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Although you can measure your arm length on your own, you will get a better measurement if you have a partner who can take it for you. Avoid hunching forward or leaning as much as possible, as either may skew your measurement. Keep your arms slightly bent, with your fingers in your pockets. Position the measuring tape exactly in the center of your neck to get the most accurate measurement. Taking your measurements over the shoulder and down the hand will give you a precise reading, particularly if you are measuring your arm length for clothing. Do not go measure down your back, as you will want to get the fullest arm length possible. Instead, go across your shoulder and down your arms. If you're not sure how to take this measurement, think about what a long-sleeved shirt seam will look like—this is approximately the length you will measure. If you're taking sleeve measurements, finish measuring where you would like the sleeve or shirt cuff to sit. This should be around or just past your wrist bone, depending on how long you prefer your sleeve length to be. If you're measuring arm length for fitness-related reasons, you may need to measure past your wrist. Measure all the way to your fingertips, stretching out your fingers as far as possible.

Summary:
Stand up straight with your arms relaxed and at your sides. Place one end of a measuring tape at the base of your neck. Measure your arm over the shoulder and down your arm. Take your measurements to the area just past your wrist bone for clothing. Continue measuring to your fingertips if measuring your full arm length.