From US Equestrian
You should change your riding helmet every five years (three years if you ride every day), even if you haven’t had a fall while wearing it. This is because the protective materials inside a helmet can deteriorate over time.
If your helmet has suffered an impact, whether from a fall or from a significant drop on the floor, you’ll need to replace it. Even if your helmet looks OK on the outside, the protective liner inside can be damaged, meaning it won’t provide the right level of coverage anymore.
CHECK THE ‘LOCK’
Your helmet should lock over the bump at the back of your head and sit snugly against your head.
It’s important to have your chinstrap fastened so your helmet stays in place. You’ll know it’s right if you can fit one finger between the strap and your chin.
THE PERFECT FIT
Your helmet should feel snug without pinching your head. Pressure just at the sides may mean a round fit will be more suitable. With your helmet on your head, but the chinstrap unfastened, place a hand on either side and gently try to rock it backwards or forwards. Your helmet should stay put – if it moves too much it’s not a good fit.