Use the templates that are linked on this page (next to where you select the size, and in the product description).
NOTE: Please measure your foot and/or use the templates. Sandal size is very often different than shoe size.
Also, if you have the Z-Trek, you may need a different size in the Z-Trail. The Z-Trek is a TINY bit longer than the Z-Trail, so if your toes come close to the edge in your Z-Treks, you may want to up one size for the Z-Trail.