Sizing is definitely out. Measure my daughter at 50cm, expected the 52 to be a little big, but still OK. Was in fact too small, ordered the 54cm instead thinking that would be plenty big enough given her measurements and the sizing if the 52cm hat. But no, that was also to small. At that point I opted to go to my local saddlery to get her measured again. No surprise they agreed with my measurements and fitted her a 51cm hat, which fits perfectly.Ordering this has cost me £7.98 in returns when actually it is the hat sizing that is out and not my ordering. I would not recommend this product, I can only assume they are seconds which have been sized incorrectly.After reading several reviews on this product, I decided to order a bigger size than my head is (I'm usually 54cm) and ordered 56cm at first. This was far too small so I had to return but then got the next size up, 57cm, which was about £30 more expensive for some reason. This one I have now does fit but looks strangely thick in my head unlike the one that was too small. Overall the product is good quality and has the necessary safety requirements but just be warned you should definitely order up!I was checked at my local equestrian centre and this helmet and size were correct. Unfortunately there seems to be a discrepancy in sizing as this is way too small. Not worth the hassle, will order elsewhereI was worried about the size being too small from what the other reviews had said, but I decided to go for a 58cm hat as it was the cheapest option by about £30 at the time, and that's what my head size measures in at. It arrived today, and the hat fits perfectly! I think if you are nervous about the sizing not being right, see if you can get fitted by a specialist with this type of hat to see what size fits you best.I'm so happy with the overall product, and I'm glad my worry was for nothing.Great fit, great price{'the black one was £60 dearer than the blue} plus conforms to the EU 'fun police' standard.Delighted with this bunnet and would buy again from the sellers. Had fun with friends using the Amazon Locker for the first time, great idea and will use the locker again.