bike tires are usually square, as in the width of the tire= height of tire or profile=100%........so you add double the width plus 622mm(which is the standard ERD of a 700c rim) to get the diameter of the wheel with tire mounted
ERD= estimated rim diameter
so for 23's that would be 46+622= 668mm = 26.3 inches
27" wheels have an ERD of 630mm usually
IIRC with mtb 26" wheels the 26" measurement is accurate with tires that are 1.75"