When choosing hockey pants, there are two distinct styles to choose from - pants and girdles. The choice between the two styles is completely based on personal preference. Pants are the more traditional style, with a bigger fit and more volume, while girdles will fit tighter to your body.
Aspects of fitting both styles are mostly the same. You want to be sure that the belt is sitting up on your waist and above your hips. This will ensure that you have optimal movement and that your kidneys and lower ribs will be properly protected.
If possible, you'll want to try on any pants you are considering with your shoulder pads and shin pads on. This will give you the most accurate feeling of how they will fit in a game. As with anything else, you want to be sure the pants do not interfere with your shoulder pads, but also that you do not have any protection gaps. You want to test the movement of the pants, take some strides, squat, jump, dance, whatever you need to do to be sure you have the freedom of movement that you'll need on the ice.
Deciding the proper length is the first place where there is a real difference between a pant and girdle. With pants, when you stand upright the bottom of the pant should fall somewhere about a third to halfway down the knee cap of your shin guard. This slight overlap will ensure that you are fully covered when in stride. When your leg is bent, the pant is going to cover down to the top of your shin pad, again for the purpose of keeping you fully protected.
With a girdle, there is less movement in stride since it is tighter to the leg. There isn't enough room to overlap with the shin guard. For these reasons, the girdle will need to be fit slightly shorter. Without that extra play to fall over the shin guard, it becomes more important to be sure that the two pieces of gear are not interfering or displacing one another. If purchasing a girdle, you will also need a shell, or you will look pretty ridiculous out there.
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Check out this video on fitting hockey pants.