How do I find out my helmet size? If your head is 18.5" - 20.5" in circumference - order and S1 Mini Lifer Helmet. If your head is 21" - 23.5" order an S1 Lifer Helmet. How do you measure your head for a helmet fitting? take a string and measure the circumference of your skull - measure 1/2 inch above your eyebrows. Lay the string down and measure with a ruler. Doing this with a soft tape measurer is ideal but most people don't have those lying arounf |