We recommend measuring yourself before purchasing, minimizing the potential of returns/exchanges.
Whether measuring with a soft measuring tape or a string and yard stick, make sure not to wear bulky clothing that could interfere with your numbers.
Don’t leave the tape/string loose or pull it too tight; it should sit snugly but comfortably around you. Don’t suck in, (saint andro says) it's not good for you.
Alternative: If you have an existing garment that fits you like a glove, lay it flat and measure like that.
If you have questions about sizing, don't hesitate to reach out.