First of all, read listings CAREFULLY, as always.

This guide will cover conversions for Dr. Marten (Doc Marten) AND Birkenstock footwear AND both official links are in this guide--if you notice that the information doesn't match other guides, who are you gonna trust?  The manufacturer guide or an eBay guide?  Honestly.

Here is the link to the official Dr. Marten website, USA region (no, the sizing doesn't change from region to region):  http://www.drmartens.com/

UK--------3----4----5----6------7----8----9------10----11----12----13----14----15

US-Mens--4----5----6----7------8----9----10----11----12----13-----14----15----16

US-Women5----6----7----8------9---10----11----12----13----14----15----16----17

EURO----36---37---38--39/40--41--42---43/44--45----46----47----48----49----50

Notice that Dr. Marten sizing chart combines a couple of the Euro sizes, when you get into Mens size 7 and 10/Womens size 8 and 11--the site itself says that lining, shoe shape, etc. will change fit of shoe, so go by this guide for Dr. Martens and use the Birkenstock guide for Birks.  Birks are such a differently made shoe anyway, it only makes sense!

Here is a link to the official Birkenstock website, in English: http://www.birkenstockusa.com/how-to-fit

US Mens-------5-5 1/2----6-6 1/2----7-7 1/2----8-8 1/2----9-9 1/2----10-10 1/2----11-11 1/2----12-12 1/2----13-13 1/2----14-14 1/2----etc..

BIRKENSTOCK--38-------39--------39----------40---------41---------42------------43------------44------------45-----------46---------etc.

US Womens----2-2 1/2----3-3 1/2----4-4 1/2----5-5 1/2----6-6 1/2----7-7 1/2----8-8 1/2----9-9 1/2----10-10 1/2----11-11 1/2----12-12 1/2

BIRKENSTOCK--33-------34---------35---------36---------37---------38---------39----------40---------41-----------42-------------43--etc.

(they also give kids sizes at the website and Birkenstock also carries different widths!)

US Mens widths------D-EE----------B-C

BIRKENSTOCK---Regular-------Narrow

US Womens widths---C-D---------A-B

BIRKENSTOCK---Regular------Narrow

Here's more information from the Birkenstock site:

Once you get your Birkenstocks home

1. Remove the shoes from the box and gently flex them with your hands or on the floor. Birkenstocks often feel a bit stiff at first and this will help soften them and make them feel comfy sooner.

2. Unbuckle ALL of the straps and place your feet in the shoes. Make sure your heels are ALL the way back in the heel cup of the shoes. The contours of the footbed should correspond to your own arch and the toe bar should fit comfortably beneath your toes, where your toes meet your foot.

3. Before you stand up, look to see if there is adequate space for your feet (approx. ¼” for toes and heels, and 1/8” on the sides of your feet). You’ll want a little extra room because your feet will expand when you stand and walk.

4. Buckle the straps starting at your toes and working up. The straps should be a bit snug but not tight. The support will come from the footbed, not from tightly buckled straps. It’s just right if you can wiggle one finger between the strap and your foot.

5. Now take a walk around the house and see how they feel. They might feel a bit stiff. That’s normal. As soon as you start wearing them a bit they will start to soften and shape to your foot.

6. Once you feel comfortable with them around the house then go ahead and start wearing them outside. Make sure to get used to them slowly though. Allow your feet and your body to gradually get used to the new support, the straps and the buckles. We recommend a few hours for the first 5 or 6 wearings. After that you may never want to take them off.

The websites are also helpful research tools when buying/selling--lets you know retail price of new item.

Hope this helps!