Rash guards are supposed to fit tightly like a second skin, so they don’t get in your way. However, the fabric is quite stretchy. When wet, the fabric will loosen up and stretch even more.
Rashguards that will be worn under wetsuits should fit tightly. If you don’t need a tight fit for under a wetsuit or active sports, feel free to order a size up.
Please remember the shirt bottom of the rash guard features a rubbery, silicone strip to prevent it from riding up. If you tend to wear smaller tops than bottoms, ensure the low-hitting strip at the shirt bottom won’t be too tight or restricting. If you have large hips and don’t need a tight rash guard for extreme sports, you can buy the next size up. Rash guards have a fit similar to wetsuits, so the same size usually works for wetsuits.