You will want to fill the ball to a level that provides you with a comfortable yet supportive cushion. The amount of inflation will depend on your height and body weight. Ideally, if you are sitting on the Evolution Chair with your feet flat on the floor, your hips and knees should form an angle slightly greater than 90 degrees with your hips slightly higher than knees.
To inflate with the provided pump we recommend filling the stability ball approximately 80% on the first day. Let it sit for 24 hours to stretch before filling to 100% or so that you can dent the ball about two inches (5cm) if you stick your finger in the ball.
If your are 5 feet tall, you would not inflate the ball as much as someone who is 5 feet 11 inches. Likewise, a 250 lb individual would inflate the ball more than a 150 lb individual of equal height. Regardless of your height or weight combination, you should not exceed the maximum diameter of the ball. If your ball is underinflated, the ball will not be as firm, but you will still benefit from the Evolution Chair.
We recommend using an air compressor for the initial inflation. See below for inflation using provided hand pump.
Inflate the ball until it is firm and measures from 20" — 21" high (52cm — 54cm) when measured perpendicular from the floor to the top of the ball on the first day. Let the ball sit and rest for 24 hours to allow the material time to stretch and relax, the following day gradually release air until your personal desired firmness level and correct ergonomic height is achieved.