A UK size 8, European 42, US 9 and Japanese 27 can all be used to describe approximately the same foot.
Yet these numbers do not speak the same measurement language. A clinician cannot simply place a ruler against a foot, read a length and discover that it measures “42”.
That apparent contradiction highlights something easily overlooked in footwear and, more importantly, in the assessment and manufacture of custom foot orthoses: traditional shoe sizing was not designed to provide a precise description of an individual’s foot.
For clinicians embracing digital assessment, foot scanning and custom manufacturing, that distinction matters.
From barleycorns to Paris points
Some of today’s most familiar shoe-sizing systems carry the legacy of measurement conventions that developed centuries before digital scanners, CAD software or additive manufacturing existed.
The traditional British system is associated with the barleycorn, an old English unit equal to one-third of an inch. The barleycorn has deep historical roots in English measurement, with the statute of Edward II in 1324 famously describing an inch in terms of three barleycorns placed end to end.
European sizing developed differently. Continental sizing commonly uses the Paris point, equivalent to two-thirds of a centimetre.
US sizing subsequently developed from the English tradition but with differences in its starting point and conventions.
The result is the familiar situation in which one person’s footwear can carry dramatically different numbers depending upon which system is being used.
Even apparently more intuitive centimetre-based systems do not eliminate all the complexity because footwear sizing involves more than simply recording the longest dimension of the foot.
The number describes footwear—not the complete foot
The bigger issue for orthotic practice is what shoe sizing doesn’t tell us.
Two people who purchase exactly the same shoe size can have substantially different feet.
They may differ in:
- forefoot width and shape;
- heel width;
- arch morphology;
- instep height;
- toe configuration;
- left-to-right asymmetry;
- plantar contours;
- overall foot volume.
A single shoe-size number cannot capture this three-dimensional anatomy.
There is another complication. Shoes are manufactured around lasts—three-dimensional forms used to define the internal shape and characteristics of footwear.
Last design varies between manufacturers, footwear categories and individual models. Consequently, someone who comfortably wears one size in one brand may need a different size in another without their foot having changed at all.
Consumers experience this every day.
“I’m normally a 42.”
“I’m an 8 in these but an 8½ in those.”
“That brand comes up narrow.”
These aren’t necessarily inconsistencies in the person’s foot. They reflect the limitations of trying to represent the relationship between a complex anatomical structure and differently shaped footwear with one simple number.
Why this matters much more in custom orthotics
For mass-produced footwear, standardised sizing is necessary. Manufacturers need a practical way of grouping millions of consumers into manageable production categories.
Custom orthotics have a fundamentally different objective.
The clinician isn’t trying to determine which standardised mould most closely resembles the patient.
The objective is to understand the individual foot.
Historically, clinicians have achieved this through combinations of physical examination, measurements, impressions, foam boxes, plaster casting and other techniques.
Digital technology adds another possibility: capture the actual three-dimensional geometry of the patient’s foot.
A modern 3D scan can provide a digital representation of anatomical shape that can subsequently form part of a CAD/CAM workflow for designing an individualised orthosis.
Instead of asking:
“Which standard size is closest?”
the process can increasingly ask:
“What is the actual geometry we are trying to accommodate or influence?”
That is a significant conceptual shift.
From fitting people to categories to designing for individuals
This doesn’t mean shoe size suddenly becomes irrelevant.
The finished orthosis still has to work within footwear. Clinicians need to understand the available internal volume, shoe style and practical relationship between the device, foot and footwear.
Nor does scanning replace clinical assessment. A beautifully accurate 3D model does not independently determine diagnosis, prescription, material choice or the clinical objectives of an orthosis.
But shoe size and foot geometry answer very different questions.
Shoe size is a category.
A foot scan is anatomical data.
Confusing the two risks using a convenient commercial classification as though it were a meaningful description of individual anatomy.
Digital manufacturing makes the distinction increasingly important
The growth of 3D scanning, CAD/CAM, CNC milling and additive manufacturing is changing what is technically possible within foot orthotic workflows.
Once individual geometry can be captured digitally, clinicians and technicians no longer need to rely solely on standard size categories as proxies for shape.
The digital model can move through a workflow from capture → clinical modification → design → manufacture, potentially allowing more patient-specific characteristics to be incorporated into the finished device.
For the wider orthotics sector across the IMEA region, this is particularly interesting.
Digital workflows could eventually enable the expertise responsible for assessment and prescription to be geographically separated from some stages of design and manufacturing. A patient could potentially be scanned in one location, the digital file processed elsewhere and the resulting device manufactured through a centralised production facility.
That creates opportunities for clinics that may not have the space, machinery or technical workforce required for a conventional orthotic laboratory.
The humble shoe size still has its place
After centuries of development, shoe-sizing systems aren’t going anywhere.
They remain an extraordinarily efficient solution to the problem they were designed to address: mass-producing and selling footwear to populations with differently sized feet.
The mistake is expecting them to tell us considerably more than that.
For custom orthotics, the interesting question isn’t whether someone is a UK 8, EU 42, US 9 or Japanese 27.
It is what the person’s foot actually looks like, how it functions, what clinical intervention is required and how the resulting orthosis will interact with their footwear.
Digital scanning increasingly allows the profession to separate those questions.
The history of shoe sizing can therefore be viewed as an interesting contrast with the direction of modern custom orthotics.
For centuries, footwear manufacturing has asked:
Which mould should this person fit?
Custom digital orthotics can reverse the question:
How should we design the device around this person?
And once you can answer that question, the number printed inside the shoe becomes considerably less interesting.
- ISO – Footwear sizing and terminology standards
- International Society for Prosthetics and Orthotics (ISPO)
- World Health Organization – Assistive Technology
- International Organization for Standardization (ISO)
- IMEA CPO