ankle bone. Record the measurement, then repeat on the other leg. Taking two measurements per side and averaging them improves accuracy. Research in Clinical Orthopaedics and Related Research found that this averaged two-measurement approach has acceptable validity and reliability as a screening tool.
A few tips that improve accuracy: make sure your pelvis is level and not rotated, keep both legs in the same position, and use the same spot on the ankle bone each time. Even small shifts in landmark placement can introduce a few millimeters of error, so consistency matters more than precision on any single attempt.
The Block Test
A simpler option that doesn’t require finding exact landmarks is the standing block test. Stand barefoot on a hard floor with equal weight on both legs. Have someone slide thin, measured blocks or stacked magazines under the foot of your shorter leg until your pelvis feels level and your stance feels even. The total height of the material you added approximates the discrepancy. This method is less precise than the tape measure technique, but it gives a practical, functional number because it captures how your body actually compensates while weight-bearing.
Why X-Rays Give a Different Answer
Your surgeon likely measured your leg length on X-rays before and after surgery. The gold standard for imaging is a full-length standing X-ray that captures everything from the hip center to the ankle center on both sides. In practice, most surgeons use a standard pelvic X-ray instead, measuring from a reference line drawn across the bottom of the pelvis (the “teardrop” line) to the center of the femoral head. This is faster and widely available, though slightly less precise than a full-leg film.
Imaging measurements and tape measurements don’t always agree. An X-ray captures pure bone length, while a tape measure is influenced by soft tissue, how relaxed your muscles are, and whether your pelvis is tilting. Both are useful, but they answer slightly different questions. If your tape measure screening suggests a meaningful difference, an X-ray can confirm the exact skeletal discrepancy.
How Much Difference Is Normal
Leg length discrepancy after hip replacement is classified by severity: mild is less than 10 mm, moderate is 10 to 20 mm, and severe is more than 20 mm. Most patients tolerate differences under 10 mm without significant problems. In fact, surgeons sometimes intentionally leave the operated leg very slightly longer (a few millimeters) because this adds stability to the new joint.
That said, there’s no universal cutoff for what counts as “acceptable.” Some people notice and are bothered by a 5 mm difference, while others adapt easily to 15 mm. The threshold depends on your biomechanics, your sensitivity to asymmetry, and your activity level. If a difference feels wrong to you, that perception matters regardless of what the number is.
When a Shoe Lift Helps
Most experts agree that a discrepancy over 20 mm warrants correction, typically with a shoe lift or built-up sole. For differences under 20 mm, opinions diverge. Some clinicians consider anything below 20 mm insignificant, while others have found that correcting differences as small as 10 mm, or even less, can relieve symptoms like low back pain, hip discomfort, or an uneven gait. If you’re symptomatic with a moderate discrepancy, a heel lift trial is low-risk and can tell you quickly whether correction makes a difference for you.
For discrepancies under about 10 mm, an off-the-shelf heel insert inside your shoe is usually enough. Larger corrections may require an external build-up on the sole of the shoe, which a pedorthist or orthotist can fabricate. It’s generally better to start with a partial correction (roughly half the measured discrepancy) and adjust upward, rather than correcting the full amount immediately. Your body has already started adapting to the new length, and an abrupt full correction can create its own discomfort.
Why Early Measurements Can Be Misleading
In the first weeks and months after surgery, your pelvis, spine, and surrounding muscles are still adjusting. Pelvic alignment is dynamic after hip replacement, and changes in pelvic tilt can persist up to a year. Muscle guarding, swelling, and altered gait patterns all influence how long your operated leg “feels,” sometimes making a well-balanced leg seem longer than it actually is. Many patients who perceive a significant difference at six weeks find the sensation diminishes by three to six months as soft tissues heal and their body recalibrates.
This doesn’t mean you should ignore an obvious difference early on. It does mean that a tape measure reading taken at four weeks may not reflect your final situation. If you’re tracking your own measurements, repeating them at regular intervals (say, six weeks, three months, and six months) gives a clearer picture of whether the discrepancy is real and stable or gradually resolving on its own.
What the Measurement Tells You
A home tape measurement is a screening tool, not a diagnosis. It can confirm that a difference exists and give you a rough number to discuss with your surgeon. It cannot tell you whether the discrepancy is coming from the hip joint itself, from pelvic tilt, or from changes further down the leg. If your screening measurement shows a difference of more than about 10 mm, or if you’re experiencing persistent limping, low back pain on the opposite side, or a feeling that your operated leg is noticeably longer or shorter, that’s worth bringing to your follow-up appointment with specific numbers in hand. Having your own measurements gives your surgeon a starting point and shows how the discrepancy translates into what you’re actually feeling day to day.