What Shoe Sizes Actually Convert
A shoe-size number is not a measurement of your foot. It is a measurement of the shoe's last — the carved form a shoemaker builds the shoe around — and different countries have used wildly different conventions for centuries. The United States system increments by one third of an inch (a "barleycorn") per full size. The United Kingdom uses the same barleycorn step but starts counting from a different baseline, which is why a UK adult size is consistently about one size smaller than the equivalent US men's size. Continental Europe uses the Paris point — exactly 2/3 of a centimetre per size — codified in the Mondopoint family of standards. Japan, Korea, and most of Asia simply use the foot length in centimetres, which is the most physically meaningful but less commonly used in marketing copy.
Because these systems all measure different reference points (the shoe last, the insole, or the foot itself, with different built-in clearance allowances), no conversion table is exact across every brand. Most published charts target a half-size accuracy, and the real-world variability between brands routinely exceeds that. The international standard ISO 9407:2019recommends Mondopoint — millimetre foot length and width — as the unambiguous reference, but the consumer market still buys mostly by US, UK, or EU numbers.
The Conversion Formulas
The relationships between systems are linear once you fix a starting point and a step size. The constants below are widely used in industry charts and match the values published by Brannock and major footwear retailers.
US men's size = (foot length in inches − 7.333) × 3 + 1
US women's size ≈ US men's + 1.5 (same foot length)
UK adult size ≈ US men's − 1
EU size (Paris point) = (foot length in cm + ~1.5 cm allowance) × 1.5
JP size = foot length in cm
One US full size is one barleycorn ≈ 8.47 mm. One EU size is 6.67 mm. So an EU step is slightly finer than a US step, which is why you sometimes see EU charts list two adjacent EU values for a single US size.
How to Convert Step-by-Step
- Measure both feet (described in the next section) and use the larger of the two as your reference.
- Decide whether you want men's or women's sizing — most US and UK adult shoes split by gender, while EU and JP do not.
- Look up the foot-length row in a sizing chart and read the corresponding number for your target system.
- If your foot length falls between two rows, size up. Half a size is rarely too loose; half a size too small is genuinely painful.
- Always cross-check against the specific brand's size guide before ordering. Brand last shape varies far more than the national-system gap.
Worked Examples
Example 1 — Adult man, foot 27 cm
A 27 cm foot maps to US men's 9, UK 8.5, and EU 42. Japan simply uses 27 cm. The Brannock device — the metal foot-measuring tool every traditional shoe shop owns — would point to roughly the same numbers, with width marked separately as D (medium).
Example 2 — Adult woman, foot 24 cm
A 24 cm foot is approximately US women's 7, UK 5, and EU 37.5. The same foot in men's sizing would be US men's 5.5 — which explains why women buying gender-neutral sneakers (running shoes, basketball shoes) usually need to subtract about 1.5 from their women's number to land on the correct men's size.
Example 3 — Child to adult transition
A child wearing US Big Kid 6 (around 24.1 cm foot) will fit a US men's 6 and a US women's 7.5 in the same actual shoe. The kids' size scale runs up to US 7 and then directly hands over to the adult scale. This overlap is why teenage shoppers regularly find adult shoes cheaper for the same fit.
The Brannock Device, Mondopoint, and ISO 9407
The metal foot measure you have stepped on at every shoe shop is the Brannock device, patented in 1927 by Charles F. Brannock of Syracuse, New York. It measures three things at once: heel-to-toe length, heel-to-ball length, and width. Most American foot length is measured by the heel-to-ball reading, which corresponds to where the shoe's flex point should sit, not the longest toe. That subtle distinction is why Brannock-fitted shoes feel right while online charts sometimes recommend a size that is half a size off.
Mondopoint is the international standard sizing system specified by ISO 9407:2019. It expresses a shoe size as foot length in millimetres followed by foot width in millimetres — for example, "270/100" for a 27 cm foot of medium width. Mondopoint is used by the military, ski boots, and skating boots almost universally because it is unambiguous across every country. Consumer footwear has been slow to follow because consumers do not know their Mondopoint number off the top of their heads, and brands have decades of marketing tied to legacy sizing scales. Some specialist running shops in Japan and Germany have begun listing Mondopoint alongside national sizes; expect that to spread over the next decade as foot-scanning apps make the measurement trivial.
How to Measure Your Foot
- Measure in the evening, when feet are slightly swollen — that is the size your shoes need to accommodate during real wear.
- Wear the socks you plan to use with the shoes. Sock thickness can change effective size by half a size.
- Place an A4 sheet on a hard floor against a wall. Stand on it with your heel touching the wall.
- Mark the tip of your longest toe (often the second toe, not the big toe).
- Measure heel-to-mark in centimetres. Repeat for both feet and use the larger measurement.
- Add 0.5 to 1 cm of toe clearance for athletic shoes and running shoes; add 0.3 cm or less for dress shoes.
Common Misconceptions
- "A US 9 is a US 9 in every brand." No. The same labelled size from Nike, Adidas, and New Balance can vary by half a size in length and considerably more in width.
- "Women's and men's sizes are the same number." They are not. US women's ≈ US men's + 1.5 for the same foot length.
- "EU sizes are based on centimetres." Indirectly. They use the Paris point (2/3 cm) plus a fixed clearance allowance, so EU 42 is not 42 cm of foot — it is roughly 26.7 cm of foot.
- "Half sizes are decorative." A US half size is about 4.2 mm. That is the difference between toes touching the cap and toes that have room to splay during a long run.
- "Width does not matter much." It can matter more than length. New Balance, Brooks, and several work-boot brands offer 2E and 4E widths because a wide foot in a D-width shoe is the leading cause of bunions and toe-numbness during long wear.
Frequently Asked Questions
Why are US men's and women's sizes different?
Historical convention. US women's sizing starts at a smaller foot length and is offset by roughly 1.5 sizes from men's. A US women's 8 is approximately a US men's 6.5 for the same foot.
How accurate are these conversions?
Within about half a size, which matches industry reference charts. Brand-specific last shapes routinely create a wider spread than the system gap, so always check the brand size guide.
What is Mondopoint?
An ISO standard that expresses size as foot length in mm and foot width in mm. It is the unambiguous international system, used by the military, ski boots, and skating boots, but rare in consumer fashion footwear.
How do I find my width?
Trace your foot on paper at the widest point of the ball of the foot, then measure the trace. US widths progress from B (narrow) → D (medium) → E → 2E → 4E (extra wide). Brand-specific width charts will tell you the millimetre threshold for each.
Should I size up or down?
When in doubt, size up — especially for running, hiking, and any all-day wear. Toes need a few millimetres of space, particularly during long activity when feet swell.
Is my data stored?
No. Every calculation runs entirely in your browser. Nothing you type is uploaded, logged, or retained.
References
- International Organization for Standardization. ISO 9407:2019 Footwear — Sizing — Mondopoint system of sizing and marking.
- Brannock Device Company. Brannock Device User Guide — heel-to-ball measurement methodology, Syracuse, New York.
- U.S. Patent 1,963,068, "Foot-measuring device," Charles F. Brannock, granted 1934.
- European Committee for Standardization. EN 13402 Size designation of clothes (foot dimensions appendices).
- Japanese Industrial Standards Committee. JIS S 5037 Sizing systems for footwear.