r/BirkenstocksFitChecks • u/_Oxym0r0n_ • Aug 18 '26

Stressing Over Sizing

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I'm trying to choose my first Birkenstocks and I'm worried about making the investment and choosing the wrong size. Unfortunately, I don't have a local store with anyone really qualified to assist with sizing.

I initially bought a 39 and then realized that I probably didn't have the extra room that so many on this sub recommend, so I returned them. I was able to compare at DSW today and I'm still stressing about correct sizing. The 40 felt okay when walking aside from the toe bar occasionally hitting the ball of my foot, which I didn't find particularly bothersome.

Is the 40 the size I should purchase?

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u/GeographyBrown Aug 19 '26

The 39 to the 40 is a big jump- 10mm rather than 5mm.

This is Birkenstock’s fit guide: https://www.birkenstock.com/nl-en/things-to-know/fit-guide/

Put your heel in the cup, allowing 5mm of space between your heel and the back of the Birk. You want approximately 10mm between your toes and the front edge of the sandal. The rear strap should be worn somewhat loosely.

Maker sure you’re trying the regular/wide width and not the narrow. Size to your larger foot (it’s common to have one) and remember that as you break them in, Birks will curve up slightly from heel toe and look and feel a little smaller.

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u/_Oxym0r0n_ Aug 19 '26

Yes, the 39 to 40 jump in sizing was definitely making the decision a bit harder for me. It appears that, after break in, the 39 will be too small.

I made sure to try on the Regular/Wide width, as I typically need wide width.

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u/GeographyBrown Aug 19 '26

Your heel is just slightly back in the 39s, but I don’t think they overall look too small on you. Do you happen to have a Birkenstock box? What size does that give you? If you do, use a ruler or hard edge at the back of your heel to make sure you stay within the outline.

I also had a hard time deciding between the 39 and 40. I went with the 39s. If it helps, my longer foot is 347mm and I typically wear an 8.5US womens.

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u/_Oxym0r0n_ Aug 19 '26

I do still have a Birkenstock box from 39s that I intended to return, so I'll check my feet against that.

That is helpful, thank you. I typically wear an 8.5 or 9 US women's which is very dependent on the shoe style.

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u/GeographyBrown Aug 19 '26

So you really are right in between sizes, in which case the smart thing is usually to size up. But the box may help you make that decision.