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Use a pencil to mark your height on a wall, then use a tape measure to get an exact measurement. Next, get your current weight using a bathroom scale.  While the individual measurements associated with the suit are also important to get the right fit, your height and weight can help you estimate your fit. This is especially important if you're going to have your suit tailored, because height and weight helps tailors determine your “drop” of the suit, which is the difference between the size of the pants and jacket. Choose the type of shirt that you’d normally wear under a suit. This will help you avoid getting a jacket that’s too snug. For example, you might put on an undershirt and a white dress shirt if that’s what you normally wear. The over-arm measurement will help to find the width of the entire jacket, from shoulder-to-shoulder. Start your tape measure at the joint of one shoulder. Then, run it across the top of your shoulders, behind your neck, and to the joint of your other shoulder. When determining measurements, don't pull the tape measure tight. Instead, make sure it fits snugly around your body. This measurement is essential to get the right body-fit and a flattering cut for your jacket. To take it, raise both arms and run the tape measure around the fullest part of your chest, underneath both your arms. Go all the way around your body. Then, lower your arms to secure the tape measure and record the measurement.  Your nipples are typically where the widest part of your chest is. Don't flex your chest muscles while taking this measurement, as this could make your suit fit too big. Place the end of your tape measure at the shoulder seam of your dress shirt. Then, slide the tape measure down your arm toward your wrist. Take the measurement at your wrist to find your proper sleeve length. You may also need to provide your coat in-sleeve measurement for a custom-made suit. This is the measurement of the underside of your arm. To get it, place the end of the tape measure in your armpit, then pull it down to your wrist and record the measurement. Put your arms down at your sides, with your hands relaxed over your hips. Then, position the tape at the base of your neck and run it down the front of your body. Stop when the tape reaches your thumb and take the measurement. Your jacket will probably end around your upper thigh area, depending on how long your arms are. Wrap your tape measure around the base of your neck, just above your collarbone. The tape should feel snug but not tight. Then, record the measurement. This will help you buy the right size dress shirt for layering under your suit.
Check your height and weight. Wear a dress shirt while measuring so your jacket will fit over it. Measure shoulder-to-shoulder to get your over-arm measurement. Raise your arms and measure around the widest part of your chest. Measure your arm from your shoulder down to your wrist. Get your jacket length by measuring from your neck to your thumb. Measure around your neck under your Adam’s apple.