Do NOT just choose your regular lab coat size. These will fit differently. Measure!
How to measure: (Returns are not free, spend time on this!)
Using a tape measure or a string and ruler, measure the circumference of your body without pulling it tight. Wear your normal lab clothes and include those in your measurement.
Measure the widest part of your chest, under your armpits, with a big inhale.
Measure your waist, usually about 6-8 inches below your armpits.
Measure the widest point of your hips with your feet shoulder width apart.
Choose the correct size:
Below are the maximum measurements each lab coat size will accommodate. Make sure all 3 of your measurements are at or below these numbers. For a loose fit, go up one more size. The "Waist" numbers are before using the belt, which pulls in up to 8 more inches.