1. Measure your chest
With good posture, wrap a measuring tape under your arms around your back and take the measurement at fullest part of your chest. Be sure to keep the measuring tape parallel to the floor.
2. Measure your waist
Place the measuring tape at the seam where the collar band attaches to the yoke (the fabric that sits across the shoulders) and measure straight down the middle of the back of the shirt to the hem at its longest point.