Measuring your head can be broken down into 3 real easy steps. Please note, it might be easier if you get a buddy to give you a hand on this one.
Step 1. Grab yourself a tape measure, make sure its a cloth measuring tape as a builders tape could be a little tricky.
Step 2. Start at the center point of your forehead and bring the tape measure around your head, make sure you have the tape measure just above your eyebrows and ears.
Step 3. Once you have done the above step, make a note of your measurement. It might be worth doing two measurements just to make sure you have done so correctly and ensuring your results are consistent.
When you have your measurements, enter them in and the sizing chart will give you an indication on the size needed.