How Much Bigger Is a Half Size in Shoes? US, UK and EU Compared

A half size is 1/6 of an inch, or about 4.2 millimeters, in the traditional US and UK systems. That is a very small distance, yet it often decides whether a shoe feels right. Brand charts round it to 5 millimeters (0.5 cm), and the size difference you feel in a real shoe can be larger or smaller, depending on the shoe.

This guide gives you the exact numbers in inches, millimeters and EU Paris points, shows real half-size steps from the brand charts in our data, and explains when a half size is the right fix and when it is the wrong one.

The math: how long is a half size?

The US system is based on the barleycorn, an old unit equal to one third of an inch. One full shoe size is one barleycorn, which is 8.47 mm. A half size is half of that, 4.23 mm. The UK system uses the same barleycorn step, so the distance between two UK sizes is also 8.47 mm. Only the starting point differs, which is why a US 9 is not a UK 9.

The EU system works differently. It counts in Paris points, where one point is two thirds of a centimeter (6.67 mm). EU sizes therefore do not line up one-to-one with US or UK steps, and that explains the odd gaps you see on conversion charts. A full US size is about 1.27 Paris points, so one EU number does not equal one US size.

Why brand charts do not show 4.23 mm

Look at the foot lengths in the standard chart below. Every half size adds 0.5 cm, not 0.42 cm. Charts round to a convenient figure, and manufacturers also build their lasts to their own tolerances.

Read down the US column and the foot length rises by 5 mm at every half size. Read down the EU column and the step alternates: from US 8 to 8.5 the EU number rises by 1, from 8.5 to 9 by 0.5, and from 9.5 to 10 by 1 again. That is rounding at work, because EU sizes are published in whole and half numbers while the real steps are 6.67 mm.

Brand differences in half-size steps

Brands share the same idea but show it differently. Adidas moves EU sizes in thirds. Look at what that does to the numbers.

On the Adidas chart, US 8.5 and US 9 have the same foot length, 26.5 cm, even though the EU number rises from 42 to 42 2/3. In other words, two neighboring sizes are listed as the same length, which tells you that the difference between them is within rounding. New Balance and Asics also round to their own steps. Asics has a few spots where the foot-length column moves by only 0.25 cm, near men's US 10.5 to 11 and 11 to 11.5.

The takeaway is that a chart cannot tell you how a particular shoe will feel. It can tell you which neighbors are close. For a brand-by-brand look, see our brand size comparison, and for a full table, the shoe size conversion chart.

Does a half size make a noticeable difference?

Yes, when the shoe is already almost right. A 4 mm change is minor if you are two sizes off, but it can be the difference between a toe that grazes the front and one that sits comfortably. Many people can feel a half size in the toe box of a snug shoe, and some notice it more in the heel.

It also depends on the shoe. A soft mesh runner forgives half a size better than a stiff leather boot. A stiff upper does not give, so the extra millimeters are the whole story. In a knit sneaker, the fabric moves and the same half size gets lost.

Does a half size make a shoe wider?

Slightly. As shoes get longer they also get a little wider, even in the same width code. New Balance publishes numbers for this: in its men's guide, a 9D and a 9.5D differ by 5 mm in listed foot length but only about 1 mm in width. A 9 in 2E has the same length as the 9D and about 3 mm more width.

That is the point most people miss. If your only problem is a tight forefoot and the length is right, a half size up buys you very little width and adds length you do not need. A wider fitting, where available, solves the right problem. Our shoe width chart explains D, 2E and the other codes.

When to go up half a size

• Your toes touch the front of the shoe when you stand and push forward.
• You get pressure on the nail of your longest toe after twenty minutes of walking.
• A snug sneaker, a boot with a stiff toe cap or a shoe with a lot of padding is the only issue, and the heel still holds.

When to go down half a size

• You have more than a thumb's width of space in front of your toes and the heel slips when you walk.
• The shoe is known to run long. Heritage leather boots are the classic example, and many are fitted half a size below sneaker size. See our Red Wing guide.
• The shoe creases ahead of the ball of your foot instead of under it.

When a different size is the wrong answer

Half a size cannot fix a shoe that is the wrong shape. If the problem is a squeezed little toe in a shoe with good length, look at a wider width or a roomier model. If your heel slips already, a longer shoe makes that worse. If the instep is tight, you may need a different lacing pattern, thinner socks or a different last.

Fix the cause before changing the number. Our foot measuring guide helps you find your length and width, which makes it much easier to judge a half-size move.

Reading the steps in practice

Take a men's US 9 on the standard chart: 27 cm, UK 8, EU 42.5. The next half size, 9.5, lists 27.5 cm, UK 8.5 and EU 43. Everything moves by a half step except EU, which moved by half a point. The next one up, US 10, jumps to EU 44. If you are comparing two shoes and see EU 43 and EU 44, that is one full US size apart, not a half.

The same logic applies on women's charts. A women's US 8 lists 25 cm and EU 39, and a women's 8.5 lists 25.5 cm and EU 40. Half a US size can show up as a whole EU number, so do not read the EU column as if it had the same granularity as the US column.

If you are shopping across systems, convert your foot length first and only then pick a label. Our US to EU guide shows the full mapping, and it is a safer route than converting size labels one to one.

Other things that are worth half a size

A half size is small enough that everyday variables rival it. Your feet swell over the day, and many people see a difference of a few millimeters between morning and evening, so try shoes on in the afternoon or evening. Most people also have one foot slightly longer than the other, so fit the larger foot. A thick athletic sock can take up roughly the same space as a half size, which is why a pair that felt perfect with thin socks can feel snug on a cold day.

Insoles matter as well. A thicker aftermarket insole removes volume, and in a roomy shoe it can do the work of going down half a size without touching the length. Combine all of this with the shoe's own character, and you can see why two people in the same size can disagree about the same pair. The number only tells you where to start the fitting, not where it ends.

How to choose between two sizes

When two sizes both seem acceptable, run through a short list. Check the heel: choose the size where the heel stays put. Check the toes: choose the size with space for the toes to move. Check the sides: choose the size without pressure. If both pass, go with the smaller size in a knit or soft shoe and the larger one in a stiff leather shoe, since stiff materials rarely stretch lengthwise. You can also use our shoe size calculator to turn a measured length into a size before you make the call.