Most popular gauges/scales for model railroading

Scale	Ratio	Model gauge	Notes
Z	1:220	6.5 mm (0.256 in)	NMRA does not give any other dimensions for Z-scale apart from the gauge The s.g. is set nominally to gauge=6.5 mm; more exact to 1:220 would be 6.52 mm (0.257 in)
Nn2	1:160	0.177 in (4.5 mm) 4.5 mm (0.177 in)	narrow gauge
Nn3	1:160	0.256 in (6.5 mm) 6.5 mm (0.256 in)	narrow gauge
N	1:160	0.353 in (8.97 mm)	standard gauge
TT	1:120	0.470 in (11.94 mm) 0.472 in (12 mm) 12 mm (0.472 in)	standard gauge
HOn2 or 3.5 mm	1:87.1	7 mm (0.276 in)	narrow gauge
HOn30 or 3.5 mm	1.87.1	9 mm (0.354 in) (0.353 in.)	narrow gauge--N scale track can be used
HOn3 or 3.5 mm	1:87.1	10.5 mm (0.413 in)	narrow gauge
HOm or 3.5 mm	1:87.1	12 mm (0.472 in)	narrow gauge--TT scale track can be used
HO or 3.5 mm	1:87.1	0.65 in (16.5 mm)	standard gauge
OO or 4 mm	1:76.2	0.75 in (19.05 mm)	standard gauge
Sn3 or 3/16″	1:64	0.563 in (14.3 mm)	narrow gauge
S or 3/16″	1:64	0.883 in (22.43 mm)	standard gauge
On2 or 1/4″	1:48	12.7 mm (0.5 in)	narrow gauge
On30 or 1/4″	1:48	HO-track	narrow gauge
On3 or 1/4″	1:48	0.75 in (19.05 mm) 19.4 mm (0.764 in) (?)	narrow gauge
O or 1/4″	1:48	1.25 in (31.75 mm)	1.177 in (28.9 mm) is true standard gauge
No. 1n3 or 3/8″	1:32	1.125 in (28.6 mm)	narrow gauge
No. 1 or 3/8″	1:32	1.766 in (44.85 mm)	standard gauge
Fn3 or 15 mm	1:20.32	No. 1-track	narrow gauge
F or 15 mm	1:20.32	2.781 in (70.69 mm)	Identical to Proto 20.32 except the wheel flange depth
3/4″	1:16	3 1⁄2 in (89 mm)	standard gauge
1″	1:12	4 3⁄4 in (121 mm)	–