To measure chest size: Measure from armpit to armpit and multiply by 2. For example-if you measured 20in between your armpits, and multiplied by 2, that  would give you a 40in chest measurement. You would need to purchase a size medium.

To measure pants size: Waist- Lay down a well-fitting pair of pants and measure from one side of the waist to the other and multiply by 2. Inseam- Measure starting from the crotch seam intersection and go down the inner leg seam to the bottom of the hem. Reference these measurements with the chart.

To measure hat size: Measure the circumference of your head slightly above your ears. Reference your measurement with the above chart. 

When purchasing a jacket or Gore-Tex pants you may want to size up if you plan on wearing layers underneath.