Bicycle Helmet Fitting Check List

Size

Check to see if the head size and the helmet size fit well in circumference.
Put your fingers around the edge of the helmet. If there is too much room on either side or the front and the back of the head, the helmet is too big. Find a smaller one.
Put a finger through one of the top vent of the helmet. If the helmet sits high on the head and there is a gap between the top of the head and the inside top of the helmet, the helmet is too small. Find a bigger one.
Check to see the shape of the helmet fits the head shape and make minor adjust.
If the helmet fits well in the front and the back, but a little tight on the sides, replace thicker comfort pads with thinner ones on the sides. Those comfort pads can be used to make minor adjustment for a better fit.

Straight

Check and make sure the helmet is not on the head backwards.
If there is no label to show the front and the back of the helmet, the back of the helmet usually has one port that hold two back straps together or two ports that are closer than the two ports in the front. The wider strap ports in the front help to position the straps away from the eyes when buckled.
Check for the correct position of the helmet.
Make sure the helmet is level on the head, not tilted to either side. The front of the helmet should be low, just above the eye brows. Looking upwards, the wearer should see the front edge of the helmet.

Strap

Check and make sure that the straps are not twisted or tangled.
Put the side straps around the ear, one in the front and one behind the ear.
Position the side buckles right under the ear lobes. Adjust the straps one at a time individually. The front straps can be a little shorter than the back ones so that they will hold the front of the helmet low over the forehead.
Buckle up under the chin snugly but comfortably. With the helmet properly fitted and buckled on your head, you should not be comfortable eating a large sandwich. You should be able to open your mouth wide, you will feel the strap under the chin and the helmet being pulled down on your head. If you can shift the helmet off the forehead, or if the helmet moves on your head when you shake your head from side to side, the helmet is either too big or the straps are too loose.

Stickers

Make sure the helmet has a sticker stating that the helmet meets the CPSC helmet standard, which is mandatory in the U.S. If the helmet has a blue Snell sticker, it means that the helmet not only meets the CPSC standard but also the more stringent Snell Standards.