Smith Helmet Sizing
Getting the right lid for your skull isn't tough, you just need to measure correctly. It is important that you get an accurate measurement, it is possible to make reading mistakes of up to 2cm, so be sure to measure the biggest circumference of your head accurately.
The following should insure that you get the right fit: Take a soft flexible tape measuring roll (or print out one below) and measure the circumference of the forehead an inch or two above the eyebrows (or wherever is the biggest.) Be sure to take the reading OVER your hair, unless you are going to cut it all off before you wear the helmet.
Have a friend read the distance or grip the location with your fingers and read the number yourself (be sure to read to at least 1/8" or 1/4cm.) Repeat 2 or 3 times to be sure you have measured correctly. Convert from inches to centimeters by multiplying by 2.54 (that is 24 inches = 24X2.54 cm.)
You should always IGNORE the listed letter size of a helmet and concentrate on the actual cm sizing information that all helmet companies provide. All helmets, unless otherwise stated, list the largest head that will fit that particular helmet. The range of sizes of noggins that a helmet will fit is usually about 2cm.
When near the borderline it is always difficult to choose a size, if you are adverse to a tight fit choose a size up and use adjustment fit pads if necessary