Start with your foot, not your old shoes
Shoe sizes are confusing because several systems coexist, each with its own unit and starting point, and brands interpret them differently. The most reliable starting point is the length of your foot. From it, the calculator estimates your size in the main systems for adults.
How to measure
- Tape a sheet of paper to the floor against a wall.
- Stand on it, heel touching the wall, with your weight on the foot and wearing the socks you plan to wear.
- Mark the tip of your longest toe, which is not always the big toe.
- Measure from the wall to the mark.
- Repeat for the other foot and use the longer measurement.
Measure late in the day: feet swell by up to half a size between morning and evening.
The formulas
With L the foot length:
- US men’s ≈ 3 × L (inches) − 22
- US women’s ≈ 3 × L (inches) − 20.5
- UK ≈ 3 × L (inches) − 23
- EU ≈ 1.5 × (L in cm + 1.5)
- Mondopoint = foot length in mm
UK and US sizes step by a barleycorn, a third of an inch, an old English unit. European sizes step by the Paris point, two-thirds of a centimeter, and are based on the length of the last, the form a shoe is built on, which is longer than the foot to leave toe room. That is why the EU formula adds about 1.5 cm. The US formulas follow the scale of the Brannock device, invented by Charles Brannock in 1925 and still used in American shoe stores.
Worked example
A foot measuring 10.5 inches (26.7 cm):
- US men’s: 3 × 10.5 − 22 = 9.5.
- US women’s: 11.
- UK: 8.5.
- EU: 1.5 × (26.67 + 1.5) = 42.3, shown as 42.5.
- Mondopoint: 267.
Why the result is only a guide
- Brands differ: one brand’s 42 may fit like another’s 43, especially between running shoes, dress shoes and European or Asian brands.
- Width matters: US sizes come in widths such as B, D and 2E; a wide foot may need a longer size in a brand without widths.
- Use: running shoes are often bought half a size up to leave room for swelling; ski boots and climbing shoes fit much tighter.
- Children’s sizes use a different scale and are not covered here.
The best check is the brand’s own chart in foot length, then trying on in the afternoon with about a thumb’s width, roughly 1 cm, between the longest toe and the end of the shoe.