Shoe sizes are based on the length of your foot, measured in units that date back to medieval England. The core unit behind US and UK sizing is the barleycorn, a grain-based measurement equal to one-third of an inch. Each full shoe size represents one barleycorn of length, meaning going up one size adds roughly 8.5 millimeters to the shoe. European sizes use a different unit called the Paris point, equal to two-thirds of a centimeter, but the principle is the same: foot length translated into a numbered scale.
The Barleycorn Origin
In 1324, King Edward II of England issued a decree standardizing the inch as three barleycorns laid end to end, with 12 inches to a foot and three feet to a yard. That barleycorn, roughly one-third of an inch, became the increment between shoe sizes and remains the basis of both US and UK sizing today. It’s a 700-year-old system that was never redesigned from scratch, just adjusted over time with different starting points for men, women, and children.
How the Formulas Work
UK and US shoe sizes are calculated by multiplying your foot length in inches by 3 (since each size equals one-third of an inch), then subtracting a constant. The constant differs depending on the system:
- UK adult sizes: 3 × foot length (inches) minus 23
- US men’s sizes: 3 × foot length (inches) minus 22
- US women’s sizes: 3 × foot length (inches) minus 21
- US children’s sizes: 3 × foot length (inches) minus 9.75
This is why US sizes run about one number higher than UK sizes for the same foot length, and why US women’s sizes are about 1.5 to 2 sizes higher than US men’s for the same foot. A person with a 10.5-inch foot would wear roughly a US men’s 9.5 or a US women’s 11. The foot hasn’t changed; the starting point of the scale has.
Length Isn’t the Only Measurement
Shoe size numbers only capture length, but your foot is three-dimensional. Width is handled through a separate letter scale. In the US, widths range from AAA (narrowest) to EEE (widest). The standard or “medium” width for women is B, while for men it’s D. So a men’s D and a women’s B represent similar widths even though the letters differ.
There’s also a measurement most people have never heard of: arch length, or the distance from your heel to the ball of your foot. The Brannock Device, that metal foot-measuring tool found in shoe stores since 1927, actually measures both heel-to-toe length and heel-to-ball length. Your true shoe size is whichever measurement is larger. Most people have a longer heel-to-ball measurement but wear the shorter heel-to-toe size, which means the shoe’s arch support doesn’t line up with their actual arch. Over time, this mismatch can contribute to plantar fasciitis or joint problems in the big toe.
Children’s Sizes and the Transition to Adult
Children’s sizing uses a different constant in the formula, creating a separate number scale that starts at 0 for infants. Kids’ sizes run up to about a youth 7, at which point the scale resets to adult sizing. A youth size 7 is the same length as a men’s size 7. For women’s shoes, you add 1.5 to 2 sizes to the youth number, so a youth 6 translates to roughly a women’s 8.
Why Sizes Vary Between Brands
If shoe sizes follow a formula, you’d expect a size 9 to fit the same across every brand. It doesn’t, for several reasons.
Each brand designs shoes around a “fit model,” a foot shape they treat as their standard. One brand’s fit model might have a higher arch or wider forefoot than another’s. The shoe is then built on a last, a solid foot-shaped form that determines the shoe’s internal dimensions. Lasts vary not just in length but in toe shape. A rounded-toe last typically adds about 6 millimeters beyond the foot, an almond-toe shape adds around 8 millimeters, and a pointed-toe last can add 20 millimeters. That extra material at the tip changes how roomy the shoe feels even at the same labeled size.
Manufacturing introduces more variability. Mass production means slight differences in cutting, stitching, and assembly. Brands that use multiple factories may see inconsistencies depending on machine calibration. And materials matter: leather stretches and molds to your foot over time, while synthetic uppers hold their original shape more rigidly. A leather size 9 that felt snug on day one may fit differently after a few weeks of wear.
What the Number Actually Represents
Your shoe size is not a direct measurement of your foot in any standard unit. It’s a number on an arbitrary scale that corresponds to foot length through a formula, offset by a constant that changes depending on whether you’re shopping in men’s, women’s, or children’s sizes, and whether you’re using the US, UK, or European system. The number itself carries no intuitive meaning. A US men’s 10 doesn’t mean 10 inches or 10 centimeters. It means your foot is roughly 10.83 inches long, plugged into a formula with roots in a medieval grain of barley.
For practical purposes, the most reliable way to find your size is to measure both foot length and arch length, check which is longer, and use that as your starting point. Then treat the resulting number as a guideline, not a guarantee, since the last shape, materials, and manufacturing behind each shoe will shift the fit in ways no single number can capture.