How to Use the Stability Ball Size Calculator
Stability balls (exercise balls) come in diameters ranging from about 45cm to 85cm, and picking the wrong size for your body makes core work and stretching less effective — and your form tends to break down. This calculator uses the height-based sizing chart commonly published by fitness equipment brands to recommend the ball diameter that best fits you.
The guiding principle is that your knee angle should sit around 90 degrees while seated on the ball — hips level with your knees, or slightly higher. Getting that angle right makes it much easier to engage your core and hold your balance, and it keeps strain off your lower back. Shorter people need a smaller ball to hit that angle, while taller people need a larger one.
Because a stability ball's actual diameter shifts somewhat depending on how much air you put in it, it's worth sitting on your new ball slightly under-inflated at first and adjusting the air level while checking your knee angle. If your result falls right on the border between two sizes, use the FAQ below to decide based on how you plan to use it.
Frequently Asked Questions
The safest, most stable position on a stability ball is when your hips are level with or slightly higher than your knees (about a 90-degree knee angle). Getting that angle right depends on your leg length, which tracks closely with height — hence choosing a ball diameter based on height.
For core and balance work that needs finer control, size down. For stretching or comfortable seating, size up. You can also fine-tune the actual diameter somewhat by how much air you put in the ball.