12 mm

12 mm or 12 mm gauge may refer to:

Rail transport modelling

 * TT gauge, 1:120 scale with rails 12 mm apart, representing standard gauge
 * HOn3½ gauge, 1:87.1 scale with rails 12 mm apart, representing 3-foot-6-inch gauge
 * H0m, 1:87 scale with rails 12 mm apart, representing metre gauge
 * 00n3, 1:76 scale with rails 12 mm apart, representing 3-foot gauge
 * 5.5 mm scale, 1:55 scale with rails 12 mm apart, representing narrow gauge
 * On2 gauge, 1:48 scale with rails 12.7 mm apart, representing narrow gauge

Firearms

 * 12 mm caliber