Hi there!
‘gymdandee’ has given a good description on how to chose a fit ball. Another thing that I’d like to add is while trying out one, while on the back with hands and feet touching the floor, one should not feel any undue stress or laxity around the back especially in the lumbar (lower back) and thoracic (behind chest) region. You can try out the ball if it’s comfortable and is able to support your weight adequately. If you feel uncomfortable, you could switch to a slightly larger ball for some time before you lose weight and then switch back.
Hope this is useful.
Take care!
You can try it and see if it's OK! If not then buy another one.
Well my friend got me one that is 55 cm around and its for people who r 5'0 to 5'7, but the pamphlet that came with it says its weight limit is 250, but I weigh 270. Should I still try it out, or just buy a stronger one
The general rule for choosing the correct exercise ball size for core (abs/low back) exercises is to have your knees and hips bent to 90 degrees (thighs parallel to floor) when sitting on the ball. This is the minimum ball size--some people prefer a larger ball, where hips are higher than knees. Don't go by photos of models sitting on balls. Typically they sit on a ball much larger than they would train on as it photographs better.
Consider how you will use the ball and your body characteristics (see below). Also, keep in mind that if you are a novice exerciser and/or have poor balance, a larger ball will provide more stability and support, thus making exercising easier (muscles won't work as hard).
To avoid being disappointed that your ball is smaller than you expected, test out the ball size before ordering. Most people underinflate a ball because it is so firm (like a rock) inflated to its maximum diameter. To see if a 55cm ball is right for you, place a mark on a wall about 20 inches high (16 in. for a 45cm ball), and squat next to it. Consider that you will sink down a few inches (depending on your weight and amount of inflation). This is approximately how tall the ball will be when sitting on it. Get a larger size if you don't like this height!
Your Height 5' 0" to 5' 5" Ball Size 22 in. (max. height/diameter)
2250 lb. weight capacity
.
Measure from the back of your knee to the ground (in your workout footwear)
Add 2" to this measurement to allow for depression of the FIT BALL
Choose the FIT BALL that is the closest size ball after this figure