INSIGHTS
I've bought footwear across dozens of brands as a buyer, sold for a mountaineering boot brand, and spent plenty of time on the sales floor myself — and even so, shoe sizing stays confusing no matter how long you've been in the industry. Between US, UK, EU, and Japanese (JIS) systems, even with the conversion tables memorized, there's still a flicker of doubt the moment you're the one being fitted rather than doing the fitting — "wait, is this really my size?" For newer sales staff, guiding a customer through that moment with real confidence is harder still. A conversion chart alone doesn't solve this; what matters is whether the people on the floor can actually use it with confidence.
The EU system, which originated in France, is based on a unit called the "Paris point" — one size equals two-thirds of a centimeter, or about 6.67mm. The UK system is based on an older unit called the "barleycorn," derived from the idea that three grains of barley laid end to end equal one inch (2.54cm) — so one UK size equals one-third of an inch, or about 8.46mm. Both are awkward fractions in practice, so retailers commonly round them to roughly 6mm (EU) and 4mm per half-size (UK).
The most confusing part of shopping for shoes internationally is this: US size 7 does not mean you multiply the gap between one size and the next (US 7 to US 8 is about 8.5mm) by seven to get the actual foot length. It's an easy assumption to make — surely each size up means roughly a centimeter more — but sizes actually move in increments of about 8.5mm, not 10mm. On top of that, overseas sizing isn't a plain ruler that starts at zero; it starts counting at "size 1," which already corresponds to an adult foot length of roughly 19.5cm, and counts up from there one notch at a time. Work backward from the formula, and US size 1 does indeed come out to almost exactly 19.47cm.
So why does the same inch-based logic produce "UK 6 = US 7"? The reason is simple: the US ruler starts counting from a point exactly one notch (about 8.5mm) shorter than the UK ruler does. Because the US starting line sits one notch earlier, measuring the very same length of shoe always produces a US number that's one higher than the UK number. That's the real reason shoe size numbers don't translate directly into length, and it's the mechanism behind the gap between UK and US sizes.
By the math above, US 7 is designed, in theory, for a foot measuring 24.55cm. Yet in Japan, "men's US 7 = 25.0cm" is the conversion everyone treats as settled fact. Two things explain the gap.
The first is toe room — the built-in space between the end of your toes and the front of the shoe, standard practice at roughly 1cm. Someone with a true 24.55cm foot needs that cushion, or their toes hit the front of the shoe. So when the industry converts to a centimeter figure for the Japanese market, that toe-room allowance gets folded in, and the number gets rounded up to a clean 25.0cm.
That's also why REGAL, a long-established Japanese leather shoe brand, tends to feel roomier than the sneakers people usually wear. REGAL builds to a Japanese Industrial Standards (JIS) concept called "foot-insertion size," where the labeled size is meant to match the wearer's actual foot length. The toe room — about 1.0 to 1.5cm — gets added on top of that actual length, inside the shoe. A typical sports sneaker labeled 25.0cm, by contrast, is built around the idea that the internal cavity itself measures close to 25.0cm. Bring the same US 7 (a design length of 24.55cm) into Japan, and a sneaker brand files it under 25.0cm based on internal room, while a leather shoe brand calls it roughly 24.5–25.0cm based on actual foot length. Which of those two philosophies — actual length or internal room — a brand converts by changes what the customer hears at the counter.
The second factor is that Nike's and Adidas's own size charts simply took over as the de facto standard here. Nike's official men's chart lists US 7 as 25.0cm, plainly. Once the biggest global brands settled on that number, it became the shared assumption for shoe shopping in Japan.
The same logic applies to EU sizing. EU 39, on many conversion charts, gets grouped with US 7 and UK 6, landing at roughly 25cm as well. That said, this mapping shifts by half a size or more depending on the brand — no single conversion chart is the final word.
It doesn't stop there. Women's US sizing doesn't just reuse the men's scale — it shifts the starting point 1.5 sizes (about 12.7mm) shorter than men's, reportedly reflecting historical market data showing women's feet running shorter on average than men's. As a result, the gap between UK and US sizing isn't even constant: it's 1 for men's and 2 for women's.
These conversion quirks are an easy thing for an overseas footwear brand to overlook when entering Japan. It's not enough to hand over a size chart — what actually separates a workable distributor partnership is whether store staff understand the difference between actual length and internal room, and can translate that into confident, in-person guidance for the customer.
Written by Akira Kariya — 25+ years in the outdoor & apparel industry. Retail, corporate sales, and brand management before founding Replacements to help overseas and domestic brands enter and grow in Japan.
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