Wear the type of socks you'll wear with your shoes and measure feet in the afternoon (feet typically swell during the day). Distribute weight evenly before measuring.
Stand up straight on a hard surface with your heel against the wall and a piece of blank paper taped to the floor, flush against the wall beneath your foot.
Have someone mark the longest part of your foot (referred to as heel-to-toe length) on the paper with a pen or pencil, or measure yourself if necessary. Repeat with the other foot, as right and left sizes may be different.
Use a ruler to measure the heel-to-toe length you marked for each foot.