Your dress shirts really shouldn't come in standard sizes like small, medium or large. But when your shirt tag looks like a lottery ticket—15 , 32/33—it can be a bit confusing. This isn't the time to guesstimate. The numbers are precise measurements, so make sure you're sized up correctly. Below, how to ensure a perfect fit.
While you can simply measure the circumference of your neck, a safer bet would be to take a shirt that fits you well and record the length from the collar button around to the tip of the buttonhole.
Bend your arm into a 90-degree angle. Start measuring from the middle of your back (just below your neck), across your shoulder and down over your elbow to your wrist.