O16.5

On30
A Rail Truck based on the Bachmann Industries model.
Scale	1⁄4 in to 1 ft (6.35 mm to 1 ft)
Scale ratio	1:48
Model gauge	16.5 mm (0.65 in)
Prototype gauge	2 ft 6 in (762 mm)

O16.5
Peco trains at Expométric in Paris (France)
Scale	7 mm to 1 ft
Scale ratio	1:43.5
Model gauge	16.5 mm (0.65 in)
Prototype gauge	2 ft 6 in (762 mm)

0e
Saint-Paul railway depot at Suisse Toy in Berne (Switzerland)
Scale ratio	1:45 and 1:43.5
Model gauge	16.5 mm (0.65 in)
Prototype gauge	750 mm (2 ft 5 1⁄2 in), 760 mm (2 ft 5 15⁄16 in), 800 mm (2 ft 7 1⁄2 in)

On30 (also called On2 ¹⁄₂, 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 model railroaders, in 1:43.5 scale ratio by British and French model railroaders and 1:45 by Continental European model railroaders (excluding France).

On30 uses the American O scale of 1/4" to the foot, (ratio 1:48) to operate trains on HO gauge (16.5 mm/​0.65 in) track. The 30 indicates the scale/gauge combination is used to model 2 ft 6 in (762 mm) narrow gauge prototypes, although it is often used to model 2 ft (610 mm) and 3 ft (914 mm) gauge prototypes as well. This scale/gauge combination is sometimes referred to as On2 ¹⁄₂.

O16.5 in the United Kingdom is a model railway scale/gauge combination of 7 mm to the foot. This is the same scale as British O scale (1:43.5 ratio) running on 16.5 mm (0.65 in) gauge track, which is used by HO/OO gauge model railways. It thus represents the prototype gauge of just over 2 ft 6 in (762 mm), although also widely used to model 2 ft (610 mm), 2 ft 3 in (686 mm) and 2 ft 4 in (711 mm) gauge UK prototypes.