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Here is how we size helmets for our customers:
Measuring the head is a starting point for the entire helmet sizing procedure. Due to varying shapes, heads that are apparently the same size when measured by a tape may not necessarily fit the same size helmet. The circumference of the head should be measured at a point approximately one inch above the eyebrows in front and at a point in the back of the head that results in the largest possible measurement. Take several measurements to make sure you have the largest one.
A) Grasp the helmet by the chin straps, with the front of the helmet facing you
and the top of the helmet facing down.
B) Place the thumbs on the inside surface of the straps and balance
the helmet with the index fingers.
C) Spread the helmet apart with the hands, and slip down over the head.
If the helmet slides down on the head with no resistance, you have your first indication that it may be too large. Obviously, if it will not slide down over the head at all it is too small. Many people unfamiliar with helmets are reluctant to pull down if they meet resistance as the helmet goes on. Only if the helmet is impossible to put on should you move up to the next size, as helmets that go on snug generally fit very well once all the way on. Remember, most people will select a helmet that is too large for them. The eyes should be approximately in the center of the eye port with the top edge of the liner padding just above the eyebrows.
CHECKING HORIZONTAL AND VERTICAL MOVEMENT:
Now that you are wearing the helmet, look carefully at the way it fits. Check to see if the cheek pads are in contact with the cheeks. Is there excess pressure on the cheeks? Look for gaps between the temples and the brow pad. Check the back of the helmet where the neck roll (if the helmet has one) makes contact with the neck. Does it touch at all? Or is it pushing the helmet away at the rear causing it to roll down over the eyes in front? After you have made your visual check, grab the helmet in your hands, one on either side, and try to rotate the helmet from side to side. Note any movement of the skin while doing this, as well as the amount of resistance to movement remembering to hold your head steady.
Next, check movement up and down, again noting skin movement and resistance. If in either test there was little or no skin movement, and/or the helmet moved very easily, the helmet is too large. A properly fitted helmet will cause the skin to move as the helmet moves. And, it will feel to the wearer as if evenly distributed pressure is being continuously exerted around the head.
NOTE: Helmets are a little like shoes, in that they do break-in a little.
For this reason the best attitude to have when fitting is that the helmet
should be as tight as you can stand to wear it.
HELMET SIZE CONVERSION CHART
Inches Size Metric Equals
21-1/4 6-3/4 54 XX-Small
21-5/8 6-7/8 55 XX-Small
22 7 56 X-Small
22-3/8 7-1/8 57 Small
22-3/4 7-1/4 58 Medium
23-1/8 7-3/8 59 Medium
23-1/2 7-1/2 60 Large
23-7/8 7-5/8 61 Large
24-1/4 7-3/4 62 X-Large
OVER 24 1/4" - 63 XX-Large
Hope this helps.