Ski boot sizes on this page are UK 4 and 4.5 which for most brands is the same as MP 23 and 23.5 or Europe EU 37 and 37.5 or USA Men's 5 and 5.5 or USA Women's 6 and 6.5. MP refers to 'Mondo Point' and is the world system for shoe sizing. MP 23 means the foot-bed liner is exactly 23cm long. MP 23.5 is 23.5cm and so on. We recommend checking the length of the foot-bed liner on your current ski boots or on a pair of trainers if you are unsure of your size.