Choosing a Swiss ball -- also known as a stability, exercise, fitness or balance ball -- to fit you is a slightly more complicated undertaking than you might expect. How you plan to use your ball, your fitness level, your overall height, the length of your legs and your comfort in various positions on the ball are all considerations in finding the right size.
A properly inflated Swiss ball feels firm and compresses approximately 4 to 6 inches beneath your body weight. An under-inflated ball compresses further and feels squishy or soggy beneath your body weight. An under-inflated ball is less stable than a properly inflated ball because it’s more difficult to find your balance on a ball that has more surface contact with the floor. Conversely, an over-inflated ball feels drum-tight or taut and compresses very little under your body weight. No matter how you use your ball, it should be properly inflated. For example, don’t make a ball that’s otherwise too large for you more your size by slightly deflating it.
Ball Sit Test
Generally, when you sit on a firm Swiss ball with your feet flat on the floor your knees should form a 90-degree angle. This is a particularly good rule of thumb if you usually use a different ball each time you train at the gym. Ideally, your hips should also form a 90-degree angle so that your thighs are parallel to the floor and your knees and hips are level. If you can’t find a ball that places your hips level with your knees, opt for one that puts your hips slightly higher than your knees rather than one that sits them lower. Keeping your hips slightly higher is easier on your knees.
Size By Height
Swiss ball manufacturers use your height as a primary method for recommending the correct size for you. If you are under 4-foot-6, you should probably use a 12-inch ball. If your height is between 4-foot-6 and 5 feet, try an 18-inch ball. If you’re between 5-foot-1 and 5-foot-7, use a 22-inch ball, and if you’re between 5-foot-8 and 6-foot-2, you should use a 26-inch ball. At 30 inches, the largest ball is made for exercisers over 6-foot-2. Considering only your overall height doesn’t take into account your leg to torso ratio -- if you have particularly long or short legs, you might need to go up or down a size.
Match Size to Use
If you don't use your ball for different kinds of activities, you can allow its use to partially guide size selection. For example, if you use the ball as a chair, it’s most important that your knees form a 90-degree bend and your hips are level or slightly higher than your knees. If you mainly incorporate the ball in flexibility training, you might be better off with a slightly larger size. You can drape yourself more comfortably over a slightly larger ball. For strength-training workouts, you might want to consider a slightly smaller ball. A Swiss ball that’s on the small side allows for greater range of motion during exercises that require you to move the ball.