What Size Exercise Ball Do I Need?

The short answer

For a full-size stability ball, choose by your height: 55 cm if you are up to about 5 foot 6, 65 cm from there to about 6 foot, and 75 cm above that. The target is simple — sitting on the inflated ball with your feet flat, your hips should be level with or very slightly above your knees.

For a mini pilates ball, height is irrelevant. Those are sized by what you do with them: 9 inches is the general-purpose choice, 7 inches suits smaller frames and tighter placements between the knees or under the neck. The ranked options for both are in the ball roundup.

Stability ball size by height

These are the bands the major makers publish, and they agree with each other closely. Two situations push you up a size rather than down. If you are heavier, the ball compresses more under load and effectively sits lower, so the next size up frequently fits better. And if the ball is for sitting at a desk rather than for exercise, the larger of two adjacent sizes is usually the more comfortable choice.

One situation pushes you down: if you want the ball mainly for floor work — hamstring curls, planks with forearms on the ball, stretching over it — a smaller ball is easier to control and the height table matters less.

The sitting test

Height tables are a starting point and your proportions are not the average. The test that actually settles it takes ten seconds with the ball fully inflated.

Sit on top with your feet flat on the floor and roughly hip-width apart. Look at your thighs. They should be level with the floor or sloping very slightly downward from hip to knee, with your knees at about a right angle. If your knees are higher than your hips, the ball is too small. If you are perched and reaching for the floor with your toes, it is too big.

The reason the test beats the table is leg length. Two people of identical height can differ by several inches in inseam, and the ball cares about your legs rather than your total height. If you are between sizes and you have long legs for your height, size up.

Remember that a ball sitting in a box is smaller than a ball in use. Most need a day to settle after the first inflation, and almost all of them are inflated in two stages — part way, left to stretch, then topped up. Judging the size before that has happened is how people conclude they bought the wrong one.

Mini pilates balls are a different question

A mini ball is not a small stability ball. It is a soft, deliberately under-inflated ball used as support and as a destabilizer, and the sizes are chosen by placement rather than by your body.

If you are buying one mini ball, buy 9 inches. If you are buying two, 7 and 9 covers almost everything. And if you are choosing between a mini ball and a magic circle as your first prop, that comparison is here — the ball is the more useful first purchase for most home practices.