Information is from International Auto Parts' website <http://www.international-auto.com>
Metric US size Comments US/Metric ratio ====== ======= =============== =============== 7mm 5/16" Extremely loose 113% 8mm 5/16" Tight 99% 9mm 11/32" Too tight unless head is damaged 97% 10mm 7/16" Extremely loose 111% 11mm 7/16" Very good 101% 12mm 1/2" Very loose 106% 13mm 1/2" Too tight unless head is damaged 98% 14mm 9/16" Loose 102% 15mm 19/32" Very good 101% 17mm 11/16" Loose 103% 19mm 3/4" Perfect 100% 22mm 7/8" Slightly loose 101% 25mm 1" Loose 102% 27mm 1-1/16" Perfect 100% 30mm 1-3/16" Slightly loose 101% 32mm 1-1/4" Tight 99% 36mm 1-7/16" Slightly loose 101% ====== ======= =============== ===============