What size wrench do I use for a 15mm bolt?

To fit a 15mm bolt, you should use a 15mm wrench. It's important to ensure the wrench is the correct type—either an open-end, box-end, or socket wrench—to match the specific application. If you only have imperial sizes, a 9/16 inch wrench is close but may not fit as snugly. Always double-check for the best fit to avoid rounding the bolt edges.