Use a measuring tape and, as per the diagram measure your:

• Chest (widest point)
• Waist (narrowest point)
• Hip (widest point)
• Sleeve (back of neck to top of wrist), and
• In-seam (crotch to ankle bone)

Then match your results with the tables on the other tab to find the size that best fits you.

The size recommendations take into account that you are likely to be wearing mid-layers, so you don't need to worry about taking a size up.