A small difference in exercise ball sizes can greatly affect your workout because during exercise you are using them in various positions. While smaller balls can help improving the effect of certain exercises and if you have a bad back a larger ball can provide support to your body weight, choose the right ball depends on your personal height and arm size matters a lot.
In using your height in determining the right exercise ball sizes, the equipment should be level or slightly above the knees. The best approach in sizing up the exercise ball is by sitting on it. When doing so, bend your knees at a 90-degrees angle, and your thighs shoud be level with the floor. Try to locate your height and check which size you should try first.
This simple chart will help determine the right exercise ball size depending on your height.
| Height | Recommended ball size |
| Under 4'8" | 45 cm |
| 4'8'' to 5'3'' | 55 cm |
| 5'4'' to 5'10 | 65 cm |
| 5'10 to 6'4 | 75cm |
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The best determination of whether the new ball is of appropriate height is on a sitting position. At an angle of 90-degrees, your hips are level or slightly higher than the knees. When the exercise ball is too small, you might compromise the position of your hips and pelvis when sitting on it. On the other hand, a ball that is too large will not provide opportunities for your feet to touch the ground, putting you in an unstable position. This is exactly why it is very important to get the right ball size to have the right workout.
There is another approach to determining right exercise ball sizes is to calculate the length of your arms. Since the length of your leg may vary from that of other people, the arm span can accurately measure the right ball for you.
If the exercise ball will be used for several aerobic exercises and need to hold onto it, the arms cam also provide a better fit for your situaation. However, using this methhod might be a bit tricky (and require more time) as most people usually have more knowledge of their height than their arm span.
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If you prefer arm span to determine the right exercise ball for you, use this chart to help you find the right size.
| Arm Length | Recommended ball size |
| 22" to 25-1/2" | 55 cm |
| 26" to 31-1/2" | 65 cm |
| 31-3/4" to 35-1/2" | 75 cm |