Carl’s Guide to Scales and Gauges Commonly Used for Small Layouts

I’ve received a number of questions recently about the scales and gauges
used in model railroading around the world. So I’m repeating here a table
that first appeared in my book, Creating Micro Layouts. I’ve added 
a few
more scale/gauge combinations and welcome your additional suggestions.

1.75in (45mm)
7/87/8in = 1ft24 in1:13.7Very large trains; for scratchbuilders
Fn315mm = 1ft36 in1:20.3Variant of G, mainly Bachmann
Gm45mm = 1m1000mm1:22.5Metre gauge – LGB and others
Hn421/2in = 1 ft42in1:24Mainly in New Zealand/Australia
13/8in = 1ft56.5in1:32Standard gauge – Märklin and others
1.25in (32mm)7/87/8in = 1ft18in1:13.7For scratchbuilders
SM3216mm = 1ft24in1:19Mainly in garden railways
Hn301/2in = 1 ft30in1:24For scratchbuilders
1m3/8in = 1ft1000mm1:32European metre gauge
9 Mil9mm = 1 ft42in1:32Used mainly in NZ 
O (U.K.)7mm = 1ft56.5in1:43.5Standard gauge – many mfgrs
O(Europe)1435mm1:45Standard (normal) gauge – many mfgrs
O (U.S.)1/4in = 1ft56.5in1:48Standard gauge – many mfgrs
7/8in S3/16in = 1ft56..5in1:64Standard gauge
3/4in1n23/8in = 1ft24in1:32For scratchbuilders
On31/4in = 1ft36in1:48Primarily in U.S.
16.5mmGn1513.5mm = 1ft15in1:22.5A few mfgrs
Hn181/2in = 1ft18in1:24A few mfgrs
1n213/8in = 1ft20-22in1:32For scratchbuilders
1/368.5mm = 1ft24in1:36For scratchbuilders
O16.57mm = 1ft~28in1:43.5Popular in U.K.
On301/4in = 1ft30in1:48Popular – Bachmann and others
OO4mm = 1ft56.5in1:76U.K. Standard gauge, very popular
HO3.5mm = 1ft56.5in1:87Standard gauge, very popular
14mmOn2, O147mm = 1ft24in1:43.5Mainly for scratchbuilders – Roy C. Link
12mmOOn34mm = 1ft36in1:76U.K. three-foot gauge
HOm3.5mm = 1 ft1000mm1:87European metre gauge
TT2.5mm = 1ft1435mm1:120Standard (normal) gauge
10.5mmSn23/16in = 1ft24in1:64Mainly in U.S.
HOn33.5mm = 1ft36in1:87Popular in U.S.
9mmO97mm = 1ft15in1:43.5Some U.K. mfgrs – Avalon and others
On181/4in = 1ft18in1:48A few mfgrs
Sn23/16in = 1ft24in1:64Popular in U.S.
OO94mm = 1ft27in1:76Popular in U.K.
HOn303.5mm = 1ft30in1:87Popular, a number of mfgrs
HOe3.5mm = 1ft650-850mm1:87Popular in Europe
NZ1202.5mm = 1ft42in1:120New Zealand TTn42
N.075in = 1ft56.5in1:160Standard gauge, very popular
3.5mm = 1ft22-24in,400-650mm1:87For scratchbuilders
Nn3, Nm.075in = 1ft36in, 1m1:160U.S. three-foot, Europe metre gauge
Z56.5in1:220Standard gauge – Märklin and other