For a life jacket to work properly, not only does it need to be on your body, it should fit right as well. Not only is a properly fitting life jacket more comfortable (meaning your more likely to wear it) but it functions better as well. Life jackets that are too small or large can ride up when your in the water - or even come off. 

It's easy to check a conventional life jacket for fit - just put it on. The jacket should fit you snugly with all zippers, straps, ties and snaps correctly secured. Ease yourself into the water or walk into water up to your neck. Lift your legs and tilt your head back, in a relaxed floating position. Your mouth should be out of the water and you should float comfortably without any physical effort. If the life jacket rides up, try securing it tighter to your body. If it still rides up, you may need a different style. You should be comfortable and able to swim without significant restriction. You should have someone else to act as a lifeguard or assistant if you are uncomfortable with being in the water or are trying this activity for the first time.

You can test an inflatable life jacket for fit too. If you do not wish to test the CO2 inflation system, remove the CO2 cylinder (and if the device has an automatic feature, remove the water-sensing element). Put your life jacket on and fully inflate it. Then test it like a conventional life jacket. Because of the design, ride-up is generally not an issue with inflatables. The amount of buoyancy provided with inflatable life jackets will probably require the user to swim using some form of side or backstroke, as it will be difficult to swim on your stomach when the jacket is inflated.

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