Birkenstock to ASICS shoe size

The short answer

Birkenstock 40 is ASICS 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 ASICS. EU sizes on the left, US women's on the right.
Birkenstock	Birkenstock builds it for	ASICS, by chart	Same label	ASICS builds it for	ASICS also prints
35	not published	4 or 4.5	not published	foot length 21.5 cm	UK 2 · EU 34.5
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 cm	UK 4 · EU 37
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 25 cm	UK 6 · EU 39.5
40	not published	9 or 9.5	not published	foot length 25.8 cm	UK 7 · EU 40.5
41	not published	10 or 10.5	not published	foot length 26.5 cm	UK 8 · EU 42
42	not published	11 or 11.5	not published	foot length 27.5 cm	UK 9 · EU 43.5
43	not published	12 or 12.5	not published	foot length 28.5 cm	UK 10 · EU 44.5
44	not published	13	not published	foot length 29 cm	UK 11 · EU 46
48	not published	not made	not published	not published	not published

Tinted rows change label or sit between two sizes. 1 sizes have no counterpart on the ASICS 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.

Open in the converter Add an item you own

Sources

Birkenstock size guide, read on 2026-09-18.
ASICS size guide, read on 2026-09-17. ASICS on TrueToSize

ASICS to Birkenstock