Measuring your head is extremely easy. All you need is a seamstress' tape measure or a strong piece of twine or string that does not stretch. It's important that what you use to measure doesn't stretch because you'll be measuring to a tolerance of 1/4th inch.
Wrap the tape measure or twine around your head where you will be wearing your hat. Pull it snugly, as you would want your hat to fit your head. Record the measurement from the tape measure, or if you're using twine, lay it out against a regular tape measure and record the length.
Measure your head at least three times until you get a consistent measurement to within 1/4th inch each time.
Use the conversion chart below to convert the length measurement to a hat size.