Having the right width handlebar can make a huge difference in how your body sets up on the bike. A bar that us too narrow or too wide can adversely effect far more than hand, arm, shoulder and neck comfort, but can add to spinal pain and change how your pelvis aligns on the saddle, which effects your pedaling, power and more.
To measure, you’ll need a tape measure (preferably metric) and a friend to do the measuring and to help guide you. It’s best to wear something that is snug fitting rather than baggy.
What we are looking for is a natural alignment of your shoulders, arms, and hands. To get in the swing of things, start by standing straight, feet slightly apart, squaring your shoulders, with your arms at your sides, thumbs facing up. Then, keeping your head up, facing forward (not looking at your hands!) swing your arms together paying attention not to roll your shoulders. Try swinging your arms angled slightly away from your body, return to neutral and then swing your arms directed inward (again trying to keep your shoulders square) before returning to neutral. You will notice that swinging your arms in a neutral position seems more natural... because it is. The neutral position is what we want to measure.
Now, remembering to keep your shoulders square, head up, facing forward, bend slightly at your knees, pivot slightly forward at your hips and swing your arms together until you feel that neutral position. Then, have your friend measure the distance between the base of your thumbs. Repeat this three times and average the three measurements.