If there's one number that every retiree needs to know, it's their primary insurance amount for the purposes of Social Security. This is the amount you will receive each month if you apply for benefits at full retirement -- which, at present, is 66 years of age.

You can think of this figure as a pivot point. If you elect to receive benefits before then -- at 62, for instance -- then your actual benefits will be less than your primary insurance amount. By contrast, if you wait until 70 years of age to begin receiving benefits, then they will exceed your primary insurance amount.

The importance of this figure is evident when you consider that nearly half of unmarried retirees rely on Social Security for 90% or more of their income. Or that more than a third of Americans have no money set aside specifically for retirement.

With this in mind, Motley Fool contributor John Maxfield delves into the primary insurance amount in the following video, further explaining both how it works and how it's calculated.