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The best way to measure your neck is with a measuring tape. You can ask someone to do it for you, or do it yourself.
If you measure your neck yourself, take the measuring tape and put the measuring tape from the back to the front. Put the measuring tape in the middle of your neck. It's important the head and chin are in a comfortable position. Don't look down or move it. This will result in an unaccurate measurement. Hold the first side with one hand. Take the other side and put it right on top of it. Now you have your measurement of your neck.