The right size of the helmet is determined on the basis of the circumference of the head. To determine the correct size of the helmet, measure your head circumference with a measuring tape. Place the measuring tape about 2.5 cm above the eyebrows. Pass the measuring tape behind your head under the nodule at the back. Then you take the measuring tape back to your eyebrows. The number of centimeters that you measure + 1 cm is equal to the correct size helmet.