You will want to use a soft cloth or flexible plasticrubber tape measure such as is used in sewing. Measure the circumference of your chest.

Pattern Play Size Matters Knitting Interweave
Proper way to do body measurements. Use the tape to circle. Use the correct type of tape measure. Do not use a metal measuring tape such as is used in construction it will be inaccurate. Check out the easy to follow instructions for getting accurate measurements for each body part below. All you need is the height and weight of a person to find their bmi. If you have a short time and you want to take a quick measurement of your body just focus on the following areas.
How to measure your chest wrap the measuring tape around your chest at the nipple line with your arms down. Pull the tape tight. One of the most well known measurements for the human body is called body mass index bmi. Here are some tips for taking accurate measurements. Chest measure under pecs or breast as high as you can hips measure around the wide part of hipbones waist measure waist from most narrow points especially around the navel arms. Body measurements tips generally take measurements in an unflexed state and before your workout not after.
How to measure your arms put your thumb on the opposite shoulder and stretch out your pinkie to find the center of. Keep the feet together relax your muscles and stand up straight. Place one end of the tape measure at the fullest part of your bust wrap. Measure the circumference of your waist. Consistency is essential measure under the same conditions and using the same equipment each time. When taking body measurements you need to use the correct type of tape measure.
In other words it doesnt take into account how much muscle or fat you have but just your weight relative to your height.