Methodology

Most converter sites hide their math. Ours is short enough to explain completely — and its limitations are part of the answer.

The foot-length model

Every named size system is a grid laid over one physical quantity: the length of the foot the shoe fits. Our engine therefore stores each size in each system as a documented foot-length range in millimetres, and every conversion runs in two steps — your input resolves to a foot-length range, then every target system reports the size whose range overlaps it. This is why the same engine serves both directions of every pair with one table per system.

The scale relations we encode

• UK: adult size ≈ 3 × foot length (inches) − 23, grading in barleycorns (⅓ in); children's scale 0–13.5 uses ≈ 3 × foot − 10.
• US men's: the UK grid shifted one size up (≈ 3 × foot − 22).
• US women's: common retail scale = men's + 1.5 (≈ 3 × foot − 20.5); the FIA scale uses +1 — a documented convention split we flag on every women's result.
• US kids: ≈ UK kids + 0.5 (row-level variance across published charts).
• EU: Paris points (⅔ cm) with a functional last allowance — size ≈ 1.5 × foot (cm) + 2 — which is why EU boundaries never align perfectly with US/UK.
• Mexico: a barleycorn variant ≈ 3 × foot − 23.5 (about half a size below UK); centimetre marking is also common.
• Australia: composite — men's and kids' follow UK numbering; women's follows US women's.
• Mondopoint / JP: foot length in millimetres exactly (ISO 9407).

These relations reproduce the published conversion tables row for row — that equivalence is enforced by an automated golden-value test suite that runs on every build.

Sources and cross-checking

The scale definitions follow the conversion relations documented in the international standard family (ISO 9407 Mondopoint and the ISO/TS 19407 conversion tables), as documented in public reference material, and every load-bearing boundary is cross-checked against at least one independent retailer or manufacturer chart. Where the standard and the charts disagree — they do, by up to half a size — the conversion ships flagged as approximate. The full list with access dates lives on the sources & verification page.

What "approximate" means here

Every result that lands on a named size (US, UK, EU, MX, AU) carries an approximate flag. This is not hedging: it is the truth of the domain. Two brands' EU 42 can sit half a size apart; women's US has two competing scales; children's charts are rounded. Only conversions between physical units (mm, cm, inches) are exact. When your input falls on a boundary we show both candidate sizes rather than silently rounding — sizing up is usually the comfortable choice.

How to measure your foot

1. Do it in the evening — feet swell slightly during the day.
2. Stand on paper with full weight on the foot, and trace both feet.
3. Measure from the heel to the longest toe of each tracing (it is not always the big toe).
4. Use the larger of the two measurements with the cm converter.

A careful traced measurement is accurate to about 2–3 mm — smaller than half a shoe size, and more reliable than any label you already own.

Verification cadence and limits

Dataset version 1.0.0 (generated 2026-10-09) powers this site. Sources are re-verified on a six-month cadence; changes are logged, never silently applied. Known limits: brand-specific grading (Nike vs Adidas at the same EU number), width fitting, and last shapes are outside what any size number encodes — for exact brand fit, the brand's own chart is the final word, and Phase 3 of this project will add verified per-brand tables with visible provenance.

Testing

The engine ships with an automated suite covering golden values from published tables, round-trip stability, monotonicity (longer feet never map to smaller sizes), full system coverage, boundary determinism, kids/adult category isolation, and provenance integrity — a data entry without a source fails the build outright. Launch was gated on this suite passing.