You can do so by measuring your head. There should be a sizing chart on the website of what ever company you are purchasing from. Here is sparco's http://www.sparcousa.com/resourceFiles/15.pdf
Find a store which has the helmet, and go in there and try it on. I'd never buy a helmet without trying one on first. For example, Medium was the correct size for me in both the Arai GP5 and Bell Racestar, but each still had a slightly different shape. In the end the Arai fit me best.
Go to Autocraze and try them on. Different brands fit differently. Motorcycle sizes are the same as well if you want to stop at a bike shop to find your size. Don't buy a bike helmet for the car though. They have different safety standards. However, if you are not going to take my advice, the number you are looking for is the diameter or circumference (depending on brand) of your head where the band of a hat would go.
William, James is spot on. Never buy a helmet online unless you have tried on the same model somewhere else first. It's not like buying a pair of socks Regards, JK
I'll reiterate what has been said: try the exact helmet on in a store before buying it online. I spent nearly $400 on a Simpson helmet last year (and as you know that's a cheap helmet - it was on discount) and it fits my crown fine but it is ridiculously tight on my face. It mashes my cheeks into my eyes and mouth and I get a headache in minutes. Here's to hoping the foam breaks in over time, but who knows - I ordered it for the rampage last year and it showed up a month late, after the event. I haven't used it once!* *Although I did miss an FCA track day on Monday. Darn family visiting! Soon as you move to Vegas they all pour out of the woodwork!!
Another reiteration - always put the helmet on before buying. I always then then firmly slap myself on the cheeks with hands moving front to back. I tried two helmets that were same size and did this on the first one - realised that in an accident I would lose my lower jaw (For the record I do not have a Lionel Ritchie shaped chin). If you just go online and measure the crown then you can be in more danger than you realise.
All that being said, if you can't find a place locally to try them on, I've talked to dozens of folks, and the Bell and GForce series seems to be the most universal fit (though still not for absolutely everyone). When we bought "loaner" helmets for our FCA Southeast Region events, we bought Bell and GForce. I personally have Bell helmets.
I Just bought a GForce on line without trying one on first. Extra large and fits me fine. Unbeatable price too. Image Unavailable, Please Login
Definately try it one on first, it'll save you a lot of pain later on. Edit: I also have a GForce, great helmet for the price. (I got mine from SafeRacer, with an additional discount since I am an SCCA member.)
There giving them away at Gforce Outlet Store http://www.gforceoutlet.com/index.php?cPath=24&osCsid=de3242f517d71b8588e400de505a0bb1
Now you just have to have it custom painted! Come visit www.helmet-painting.com (and yes, I'm an FChat sponsor).
One very important point, you must also take into account the driver's ego when measuring for a helmet. The actual helmet may need to be 2-3 sizes larger to accomodate for the "wishful thinking" factor.
Mike C I have thought about it, your work is outstanding. Is there a discount for Fchatters? QUOTE=Mike C]Now you just have to have it custom painted! Come visit www.helmet-painting.com (and yes, I'm an FChat sponsor).[/QUOTE]
William--take the time to try them on. Fit is most important and all brands fit differently. For quality, Arai is my choice, but the Bell helmets coming out of Europe are a close second.
Since I do this as a hobby business rather than my career, you'll find that the paint jobs are already considerably less (usually $50-$100 less) than other paintshops. And you'll get a lot more personal attention, I promise!
I had my helmet custom painted sometime ago by a professional and indeed it was more expensive than Mike C's for what I've seen in other threads. Quality wise I think Mike C's is up there with the best !
Like everyone says, always try on. The measurement is based on the circumference of your head from forehead to back of skull (largest measurement) but does not account for the shape of your head (round, wide or long). If I wear a Shoei the same size as my Arai my nose gets bent out of shape. Some helmet manufacturers have different sizes of chin pads that can be placed inside a helmet to change that areas size and it is possible to have the helmet adjusted to your head size by rubbing the liner to shape with abrasive paper but this should definately be left to a decent helmet engineer who knows what he is doing. One more tip, if you think a helmet is expensive then buy one of a similair value to the value you attach to your head!... And finally, the size and fit of the helmet should be selected with the type of driving you are going to be doing in mind. If its extremely high G's then getting a size smaller than normal isn't unusual. Plus dont forget to try the helmet on over a fireproof balaclava if you intend to wear one. Different makes of balaclava also have different thicknesses etc.
