Sizes methodology

The Sizes cluster covers three body-fit categories — shoes, bras, rings — each with its own regional conventions and own anchor measurement. The underlying physical quantity is exact (foot length, band circumference, finger diameter); the regional labels on top are arbitrary and vary by manufacturer.

Shoes — Brannock-derived

Our shoe-size converter is calibrated against the Brannock device, the metal foot-measuring tool every US shoe store uses. The Brannock translates foot length in barleycorns (an English unit equal to ⅓ inch, in use since the 14th century) into US sizes; each increment is one barleycorn. Mondopoint (the international system) uses millimeters of foot length directly — a Mondopoint 270 is a 270 mm foot.

We store sizes as canonical foot-length-in-mm and translate to US, UK, EU, JP labels at display time. Men and women have separate scales because US shoe sizes for the two genders differ by ~1.5 sizes at the same foot length, for historical reasons that no longer have a fitting rationale.

Bras — modern band rounding

The pre-1990 “+4/+5 rule” (add 4 to your underbust for even-band sizes, 5 for odd) systematically produced loose bands because it was calibrated to corsetry- era materials. The modern method: round your underbust measurement to the nearest even inch — that’s your band. Cup is derived from the inch-by-inch difference between bust and band (1 inch = A, 2 = B, etc., up to K).

The crosswalk to UK and EU sizes is a lookup table — UK keeps adding letters past D (DD, DDD, E, F, FF, G, GG…) while the US skips around (D, DD, DDD/E, F, G), so the letter equivalence isn’t algebraic. The table in our lib matches published crosswalks from major retailers (Nordstrom, Bravissimo).

Rings — inner diameter as the universal anchor

Every regional ring sizing system is a label on inner diameter, the inside-edge straight-line width of the band:

• US: integer + half sizes (3, 3.5, 4, 4.5, …) increasing in roughly 0.4 mm steps.
• UK: letters with halves (F, G, H, I, J ½, K ½, …) on the same diameter scale, no algebraic relation to US.
• EU and JP: inner circumference in millimeters, equal to π × inner diameter.

Our table covers US 3 to 13 (14 mm to 22.2 mm inner diameter) and emits the matching UK / EU / JP labels. Wide bands (over ~6 mm) fit more snugly than thin bands at the same nominal size — see our ring sizing guide for the adjustment.

The universal caveat: brand variance

Even within one region, the same nominal size from two brands can vary by half a size or more. This is true in all three categories. The conversions on this site are accurate between systems; matching a number to a specific manufacturer is a separate problem we don’t try to solve.