Grab a tee the recipient wears often and lay it flat. Measure these two spots:

• Chest: 1 inch below the armpit, across the width.
• Length: from the top of the shoulder seam to the bottom hem.

Double the chest measurement to get the full circumference. Compare that to the Windbar chart's chest measurement. Now do the same for length. If the numbers match, you're golden. If they're off by more than an inch, adjust your size choice accordingly.