It seems like waist circumference is easy as a pie. Just get the measuring tape and do it. However, most people do it wrong. Read this Habio article to find out how to measure waist size correctly.
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Does actual waist size differ from jean size? When I measured my waist (belly button area) it was 3 inches bigger than the size I wear in jeans - Quora
How To Measure Your Waist To Determine Your Jeans Size
How To Measure Your Waist To Determine Your Jeans Size
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