The head circumference determines the size strap you will want to use as well as if you will want to use any silicone extension rings.
This is how you measure head circumference:
Average head circumference for CHILDREN is 19 - 21.5 inches
Average head circumference for ADULTS is 21.75 - 22.5 inches
The best width of the goggles will be determined by the width of the head and the space between the eyes. This varies dramatically and is the reason why so many different goggles exist on the market. There is not one perfect goggle, only the one that fits your face best. Frogglez come in 2 sizes to accommodate the vast majority of faces and are designed so that you can attach the strap to almost any goggle should you find one that fits your face perfectly.
Most goggles will either leak at the far edges on the side of the face due to being too wide or right next to the bridge of the nose if there is not enough support yet for the sealing edge.
Frogglez are designed to put gentle pressure in the sealing edge of the face where the goggles tend to leak providing a superior fit. This seal is best if no extension rings are used.
Measuring the distance between the temples gives a good estimate on which goggle size will fit best.
SMALL GOGGLES: 4 inches or less
MEDIUM GOGGLES: 4+ inches
HALF MASK: 4.5+ inches (does not work well without a bridge between eyes)
These innovative rings allow for the increase in size of all Frogglez Goggles! Adding an extension ring is easy and also makes attachment to other goggles simple.