First measure your middle circumference (A) across your belly button.
Then check the chart what size you would require.
You can either choose for a Full-Wrap solution (front and back panels close, just touching on the side) of for an Overlap solution (front and back panels have around 2” overlap).
The Overlap choice is recommended for maximum protection.
Now measure your chest height (B), top of your breastbone to top of your belly button and see if it matches with the vest size you determined during in step 2.
In case your chest height is less or more than the chest height in the table you can either choose to go up or down one size. Your preference for the side coverage (Full-Wrap or Overlap) should be leading for your final size choice.