Birkenstock to Under Armour shoe size

The short answer

Birkenstock 40 is Under Armour 9 or 9.5 by the two brands' own charts.

At least one of the two brands publishes no foot length, so the rows line up the US size both charts print. Treat it as the label conversion, not a fit.

Every Birkenstock size read across to Under Armour. EU sizes on the left, US women's on the right.
Birkenstock	Birkenstock builds it for	Under Armour, by chart	Same label	Under Armour builds it for	Under Armour also prints
35	not published	not made	not published	not published	not published
36	not published	5 or 5.5	not published	foot length 22.5 cm	UK 3 · EU 35.5
37	not published	6 or 6.5	not published	foot length 23.5 cm	UK 4 · EU 36.5
38	not published	7 or 7.5	not published	foot length 24 cm	UK 5 · EU 38
39	not published	8 or 8.5	not published	foot length 24.5 cm	UK 6 · EU 39
40	not published	9 or 9.5	not published	foot length 25.5 cm	UK 6.5 · EU 40.5
41	not published	10 or 10.5	not published	foot length 26.5 cm	UK 7.5 · EU 42
42	not published	11 or 11.5	not published	foot length 27.5 cm	UK 8.5 · EU 43
43	not published	12	not published	foot length 28.5 cm	UK 9.5 · EU 44.5
44	not published	not made	not published	not published	not published
48	not published	not made	not published	not published	not published

Tinted rows change label or sit between two sizes. 3 sizes have no counterpart on the Under Armour chart. Read from the two charts only; no formula and no averaging.

What wearers say

Charts describe a garment. People who own both brands describe a fit, and their answer ranks above this table once five of them stand behind it. Pick your size in the converter to see whether there are enough yet.

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Sources

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

Under Armour to Birkenstock