To measure your feet stand on a level floor with the back of your heels against a straight edge or wallngest toe. Take the measurement (in millimeters) from the ruler where the flat edge crosses (see image below). This is your foot length measurement.
If your foot measurement is halfway between sizes, select the larger size.
You may find one foot is longer than the other, this is quite normal, please use the larger size when making your shoe size selection.