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Altra to Under Armour shoe size

The short answer

Altra 6 is Under Armour 5 by the two brands' own charts. 4 of 13 Altra sizes land on a different label at Under Armour.

Both brands publish the foot length each size is built for, so every row matches a foot, not a label.

On 3 sizes the charts land two or more sizes from the same label. Brands do not measure the body the same way, so on those rows treat the chart answer and the same label as the two ends of a range.

Every Altra size read across to Under Armour. US women's sizes on the left, US women's on the right.
AltraAltra builds it forUnder Armour, by chartSame labelUnder Armour builds it forUnder Armour also prints
4foot length 20.5 cmnot madenot publishednot publishednot published
4.5foot length 21 cmnot madenot publishednot publishednot published
5foot length 21.5 cmnot made5not publishednot published
5.5foot length 22 cmnot made5.5not publishednot published
6foot length 22.5 cm56foot length 22.5 cmUK 3 · EU 35.5
6.5foot length 23 cm5.56.5foot length 23 cmUK 3.5 · EU 36
7foot length 23.5 cm67foot length 23.5 cmUK 4 · EU 36.5
7.5foot length 24 cm77.5foot length 24 cmUK 5 · EU 38
8foot length 24.5 cm88foot length 24.5 cmUK 6 · EU 39
8.5foot length 25 cm8.58.5foot length 25 cmUK 6 · EU 40
9foot length 25.5 cm99foot length 25.5 cmUK 6.5 · EU 40.5
9.5foot length 26 cm9.59.5foot length 26 cmUK 7 · EU 41
10foot length 26.5 cm1010foot length 26.5 cmUK 7.5 · EU 42
10.5foot length 27 cm10.510.5foot length 27 cmUK 8 · EU 42.5
11foot length 27.5 cm1111foot length 27.5 cmUK 8.5 · EU 43
11.5foot length 28 cm11.511.5foot length 28 cmUK 9 · EU 44
12foot length 28.5 cm1212foot length 28.5 cmUK 9.5 · EU 44.5
13foot length 29.5 cmnot madenot publishednot publishednot published
Tinted rows change label or sit between two sizes. 5 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

Under Armour to Altra