When sitting on the Ball, your knees should be at a 90º angle or just slightly lower than the level of your hips. Following are some general guidelines based on your height; however, you may wish to choose a different size Ball depending on the proportion of your legs to your torso (i.e. longer legs may necessitate a larger Ball). If in doubt, choose a larger size Ball so that you may under-inflate it.
5’ to 5’6” 55 cm
5’7” to 6’2” 65 cm
over 6’2” 75 cm
Note: You may find it desirable to under-inflate your Ball in the beginning when it is the most firm. Over time your Ball will likely gain elasticity and become softer. When this happens, you may need to add more air to reach the preferred height while seated.