Arc'teryx to Under Armour clothing size

The short answer

Arc'teryx S is Under Armour SM by the two brands' own charts. 5 of 6 Arc'teryx sizes land on a different label at Under Armour.

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

Every Arc'teryx size read across to Under Armour. Letter sizes on the left, Letter on the right.
Arc'teryx	Arc'teryx builds it for	Under Armour, by chart	Same label	Under Armour builds it for	Under Armour also prints
XS	bust 33.1 in, waist 26.8 in, hip 36.2 in	XS	XS	bust 32.5 to 33.5 in, waist 25.5 to 27 in	US 0-2
S	bust 35 in, waist 28.7 in, hip 38.2 in	SM	not published	bust 33.5 to 36 in, waist 27 to 29 in	US 4-6
M	bust 37 in, waist 30.7 in, hip 40.2 in	MD	not published	bust 36 to 38 in, waist 29 to 31 in	US 8-10
L	bust 40.2 in, waist 33.9 in, hip 43.3 in	LG	not published	bust 38 to 41 in, waist 31 to 34 in	US 12-14
XL	bust 44.1 in, waist 37.8 in, hip 47.2 in	XXL	XL	bust 44 to 47 in, waist 37 to 40 in	US 18
XXL	bust 48 in, waist 41.7 in, hip 51.2 in	3XL	XXL	bust 47 to 50 in, waist 40 to 43 in	US 20

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

Arc'teryx size guide, read on 2026-09-18.
Under Armour size guide, read on 2026-09-18. Under Armour on TrueToSize

Under Armour to Arc'teryx