Measure from the bridge of your nose to the tip of your chin. It may be helpful to have a friend assist you. 

Measuring from the height of the bridge of your nose (usually just below your eye line) down to the tip of the chin, and do not measure past the tip of your chin.

Match your measurement with the given sizes below to find the correct sizing for you. We offer 3 different sizes of masks Medium, Large, and X-Large.
Medium 3.5"-4"
Large 4"-4.5"
X-Large 4.5"+

If your measurements lie in-between sizes, we recommend using your weight as a secondary reference.
Medium up to 140 lbs
Large 140 lbs - 200 lbs
Extra Large 200 lbs and up

When applying your mask, place the top most part of the mask on the very top of your nose, then lower to cover your mouth and chin. Finish by fastening the strap(s) in a comfortable position.

A proper seal is only required on the filter, not the mask shell. The filter (inside mask) is creating a seal on the users face while the mask shell has some slack. This is acceptable. As long as the filter is creating a seal, the mask is deemed effective.

*RZ filters are designed to match your chosen mask size, i.e. a Medium (M) filter fits a medium mask, a Large (L) filter fits a large mask, and an Extra Large (XL)