Your upper body should be vertical with your ears, shoulders and pelvis aligned. Don’t lean in any direction to avoid creating a counterbalance.

If the ball fits your body but compresses more than 6 inches beneath you, it’s not the right size ball for you; it’s a larger ball that’s been under-inflated. You can exercise on such a ball, but may find that the extra-soft ball feels awkward and makes balancing too easy. Don’t over-inflate a smaller ball to make it fit your body; the extra pressure makes it more likely to burst. When fully inflated, you should be able to press 2” into the fitness ball with your finger. Fitness balls lose pressure with use and age. As your ball ages, you may need to inflate it more often.

You may be able to successfully use a smaller or larger sized fitness ball as you gain experience. The size of the exercise ball depends on your height and the exercises you plan to do with it. [2] X Expert Source Katie DunlopCertified Personal Trainer & Sports Nutritionist Expert Interview. 25 August 2021. Usually, if you are between 5 feet to 5 feet 9 inches tall, you should use a ball around 55cm. [3] X Expert Source Katie DunlopCertified Personal Trainer & Sports Nutritionist Expert Interview. 25 August 2021. When you do a dumbbell press on an exercise ball, ensure that your feet can touch the ground firmly. [4] X Expert Source Katie DunlopCertified Personal Trainer & Sports Nutritionist Expert Interview. 25 August 2021.

For example, if your ball has a circumference of 172 centimeters, its diameter is 172 / 3. 14 = 55cm. Your exercise ball may stretch up to 24 hours after you inflate it. Measure your fitness ball when it’s fully inflated to ensure it’s the right size for your body.

The most common ball sizes, translated to centimeters and rounded up to the nearest inch, are: 45cm = 18 inches, 55cm = 22 inches, 65cm = 26 inches, and 76cm = 30 inches.

If you don’t know the ball’s intended diameter but need to know its actual size, place the ball against the wall. Position the box so that it just touches the opposite side of the ball. Then remove the ball, and measure from the box to the wall to calculate the ball’s diameter.

The diameter of your fitness ball is the same as its height. [5] X Research source