La Sportiva shoes are built on European half sizes which are smaller increments than US half sizes. This gives us four extra sizes per size run, which gives you a better fit! More sizes = Better fit!
LA SPORTIVA FOOTWEAR SIZE CONVERSION CHART
A general rule dictates that soft shoes require a tighter fit, two to three sizes smaller than your street shoe size. We recommend you fit your trad shoes one to two sizes under your street shoe size. A thick midsole creates a platform on which you stand, while a slip-lasted slipper-type shoe relies on your crammed toes to create an edging platform.
Once you determine your needs, try on several boots in that genre. Different last types fit different feet -- that's why we make lots of lasts. If it feels good, it probably is. Remember to factor in stretch. Unlined slip-lasted shoes tend to stretch a bit more than lined shoes -- about one full size. Lined shoes stretch about a half size. Remember: a well-fit comfortable shoe will work better than an ill-fitting tighter shoe.
La Sportiva's diverse last families have been specifically designed to conform to the widest possible range of both foot shape and climbing use. We constantly strive to revolutionize climbing shoe development and to provide every individual absolute freedom of choice in finding a shoe that is perfect for both their foot AND their climbing style.
And you thought buying a new car was tough... Getting the perfect fit in a pair of hiking boots is rarely as simple as relaying your size. Most good boot fitters will measure your foot on a Brannock device. Overall length is actually a minor element of boot fitting. In the ideal world, our left foot would be a mirror image of our right, while the ball of our foot would line up exactly with the last-maker's idea of the perfect foot. This is almost never the case because other factors such as heel to ball length, volume, and width are critical in gaining a perfect fit. A good salesperson will address each of these issues.
To use our size chart to fit mountain boots, find your American size then go across to your European size, and then add one half size. When trying on boots make sure to bring the socks you would normally wear and any orthotics or custom footbeds you use. Another note: our shoes are handmade, and you can experience minor variations in sizing from pair to pair in a given size. What you’re looking for is the size that holds your toes ever so slightly away from the front of the boot while the lacing system is snug across the top of your arch. To check the length, you should be able to slip one finger behind your heel when you are standing up with the boot unlaced and your toes touching the front. The fit across the top of your arch is the single most important factor. If the lacing system here does not hold your foot in place, the boot will not be stable on your foot while you walk. If the boot is too short, your toes will get smashed walking down hill. Too long and your heel will slide up and down and give you blisters. Be patient, ask questions and make sure you’ve got the right size and the right fit and the right boot before you hit the trail.
For more information, check out: https://www.sportiva.com/sizing-technology/