That depends on what manufacturer you are measuring for. If you look at our Sizing Charts page, you can find sizing forms that have diagrams for needed measures. Using a soft tape measure (do not use a carpenter's tape measure; it needs to conform cleanly to the body), follow the diagrams using a partner to assist you for accuracy. If you have any questions to make sure you are doing it right, feel free to give us a call. We always are happy to help you with any questions you may have.
See also: Will my ballistics / armor fit into this carrier?
See also: What ballistic panel size do I need? What sizes are my panels?
See also: What size am I? What size do I need?
See also: What measurements do I need to take?