For your helmet to work properly and provide optimal protection in the event of a crash, you need to get the fit right. Any slippage on impact can cause severe head injury, so make sure you buy a well-fitting helmet that sits snugly on your head and stays put.
Take a look at each brand's size guide before buying online, and use a soft tape measure to get an accurate measurement of your head circumference. Most helmets will come in a range of sizes (usually S, M and L) and will have a range for each size. Once you know the exact measurement of your head, you can easily choose the correct size depending on where your measurement falls in those ranges.
Of course, skull shape can have a huge impact on how well and comfortably a helmet can fit your head. Therefore our key piece of advice is to try before you buy.