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.

Algorithm details: the three lookup paths

Each category resolves the user’s input through a category-specific algorithm rather than a single shared function — the anchor quantities are physically different (length, circumference, diameter) and reducing them to a common abstraction would lose information that matters at the boundary cases.

Shoes — barleycorn arithmetic on a Mondopoint store

Internal storage is foot length in millimetres (the Mondopoint quantity, ISO 9407). The US size derives from the Brannock-published table by inverting the relation us_size = (foot_length_mm − base_mm) / 8.467 mm where 8.467 mm is one barleycorn (⅓ inch × 25.4) and base_mmdiffers by gender — 213.36 mm for the US men’s 1, 211.67 mm for the US women’s 1. UK sizes use the same barleycorn step but a base 0.5 sizes below the US men’s scale; EU sizes use the Continental Paris-point system (1 point = 6.667 mm) on the same foot-length input. Half-sizes are emitted only when the residual is within ±0.25 barleycorn of the half-size line; otherwise the converter snaps to the nearest whole size to match Brannock device behaviour.

Bras — band rounding and cup arithmetic

Internal storage is two quantities: underbust circumference in millimetres (band) and bust circumference in millimetres (cup derivation). The modern algorithm: band_in = round_to_nearest_even(underbust_in); cup_letter = cup_table[bust_in − band_in] where the lookup table is the published sister-size grid (US: A, B, C, D, DD, DDD/E, F, G, H, …; UK: D, DD, E, F, FF, G, GG …). The band-rounding step is what produces sister-size adjacency: a 34D and a 32DD have the same cup volume but different band tension.

Rings — diameter as the single anchor

Internal storage is inner diameter in millimetres. US labels derive from the diameter via us_size = (diameter_mm − 11.63) / 0.4064 (the published 0.0157 inch per half-size step). UK letters come from a lookup table because the UK series isn’t algebraically continuous with the US — A starts at 12.04 mm and each subsequent letter adds 0.4064 mm with no skip. EU and JP labels are inner circumference = π × diameter rounded to the nearest millimetre.

Sources & references

See the Sources & referencesblock at the foot of this page for the primary ISO standards (ISO/TS 19407, ISO 9407, ISO 8653) and the manufacturer references (Brannock Device Co., Nordstrom) that calibrate every lookup in this methodology. Where the ISO and retailer figures diverge — primarily for half-size rounding and cup-letter notation past D — we follow the retailer convention because that’s what users see on labels.

Assumptions & limitations

• One anchor measurement per category. We assume the user provides — or the converter derives — the single physical quantity (foot length, underbust circumference, finger diameter). Width (D/EE/EEE in US shoe), cup depth nuance, and ring band thickness are not modelled.
• Adult sizes only.Children’s shoes use a different US/UK base (US 1 youth ≠ US 1 men), and infant ring sizing has its own conventions. The converter doesn’t expose a children’s mode.
• Symmetric feet/hands assumed. Foot length asymmetry of 4-6 mm between left and right is common; the converter outputs one size, leaving the user to pick the larger foot as the reference.
• No half-size emission for rings under US 4 or over US 12.The published US ring table doesn’t define half sizes outside this range; we snap to whole sizes there.
• Wide-band ring adjustment is not applied automatically. Bands wider than ~6 mm fit tighter at the same nominal size; our ring sizing guide walks through the manual +¼ to +½ size adjustment.
• No manufacturer-specific calibration.The converter outputs the regional canonical size, not a brand-specific size — see the brand variance section below.

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.