When you sit on the ball, both feet should be flat on the floor, and your knees should be bent at 90-degree angles.

Before you put your gym’s stability balls to use or buy your own, make sure you have one that’s the right size.

Your height and, to a certain extent, your weight will determine the size of ball you need. The right size exercise ball is vital to avoid causing pain or injury. For exercise ball exercises that require you to kneel or stand on the ball, a larger ball is better. Choosing a Size. It is made of plastic vinyl and elastic, filled with air, and most are burst-resistant. A yoga ball is sometimes called a fitness ball, an exercise ball, a stability ball, or a Swiss ball. Sit on the ball firmly and note the bends at Using an Exercise Ball as an Office Chair. When sitting on the exercise ball, knees and hips should be at 90 degrees. To get the most from a bouncy fitness routine, make sure to choose the correct size stability ball. The size of the exercise ball is by far your most important consideration. Before you put your gym’s stability balls to use or buy your own, make sure you have one that’s the right size. For exercise ball exercises that require you to kneel or stand on the ball, a larger ball is better. Size. For example, a 55-cm ball is the right size for people of a height ranging from 5'0" to 5'5". If your desk height is 29-30 inches and you are of average height and weight, the 65cm ball is the preferred size. The American Council on Exercise, or ACE, a non-profit fitness organization, recommends choosing a 55 cm ball if you are between 5 feet, 1 inch and 5 feet, 7 inches tall. Select the size of an exercise ball based on your height. A user whose weight-to-height ratio is higher than average will cause greater compression to the ball when they are seated. Exercise balls come in many different sizes. This is the minimum ball size--some people prefer a larger ball, where hips are higher than knees. However, consideration must be given to the height of the chair seat you will be replacing — keeping in mind the ball will sink down when you sit on it.