661 size guide

lee96

I'm looking at the 661 race lite knee/shin pads and the size guide says an extra large fits a 15.5" leg, (just above the knee) i measured around my leg with a soft tape measure and it measures 18" around. I tried my mates pads on and there a medium and they fit fine but they are 3-4 years old so have stretched i guess. Anybody got an idea of which size i need?

Thanks