A critical yet often overlooked foot measurement, longitudinal arch length is the distance from the heel to the ball joint of the foot. This distance is important because two people may have the same heel to longest toe measurement yet require different shoe sizes. Proper use of the Brannock device consists of taking three separate measurements. First, heel to longest toe is measured. Secondly, heel to ball of the foot is taken. If these measurements are the same that number is used to calculate width. If the numbers are different, an average is taken so width may be more accurately determined.
Suppose two women have the US measurement of seven for heel to longest toe. One woman's arch length is also seven however the second woman's arch length is a six. The first woman is a true seven, the second woman's width measurement would be calculated using size six and a half as that is the average between six and seven. What makes a shoe a certain size is the arch length distance. Most shoes flex or break at the ball joint, if your ball joint does not line up with where the shoe breaks the shoe is probably the wrong size for you.
Arch length can also used to help determine arch height. In the previous example where the second woman has a heel to toe measurement of seven but an arch length of six that length of six may indicate a high arch. Conversely, if a third woman had a heel to toe measurement of seven yet an arch length of eight she may have a flatter foot. Each foot should be measured independently of the other and considered separately before a purchase is made. Most people have one foot that is longer than the other, the same holds true for width.
Some people are reluctant to let another person measure their feet. Others are willing to let their feet be measured however become argumentative when the measurements are inconsistent with their beliefs. Feet change with the rest of your body as you age. Gravity pulls your arches down, and if they do not have anything supporting them your arches may flatten which increases foot size. Other factors such as pregnancy, weight gain, disease, and injury can also change arch length and shoe size.
When shopping for shoes if you doubt that the person measuring you has done their job correctly a good way to test them is to ask for the three measurements discussed at the beginning of this writeup: total foot length, heel to ball measurement and width. Expecting someone to remember these measurements off the top of their head may be asking a bit much, most people calculate shoe size and discard the other measurements however a quick remeasure will provide you with an opportunity to determine whether someone is using the metal slider on a Brannock to measure heel to ball of the foot.