Reiss to Under Armour clothing size

The short answer

Reiss 4 is Under Armour XXS 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 Reiss size read across to Under Armour. UK sizes on the left, Letter on the right.
Reiss	Reiss builds it for	Under Armour, by chart	Same label	Under Armour builds it for	Under Armour also prints
4	bust 31 in, waist 24 in, hip 34 in	XXS	XS	bust 31 to 32.5 in, waist 24.5 to 25.5 in	US 00
6	bust 32 in, waist 25 in, hip 35 in	XXS	XS	bust 31 to 32.5 in, waist 24.5 to 25.5 in	US 00
8	bust 33 in, waist 26 in, hip 36 in	XS	not published	bust 32.5 to 33.5 in, waist 25.5 to 27 in	US 0-2
10	bust 35 in, waist 28 in, hip 38 in	SM	not published	bust 33.5 to 36 in, waist 27 to 29 in	US 4-6
12	bust 37 in, waist 30 in, hip 40 in	MD	not published	bust 36 to 38 in, waist 29 to 31 in	US 8-10
14	bust 39 in, waist 31.5 in, hip 42 in	LG	not published	bust 38 to 41 in, waist 31 to 34 in	US 12-14
16	bust 41 in, waist 33.5 in, hip 44 in	LG	not published	bust 38 to 41 in, waist 31 to 34 in	US 12-14
18	bust 43 in, waist 36 in, hip 46.5 in	XL	not published	bust 41 to 44 in, waist 34 to 37 in	US 16

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

Reiss size guide, read on 2026-09-18.
Under Armour size guide, read on 2026-09-18. Under Armour on TrueToSize

Under Armour to Reiss