Size charts are fashion tools that help determine what size shirt, pants, hat or even coat is likely to best fit your body. In order to find your size on a size chart, you need to measure the appropriate part of your body with a measuring tape. You may need assistance, depending on which parts you're measuring. Once you have the measurement, it's just a matter of looking up where on the chart that measurement falls.
Read any instructions on the size chart. Those instructions might be, for instance, to put the measuring tape around your waist and pull it comfortably tight to check the waist measurement. Note whether you need metric or standard measurements.
Measure the appropriate part or parts of your body. Write down the measurement, or if you only need to have one keep the number firmly in mind.
Look for your measurements on the chart. The chart will have rows, such as "sleeve length" or "bust" followed by the ranges of measurements. Go across the chart to find your measurement (or the closest grouping of measurements if you had to take more than one) and then look up the column to see which size your measurement(s) falls into.