How a boot fits is without question the most important factor of boot selection. While trekking boots and hiking shoes should fit the same as your street shoes, general and high alpine mountaineering boots are fit a little differently. A good fit correctly addresses the three dimensions of your foot.
Watch Mountaineering Boot Fit w/ Melissa Arnot and Peter Whittaker
Boot Fit Tips
Always Size Up - one-half to one full size. More room = more circulation = warm feet
Prevent Toe Bang - Half of mountain climbing is walking downhill. Make sure your toes do not touch the front of the boot. Test for this by lacing your boot up tightly and tapping your toe behind you. If your toes feel crunched against the end of the boot, size up.
Comparative Fit - They shouldn't fit like ski boots. Mountaineering boots are made for walking, so you should have a looser, roomy fit. Tight boots are a recipe for discomfort, decreased circulation and potential frostbite.
First Impressions - Have your sock system, lace up the boots and walk, stand sit, tap your toes and scuffle around for 15 minutes?
Brands and Models - If a boot fits too tight overall, you're likely in the wrong size. If the boot is pinching in some areas but is loose in other, your in the wrong brand or model. Boot brands used different lasts, even between different models.