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Motorsports helmets come in all sorts of configurations and there is no generally standard sizing like there is for shoes. The sizing is more like t-shirts, with small, medium, large, etc labels and like clothes, different manufacturers have different interpretations of what a given size actually is when measured in inches or centimeters.
For example, with a head size of 54 cm, you can be S, 2XS or XS depending on the manufacturer. That's a wide range, so to help you key off a measurable number-based fact, i.e. your head circumference, below is a chart to show you what size you will likely be for each brands.
We do say 'likely' because different heads are of different shapes and fit and feel can be subjective. The great thing is that with our industry-leading no-BS 30-day return policy, you can always ship a helmet back if it doesn't fit. Or better yet, just order a couple of sizes and ship the one that doesn't fit back for a full refund.Click here for a guide on how to measure your head properly.
Note: we do not actively stock Simpson, but can order any model