There are four places that you need to measure to know your size: underbust, natural waist, upper hip, and torso length.
You only need to take this measurement if you plan on buying an overbust waist trainer. While topless or wearing an unpadded bra, measure around your chest at the fullest part.
To measure your underbust, find the point around your ribcage that is just below your bra. Pull the tape measure taut but not so tight that it is pulling into your skin. This is your underbust measurement.
The next step is to measure your natural waist. This is the smallest part of your waist, which usually lines up with your belly button. Measure around this part the same way you did with your underbust.
The next step is to measure your hips. To do this, measure just below your hip bone. This measurement is slightly different than when you are measuring for a dress. You aren't looking for the fullest part of your hips because the corset will not come down that far.
The final step is to measure your torso length. Sit in a chair and, sitting up straight, place the end of the measuring tape just under your bust. Run the tape down to your thigh. The distance between the two points is your torso length.
Finding Your Size
You'll want to choose a corset that is about four inches smaller than your natural waist. This will give you enough room that you can fasten the trainer, but it will be tight enough that you will see progress.
It's possible to find the waist trainer that will help you lose inches. But make sure that you take the time to measure properly because trainers fit differently than regular clothing.