Fitness Ball Size

European physical therapists first used fitness balls — also known as stability, exercise or Swiss balls — in the mid-1960s as rehabilitative aids for their patients. By the early 1990s, they had become a common piece of gym equipment in the United States. The ball brings an element of instability to your workout, which activates your core, even when you’re targeting other muscles. The ball also facilitates a wide range of motion because it moves freely and smoothly.

Size by Height

Most fitness ball manufacturers make them in five sizes. You can use your height as a general guideline for finding the size that best fits you. If you’re 4 feet 6 inches tall or shorter, a 12-inch ball is probably the right size. An 18-inch ball suits those who are 4 feet 7 inches to 5 feet tall, a 22-inch ball is made for those who are 5 feet 1 inch to 5 feet 7 inches tall, and a 26-inch ball fits those who are between 5 feet 8 inches and 6 feet 2 inches tall. If you’re taller than 6 feet 2 inches, the largest ball, which measures 30 inches, is likely to be your best fit.

Size by Arm Length

Because leg and torso lengths can vary, you can instead use your arm length to help determine the right size ball. This method is less applicable to the shortest and tallest exercisers, and is more useful for those who fall close to one size or another. Measure your extended arm from your longest fingertip to the top of your shoulder. People whose arms measure 19 inches to 21.5 inches long should use an 18-inch ball; those whose arms measure 22 inches to 25.5 inches long should use a 22-inch ball; and those whose arms measure 26 inches to 31.5 inches long should use a 26-inch ball.

Size by Fit

Sitting on the ball is the best way to gauge which size is right for you. A properly inflated ball is firm, not drum-tight. When you sit on it, your weight should make a slight indentation, without changing the ball’s shape . Your feet should lie flat on the floor, your knees should form a 90-degree angle and your thighs should be level. If your feet aren't flat on the floor, the ball is too large and therefore less stable. If your hips sit lower than your knees, the ball is too small and could compromise your form.

Size by Activity

In some cases, you may need a ball that’s slightly bigger or smaller than the one indicated by your height, arm span or a sit test. If you’re going to use the ball exclusively as a stretching aid, a larger ball is better for draping your body. If you’re using it to strength train, a slightly smaller ball is harder to control and provides a bigger challenge. Seniors, pregnant women and those with knee problems may want a slightly larger ball, because it’s easier to stand up if your hips are a little higher than your knees.