Whistles to Gap clothing size

The short answer

Whistles 12 is Gap 8 by the two brands' own charts.

Both brands publish the body bust each size is cut for, so every row matches a bust, not a letter or a number.

Every Whistles size read across to Gap. UK sizes on the left, US on the right.
Whistles	Whistles builds it for	Gap, by chart	Same label	Gap builds it for	Gap also prints
04	bust 30 in, waist 23.5 in, hip 34.5 in	000	not published	bust 30.3 in, waist 24 in, hip 33.3 in	Letter XXS
06	bust 31 in, waist 24.5 in, hip 35.5 in	00	not published	bust 31.3 in, waist 24.8 in, hip 34.3 in	Letter XXS
08	bust 32 in, waist 25.5 in, hip 36.5 in	0	not published	bust 32.3 in, waist 25.5 in, hip 35.3 in	Letter XS
10	bust 34 in, waist 27.5 in, hip 38.5 in	4	not published	bust 34.3 in, waist 27.5 in, hip 37.3 in	Letter S
12	bust 36 in, waist 29.5 in, hip 40.5 in	8	not published	bust 36.3 in, waist 29.5 in, hip 39.3 in	Letter M
14	bust 38 in, waist 31.5 in, hip 42.5 in	10 or 12	not published	bust 37.3 in, waist 30.5 in, hip 40.3 in	Letter M
16	bust 40 in, waist 33.5 in, hip 44.5 in	14	not published	bust 40.3 in, waist 33.5 in, hip 43.3 in	Letter L
18	bust 42.5 in, waist 35.5 in, hip 46.5 in	16	not published	bust 42 in, waist 35.5 in, hip 45.3 in	Letter XL
20	bust 45 in, waist 38 in, hip 49 in	20 or 18	not published	bust 45.8 in, waist 39.5 in, hip 49.3 in	Letter XXL

Tinted rows change label or sit between two sizes. Read from the two charts only; no formula and no averaging.

What wearers say

Charts describe a garment, not a fit. We do not collect fit reports for this category yet, so this table is the two brands' own claims and nothing more.

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Sources

Whistles size guide, read on 2026-09-18.
Gap size guide, read on 2026-09-18. Gap on TrueToSize

Gap to Whistles