Shoe Size Converter
Convert UK, US, EU and Mondopoint shoe sizes through actual foot length — the only honest pivot, because a UK step and an EU step are different distances and never line up.
Shoe Size Converter
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Width, arch height and last shape are not modelled and affect fit as much as length does.
- Toe allowance varies by manufacturer and by shoe type; the percentages offered here are conventions, not standards.
- UK children's sizing restarts its numbering, so child and adult UK numbers are not on one continuous scale.
In short: Shoe sizes convert through foot length, never directly. The UK and US scales step in barleycorns of 1/3 inch (8.47 mm); the EU scale steps in Paris points of 2/3 cm (6.67 mm). One UK size therefore spans 1.27 EU sizes, so whole numbers in one system land between whole numbers in the other and every published chart has rounded — differently. ISO 19407 gives EU = 0.15L + 2 and UK = 0.1181L − 23 with L in millimetres.
Formula
ISO 19407 defines both scales against foot length in millimetres. Because the barleycorn and the Paris point are different distances, one UK size equals exactly 1.27 EU sizes and the scales never align on whole numbers.
Worked Example
- Start from foot length. A 270 mm foot is Mondopoint 270.
- Apply the EU formula. 0.15 × 270 + 2 = EU 42.5.
- Apply the UK formula. 3 × 270 ÷ 25.4 − 23 = UK 8.89.
- Round honestly. UK 8.89 becomes a UK 9, but it is not a UK 9 exactly — it is nine tenths of the way there.
- Add toe room. At 5 % the internal shoe length wants to be about 283 mm, not 270.
Why no two charts agree. Take a UK 8. The ISO formula puts that at a 262.47 mm foot, which converts to EU 41.37. Take a UK 9: a 270.93 mm foot, or EU 42.64. Neither lands on a whole EU size, and the gap between consecutive UK sizes is 1.27 EU sizes rather than one. A chart author must therefore decide whether UK 8 is ‘EU 41’ or ‘EU 42’, and different authors decide differently. Both are defensible roundings of the same arithmetic. This is not sloppiness in the charts; it is a genuine incompatibility between a system built on thirds of an English inch and one built on two thirds of a French centimetre.
Strengths & Limits Of This Model
Where this engine is strong
- Pivots every conversion through foot length instead of chaining chart lookups.
- Shows the unrounded ISO 19407 figure next to the rounded size, so the 1.27 mismatch is visible rather than hidden.
- Handles Mondopoint, the one system that is self-explanatory.
Where it stops
- Cannot predict how any particular brand will fit.
- Does not cover width fittings such as D, 2E and 4E.
- Not a substitute for trying shoes on.
Practical Use Cases
Buying shoes from an overseas retailer
Translating a US or EU listing into the size you actually wear, with the unrounded figure visible so you know which way to lean.
Measuring a child's foot
Converting a measured length directly to Mondopoint and EU without going through an adult chart.
Ski boots and safety footwear
Mondopoint is the standard in both, and is simply foot length in millimetres.
Comparing brands that fit differently
Working from measured length rather than from a remembered size — pair with the Length Converter when your tape reads in inches.
Methodology & Editorial Standards
All conversions pivot on foot length in millimetres, following ISO 19407, which defines EU size as three twentieths of the length plus two, and UK size as three times the length in inches minus twenty-three. US men's sizes are UK plus one and US women's are UK plus two, both being labelling offsets on the same barleycorn step of exactly one third of an inch, or 8.4666667 mm. The EU step is the Paris point of exactly two thirds of a centimetre, 6.6666667 mm, giving a fixed ratio of 1.27 between the scales. Mondopoint, defined by ISO 9407, is the foot length in millimetres and is conventionally rounded to the nearest 5 mm. Results are shown both rounded to the nearest half size and unrounded, because rounding is where published charts diverge from one another. The fitting allowance is applied as a percentage of foot length to estimate internal shoe length; manufacturers choose their own allowance and this converter cannot know which one a given brand used. Width, arch profile and last shape are not modelled and materially affect fit. All conversion factors are exact by definition under the International System of Units, or exact by international agreement where the unit is defined by treaty. Values are held at full IEEE-754 double precision internally and rounded only for display, so chained conversions do not accumulate drift.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Shoe Size Converter — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
What is my UK size if I am a US 10?
US men's 10 is UK 9 and US women's 10 is UK 8, because the US scales are the UK scale offset by one and two sizes respectively. All three use the same barleycorn step of one third of an inch, so the offset is purely a labelling convention rather than any difference in the underlying foot length.
Why do UK and EU shoe sizes not convert cleanly?
Because they measure in different units. A UK size step is a barleycorn, one third of an inch or 8.4667 mm. An EU size step is a Paris point, two thirds of a centimetre or 6.6667 mm. The ratio is exactly 1.27, so one UK size spans a bit more than one and a quarter EU sizes and the two scales slide against each other. A UK 8 works out at EU 41.37, which is not a size you can buy.
What is Mondopoint?
The ISO 9407 system, which expresses shoe size as the foot length in millimetres, usually rounded to the nearest 5 mm and sometimes paired with a width figure as in 270/100. It is the only shoe sizing system that means something on its own without a conversion chart, which is why ski boots and military footwear use it.
How do I measure my foot properly?
Stand on a sheet of paper with your heel against a wall, put your weight on the foot, and mark the tip of the longest toe, which is not always the big toe. Measure from the wall to the mark. Do it in the late afternoon or evening, because feet swell by up to half a size through the day, and measure both feet since most people have one noticeably longer.
Should I add room to my measured foot length?
Yes. Shoes are built on a last that is longer than the foot, typically by about 5 % for everyday wear and up to 8 % for running, where the foot slides forward on impact. That is roughly 13 to 21 mm on a 270 mm foot. The size on the box refers to the last, not to your foot, and each manufacturer picks its own allowance, which is a large part of why brands fit differently at the same nominal size.
Why do the same size shoes fit differently between brands?
Three reasons compound. Different toe allowances mean the internal length varies at the same marked size. Different last shapes change how the volume is distributed even when length matches. And width is a separate dimension that most brands do not offer at all, so a foot wider than the assumed standard must size up in length to gain width, which then leaves excess length.
What is a barleycorn?
An old English unit of one third of an inch, historically the length of a grain of barley, and the basis of the entire UK and US shoe scale. Three barleycorns to the inch is why shoe sizes step by roughly 8.5 mm. It is one of the few medieval agricultural units still in daily commercial use anywhere in the world.
Are children's and adult sizes on the same scale?
In the UK the child scale runs to 13.5 and then restarts at adult 1, so the numbering is discontinuous even though the barleycorn step is identical throughout. US children's sizing behaves similarly. EU sizing is continuous from infant to adult because it is a straight Paris point count with no restart, which is one of its practical advantages.