Legacy Shoe Size Formula

oc-shoe-legacy-formula-v1 maps a selected group and input system to an assumed foot length, then applies retained US, UK, and EU equations. Regional labels are outputs of this named, nonstandard formula only.

Entries and formulas

Choose men’s, women’s, or children’s; choose US, UK, EU, or CM; then enter a positive size or assumed foot length. The starting selection is men’s US 10. The CM choice means assumed foot length in centimeters inside this formula, not a manufacturer’s CM label.

For men’s US input $S$, assumed foot length is:

$f=\left(\frac{S+23}{3}\right)2.54$

The men’s results are $US=3(f/2.54)-23$, $UK=US-0.5$, and $EU=1.5(f+1.5)$. The women’s US input uses 21 instead of 23, with $UK=US-2$. The children’s US input uses 9.67, with $UK=US-0.5$. UK inputs use their corresponding inverse constants; EU input uses $f=EU/1.5-1.5$; CM input uses the entered value as $f$.

US and UK results are rounded to the nearest half size, EU to a whole number, and assumed foot length to one decimal.

For men’s US 10:

$f=((10+23)/3)2.54=27.94\text{ cm}$

The shown crosswalk is US 10.0, UK 9.5, EU 44, and 27.9 cm assumed foot length. Selecting women’s US 10 returns US 10.0 and UK 8.0 under the women’s equations. Neither comparison establishes fit.

Fit-check workflow

1. Measure both feet as directed by the intended manufacturer.
2. Use the larger relevant measurement and note the intended model and use.
3. Treat this formula result as a navigation aid only.
4. Compare with the exact manufacturer/model chart, including its own CM-label meaning.
5. Check width and real fit; length conversion alone is insufficient.

Manufacturer CM labels can differ from measured foot length. ISO 9407 is a separate Mondopoint sizing and marking standard; this calculator has not been assessed for conformance. For a different retailer-style crosswalk, use the ring-size chart, which has its own non-universal limits.

Sources

• Nike, How to measure foot size — manufacturer-specific measurement and chart guidance.
• Nike, Footwear size chart — manufacturer-specific evidence that CM labels can differ from measured foot length.
• ISO, ISO 9407:2019 Footwear sizing — Mondopoint system of sizing and marking — identifies the separate Mondopoint standard; it does not support these formulas or establish conformance here.