To measure the circumference of your head, you will need a string. Strings are flexible and will easily assume the shape of your head. With your string do the following:-
Hold one end of the string on the middle of your forehead
Take the extended part of the string all around your head
Bring it until it rests on the same place as the end which you started from.
Hold that place tightly and cut it off.
That should be the circumference of your head
You will use the string to stretch along the frame of the helmet to see if it is the perfect fit design for your own needs.