On30 (also called On21⁄2, O16.5 and Oe) gauge is the modelling of narrow gauge railways in O scale on HO (16.5 mm / 0.65 in) gauge track in 1:48 scale ratio by American and Australian model railroaders, in 1:43.5 scale ratio by British and French model railroaders and 1:45 by Continental European model railroaders (excluding France).