Shoe sizes: why no number converts into another
A shoe size is not a length. It is a rank in a maker's scale, and the number does not measure your foot. That is why the conversion tables disagree with each other, and why this page has no converter: the arithmetic below shows there is nothing to convert.
A size is a rank, not a measurement
When a length doubles, so does the number that names it: two meters is twice one meter. Shoe sizes do not behave that way. A size 42 is not twice a size 21, and the step from 41 to 42 is not the same amount of shoe in every country.
What a size number gives you is a position in a series, and the series belongs to whoever built it. That is the single fact this whole page rests on, and it is the reason a converter would be dishonest.
The Paris point: two thirds of a centimeter, measured on the last
The Continental European scale counts in Paris points, and one Paris point is two thirds of a centimeter, that is 6.67 millimeters. Size 42 therefore names a length of 28 centimeters, since 42 times two thirds of a centimeter is 28.
But 28 centimeters is not the length of the foot. It is the length of the last, the wooden or plastic form the shoe is built around, which is longer than the foot it is meant to hold. The allowance is usually one and a half to two centimeters, so a size 42 fits a foot of roughly 26 centimeters.
The English barleycorn: one third of an inch
The English scale counts in barleycorns, and a barleycorn is one third of an inch, that is 8.47 millimeters. The name is not decorative: the grain was the customary small unit, three of them to the inch, and the scale kept the step long after it stopped keeping the grain.
Half sizes exist on this scale because the step is large. A half size is a sixth of an inch, 4.23 millimeters, which is why an English or American chart offers halves where a European one does not need them.
Why no fixed offset can link the two
Here is the arithmetic that settles it. The English step is 8.47 millimeters and the European step is 6.67, so every size you climb, the two scales drift apart by 1.8 millimeters. Over ten sizes that is 18 millimeters of drift, which is nearly three European sizes.
So a rule of the form add thirty-three that is right in the middle of the range is wrong at both ends, and it has to be. Any table that offers one number per row is rounding a moving quantity, and different tables round it differently, which is exactly why they disagree. A chart's own column headings say it: a UK size, a US size and an EU size are three different rulers, not three names for one.
The American scales, and there are two of them
American men's sizes run roughly one number above the English, and American women's roughly one and a half above the men's. Those are the usual rules of thumb and they are rules of thumb: they hold near the middle of the range and drift at the edges, for the reason the previous section gives.
Worse, the men's and women's series are two different scales, not one scale with two labels. A woman's 8 and a man's 8 are not the same shoe, which is why a chart that gives a single American column is missing the information you need.
Mondopoint, the only system that measures the foot
Mondopoint, standardized as ISO 9407, drops the ranks entirely and states the length of the foot in millimeters, often with the width beside it. A 265 is a foot of 265 millimeters, and there is nothing to look up. It is, in a single phrase, foot length and nothing else.
It is what ski boots and several armies use, and it is not a coincidence: both need a fit that works the first time, on a foot they have measured rather than guessed. Where a maker publishes Mondopoint, it is the number to trust.
What actually works, and why this page carries no converter
Measure your foot in millimeters, standing, in the evening: feet swell over the day, and a shoe fitted in the morning can be tight by six o'clock. Measure both, because they differ, and fit the larger one. Then read the maker's own chart, which is the only chart that describes the shoes you are buying.
A converter here would turn a rank into a length and hand you a number with no maker behind it. This site converts lengths, and the foot measurement is a length: that part belongs on the length pages. The size is not, and saying so is more useful than a table that would be wrong at the ends.