If you are considering buying a pair of Birkenstock sandals ... stop, you should consider 2 things before buying. These are very generously sized and in fact most people in the UK are fitted between 1 and 2 sizes smaller than they normally buy. If you do buy your normal size in these shoes they will flop around your feet and be quite unbearable, you should go and try on a pair and get them measured properly before impulse buying, once you have the correct sizing go ahead and buy them online. However there is also another important consideration and this will save you a lot of wasted money; buying 2nd hand (previously worn) Birkenstocks is NOT the best idea, these sandals mould to the wearers feet (ask anyone with a pair to show you the insides and you will see what I mean). So when you buy a previously worn pair the chances are that they will not mould to the shape of your own foot.
I own several pairs of Birkenstocks, I normally wear a roomy size 7 and all my Birkenstocks are size 5, fit wonderfully and are a real joy to wear. I hope to save people the expense of buyng a pair of shoes before considering all the facts - hope it helps.