What size exercise ball should I buy for my height?

Across the reviewers we read, the rule of thumb is a 55cm ball for users roughly 5'0"–5'5", 65cm for 5'6"–5'11", and 75cm for 6'0" and taller. Reviewers consistently note that a ball is correctly sized when your hips and knees sit at about 90 degrees. Several pregnancy-focused community threads stress double-checking the height chart before buying.