You take your measurements, check the size chart, and land exactly on the line between two sizes. It happens constantly with shapewear. And because most people do not know what to do here, they pick the wrong size and spend the rest of the day uncomfortable.
Here is the answer, and it is not complicated: almost always, size up.
Why Sizing Up is the Right Call
Shapewear is compression fabric. It is designed to stretch and conform. A piece that is slightly larger than your measurements will still smooth and shape because the fabric does the work through tension, not through being so tight it has nowhere to go. Shapewear that is too small does not give you extra shaping. It gives you a waistband that digs in, fabric that rolls down, and visible lumps above and below the compression zones. None of that is what you paid for.
The One Exception
If your measurements are very close to the lower size and you want firmer compression, some people do size down intentionally. This only works if your measurements are within about half an inch of the smaller size and you are comfortable with tighter wear for a few hours. If you are wearing it all day, do not do this. The discomfort compounds over time in a way that is hard to predict at 9am but very obvious by 3pm.
What to Check Before You Decide
Look at which measurement is between sizes. If it is your waist only, but your hips fit cleanly in one size, size for your hips. Hip fit is more critical in most shapewear pieces because that is where rolling and digging happens most. If both measurements are between sizes, size up, no debate.
Bodysuits Specifically
For bodysuits, your torso length matters beyond just your measurements. If you are tall or have a long torso, always size up even if your waist and hip measurements fall neatly into a size. A bodysuit that is too short in the torso will pull down at the shoulder straps and ride up at the crotch all day. That is worse than any fit issue from going slightly larger.
Still not sure? The Shapies size calculator takes your measurements and gives you a direct recommendation. No guessing required.