You could probably do this one of two ways. If you know the wig maker/company, sometimes they will list the measurement. They may also have a customer service number you could call. You could also buy a foam wig block/ wig head, turn it inside out, and pin it to the head and measure the cap that way. You may want to measure the wig head and measure your head to see if there's any difference in sizes, but the wig head will give you a steady base to hold it. To hold the wig head steady, you can put it on a paper towel holder or the back of a chair that has top spindles on it. HTH!
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