How Is a Leg Length Difference Actually Measured?
A leg length discrepancy can be measured in the clinic with a tape measure or a block test. It can also be measured with imaging, using a scanogram or a standing full-length X-ray. Clinic methods give a rough guide. Imaging, especially weight-bearing imaging, gives the precise detail most clinicians need before deciding on any treatment.
Key takeaways
- Tape measure tests are quick, but results can vary noticeably from one examiner to another.
- A scanogram (a long-leg X-ray taken lying down) is the traditional imaging standard.
- Standing full-length imaging captures the difference under real body weight. It shows how the whole leg chain actually takes load.
- The method matters, because a small measurement error can change the treatment plan.
- EOS technology uses about 90% less radiation than a standard X-ray, around ten times less.
If you have been told one leg is shorter than the other, your first question is probably, "By how much?" The answer depends entirely on how it is measured. Some methods are simple enough to try at home. Others need specialist imaging. Each has strengths and real limits.
What methods are used to measure leg length?
There are four main methods. Two are clinical (done by hand in a clinic room) and two are radiological (using imaging). Each one trades convenience for precision.
Clinical. Measures the distance from a bony point on the hip (the ASIS) to the ankle bone on each side.
Clinical. You stand with blocks of known height under the shorter leg until a clinician judges that your pelvis is level.
Radiological. A long X-ray or CT of both legs taken while you lie flat, with calibration markers.
Radiological. Captures both legs from hip to ankle while you stand and carry your weight normally.
How accurate is a tape measure for leg length?
A tape measure gives a useful first sign, but its results are hard to repeat exactly. The examiner places one end of the tape on the front of the hip bone (a bony point called the ASIS). They stretch it down to the inner ankle bone (the medial malleolus). The difference between the two sides is noted.
The problem is that finding the exact bony point through skin and muscle is harder than it sounds. Research shows that two clinicians measuring the same person can easily disagree by an amount that matters clinically. Body shape, the amount of soft tissue and even how tightly the tape is pulled all add variation.
A tape measure test is still worth doing. It is free and quick, and it can flag a difference that needs proper imaging. But on its own it is not precise enough to guide decisions about a heel lift, a shoe raise or surgery.
Can you measure leg length at home?
You can try, but the same limits apply. Without training in finding the right bony points, home measurements tend to be even less reliable. If you suspect a difference, it is worth having it confirmed with imaging rather than relying on a tape measure alone.
How does a leg length scanogram work?
A scanogram is a measurement taken with either a conventional X-ray or CT. You lie flat on a table. The scanner takes a long image of both legs, with calibration rulers placed alongside. The radiologist then measures the femur (thigh bone) and tibia (shin bone) on each side separately and compares them.
Scanograms are much more precise than a tape measure. They can show how much each bone adds to the overall difference. This is important if a surgeon needs to know whether the difference is in the thigh or the shin.
The main limit is that the measurement is taken lying down. Your muscles are relaxed and your pelvis may sit differently. The image does not show how the leg length difference affects your standing posture or the rest of your alignment. Magnification error can also be a problem with conventional X-ray scanograms, though CT scanograms largely avoid this.
Why does it matter if leg length is measured standing up or lying down?
It matters because your body behaves differently under gravity. When you lie down, the muscles relax and the pelvis settles into a neutral position. When you stand, your full weight presses through the joints, and the body starts to make up for any imbalance. A full-leg standing X-ray captures all of this in one image.
Traditional scanogram. Muscles relaxed, pelvis flat on the table. Good accuracy at the level of the bones, but no information on how the difference affects your standing posture, hip tilt or spine.
Standing full-length imaging. Weight-bearing, muscles working, pelvis loaded naturally. Shows both the bony leg length and its effect on your alignment from feet to spine. The body is a connected chain. A difference at the leg can cause a pelvic tilt, a spinal curve or knee strain higher or lower in the chain.
This is why clinicians more and more want standing imaging. It shows the real-world picture, not just the anatomy on its own. EOS scanning for leg length discrepancy captures both legs and the pelvis in a single upright image, with about 90% less radiation than a standard X-ray, around ten times less.
Why does the measurement method matter for your treatment?
The method matters because a small measurement error can change the plan completely. If a tape measure overstates a difference, you could be given a heel raise you do not need. If a lying-down scanogram understates the effect on your body, a real problem might be missed.
A standing measurement gives your clinician the full picture. They can see the actual leg length difference and how your body has adjusted to it. That adjustment, a tilted pelvis, a curved spine or uneven load through the knees, is often where the symptoms come from.
Before you start treatment, or change the one you are on, it is worth getting a detailed diagnosis. Knowing whether the cause sits in the pelvis, the legs or the spine changes what will actually help.
Wondering whether your leg length difference needs a closer look? A ScanAlign standing EOS imaging assessment in London captures both legs and your pelvis in one low-dose standing scan.
Book a free video consultationWhen MRI is the better tool. EOS and X-ray scanograms show bones, joints and alignment clearly. If your clinician suspects a soft-tissue cause, such as a cartilage problem, a growth plate issue or nerve involvement, MRI is usually the better choice for that detail. The two types of imaging work well together.
Frequently asked questions about leg length measurement
They usually start with a clinical exam, using a tape measure from the hip bone to the ankle. If they find a difference, they may order imaging: either a scanogram taken lying down, or a standing full-length X-ray for a more complete picture.
You can try by measuring from the front of your hip bone to your inner ankle bone on each side. But finding the right bony points without training is difficult. The result may not be reliable enough to guide any decisions.
A scanogram is taken lying flat and measures bone lengths precisely. A standing full-length X-ray captures the same bones under body weight. It shows how any difference affects the alignment of your pelvis and spine.
Yes. EOS uses about 90% less radiation than a standard X-ray, around ten times less. It is designed for full-body standing imaging and is well suited to checking leg length.
You can book a measured leg length scan starting with a free video consultation. For patients travelling from abroad, the price is confirmed during that free video consultation.
No. ScanAlign accepts self-referrals. You can book directly without a letter from your doctor.
It is the standard clinical tape measure test. ASIS stands for anterior superior iliac spine, the bony point at the front of your hip. The medial malleolus is the inner ankle bone. The distance between them on each side is compared to estimate any leg length difference.
In a block test, measured blocks are placed under your shorter leg while you stand. Blocks are added until a clinician judges that your pelvis is level. It is a simple functional test, but it relies on what the examiner sees.
Yes. A CT scanogram avoids the magnification errors of conventional X-ray scanograms. But it is still taken lying down and uses more radiation than EOS imaging. It does not show standing alignment.
The scan itself is very quick. You stand still in the EOS booth for a matter of seconds while the image is taken. The full appointment, including getting you into position, is straightforward.
Get your leg length measured accurately
A free video consultation lets you talk through your symptoms from home and find out whether a standing scan in London is the right next step. No referral needed. No obligation.
Book a free video consultationWritten by Abbas Dhami (Specialist Diagnostic Radiographer)
Sources
- Sabharwal S, Kumar A. Methods for assessing leg length discrepancy. Clinical Orthopaedics and Related Research. 2008;466(12):2910-2922.
- Illés T, Somoskeöy S. The EOS imaging system and its uses in daily orthopaedic practice. International Orthopaedics. 2012;36(7):1325-1331.
- Brady RJ, Dean JB, Skinner TM, Gross MT. Limb length inequality: clinical implications for assessment and intervention. Journal of Orthopaedic & Sports Physical Therapy. 2003;33(5):221-234.
This page is general information and not a diagnosis.
ScanAlign is the trading name of CoreMed Solutions Ltd and operates under The Harley Street Hospital's CQC licence.