What we want to measure is the space behind your heel and the shell. If you have more than 2cm (a little less than an inch) behind your heel your ski boots are too big. If you have 1.5 - 2cm of space behind your heel it's the right size.

Ski boots should be as tight fitting as possible. You need minimal foot movement inside the boot. If your boot moves with your foot, your ski will move with your boot and in turn your foot. Allowing you to exert maximum control over your skis.

They should feel snug, but not so tight it's painful. The right ski boot fit holds your feet in place and allows your skis to rest neutral, or flat, on the snow when standing. If you have heel slippage, blisters, shin bang, or you find yourself curling your toes when going downhill, then your ski boots are too big.

If your ski boots are new, they may feel a bit tight. The foam liners of your ski boots will compress and "pack out" over time.