To determine the appropriate size of a Helmet, it’s crucial to measure your head. 

It’s recommended to use a soft measuring tape, but if you don’t have one, you can use a string or a cell phone charging cable and measure it against a ruler or hard measuring tape. You need to measure the circumference of your head by wrapping the measuring tape from the middle of your forehead just above your eyebrows around your head (above your ears) and back to the middle of your forehead where you started. 

Essentially, you want to measure your head where your hat would usually sit or where you imagine the helmet fitting on your head, just above your eyebrows.