Measure the circumference of your head, just one inch above your eyebrows and above your ears (see figure). Use a tape measure. Fasten it to the point it is snug, not too loose neither too tight. Size is requested when you choose any helmet. Please provide the size in CM (below you have a conversion table).

sizing-falcon-polo-helmet
CM INCHES UK
52 20 1/2 6 3/8
53 20 7/8 6 1/2
54 21 1/4 6 5/8
55 21 5/8 6 3/4
56 22 6 7/8
57 22 1/2 7
58 22 7/8 7 1/8
59 23 1/8 7 1/4
60 23 5/8 7 3/8
61 24 7 1/2
62 24 1/2 7 5/8
63 24 3/4 7 3/4