Most shoe stores use a Ritz Stick or a Brannock Fitting Device. Brannocks are found in most stores that carry shoes. They are metal and have a right and left heel cup, a ball of the foot slide marker and an adjustable width slide. Black is for Boys and Men. Green is for Women.
Children use the smaller red one.
The width slide is almost useless as width is a circumference - a volume, not how far across. Since no measuring device can measure volume, they are extremely inaccurate. We learn to view the foot mass against the measuring device and this visual of mass gives us a place to start. Trying on shoes will give you the most comfortable fit and finally determine width. The good news is that width is an industry standard and does not vary much from company to company and style to style, except in Oblique toe, in depth shoes.