Wrap a measuring tape around your child's head at the largest portion that covers the tops of his ears. This will give you an approximate estimate of your child's helmet size.
Read the helmet manufacturer's label to find out how to convert the measurement you found of your child's head to the manufacturers' recommended helmet size. Helmets come in x-small, small, medium, large and x-large, and different helmet manufacturers have different conversion tables. Use the conversion table specifically designed for the helmet brand you are looking at.
Allow your child to pick out the style of helmet they like. There are many different colors and themes to choose from.
Have your child try on the recommended helmet size to see that it fits him properly. The helmet must not move easily from side to side on your child's head. Use sizing pads for areas of the helmet that are loose and need to be secured to your child's head.
Measure the position of the helmet on your child's head. Take 2 fingers and place them above her eyebrows, making certain the helmet sits low enough on the forehead and is level on her head.
Adjust buckles, chin straps and side straps on the helmet until they tightly secure the helmet. Only 2 fingers should fit underneath the straps.
Recheck all the adjustments to ensure the helmet doesn't easily move from side to side, backward or forward.