Model Railway Scales And Gauges Demystified


The art of scale modeling is recreating life-like objects in a smaller scale, or size. As such most modeling efforts, including railroad modeling are based on a ’scale’ model such as 1:10 in which every 10 inches equals 1 foot for example. In additional a ‘gauge’ or distance between tracks needs to be specified, and normally is reflected by the scale name but not always. In railroad modeling the most common standards that are used include:

HO    - 1:87.1
N    - 1:148 to 1:160
O    - 1:43.5
Z    - 1:122

The ‘names’ such as HO (which literally stands for Half O scale) or N that are given to a particular scale is being built also give an indication for the track gauge being used. The actual scale associated with any given name may vary depending on the common gauge being modeled although in ‘narrow’ gauge the name usually also reflects the gauge. Standard gauge is 1435 mm or 4′ 8.5” in real life and is scaled down appropriately and rounded down or up depending on the scale model definitions and prototyping being used.

COMMON GAUGES

The following is a partial list of gauges for railways in various parts of the world:

  Standard Metric Country of Origin
Broad 7′ 0-1/4" 2140mm Britain
  5′ 6" 1676mm Spain, Portugal, India, China
  5′ 3" 1600mm Ireland, Australia, Brazil
  5′ 0" 1524mm Russia, Finland
Standard 4′ 8-1/2" 1435mm Australia, Austria, Belgium, Bolivia, Bulgaria, China, Czechoslovakia, Canada, Denmark, Egypt, France, Germany, Britain, Holland, Hungary, Italy, Japan, Yugoslavia, Mongolia, Norway, Poland, Peru, Rumania, Sweden, Switzerland, Turkey, USA
BNarrow 3′ 6" 1067mm Australia, New Zealand, South Africa, Japan, Spain, East Africa, Canada, Ecuador, Java
  3′ 3-3/8" 1000mm Germany, Switzerland, Spain, India, Burma, Kenya, Malaysia, Thailand, Uganda, Chile
  3′ 1-1/2" 950mm Italy
  3′ 0" 914mm Isle of Man, Ireland, USA, Canada (WP&Y)
  2′ 7-1/2" 800mm Wales, Switzerland
  2′ 6" 760mm Australia, Wales, Austria, India
  2′ 5-1/2" 750mm Germany, Egypt, Indonesia
  2′ 3" 686mm Wales
  2′ 0" 610mm Australia, Fiji
  1′ 11-5/8" 600mm Wales, Greece
  1′ 3" 381mm United Kingdom, New Zealand

STANDARD GAUGES

The following are scales that are often used by modelers: 

Scale Name Ratio Gauge
4 1:15

3 in.

3 1:18 2-1/2 in.
2 1:24 2 in.
1 1:32 45mm / 1-3/4 in.
O 1:43.5 32mm
Scale 7 1:43.5 33mm
O 1:45 32mm
O17 1:45 1-1/4 in.
O 1:48 1-1/4 in.
Q 1:48 1-3/16 in.
Proto 48 1:48 29.89mm
S or H1 1:64 7/8 in.
OO 1:76.2 19mm
P4 / S4 1:76.2 18.83mm
EEM 1:76.2 18.8mm
EMF 1:76.2 18.2mm
EM 1:76.2 18mm
HO 1:87.1 16.5mm
QO 1:96 3/5 in.
TT 1:100 13 mm
Scale 3 1:102 14.12mm
TT 1:120 12mm
HOO 1:145 10mm
OOO 1:152 9.5mm
N 1:160 9mm
K 1:174 8mm
QOO 1:174 7.62mm
Z 1:220 6mm
X 1:304.8 4.5mm

NARROW GAUGES

The following are the narrow gauge scales that are often used by modelers:

Scale Name Ratio Gauge
SM45 1:19 45mm
SM32 1:19 32mm
2N3 1:24 45mm
2M 1:22.5 45mm
G 1:22 or 1:29 45mm
GN15 1:22 16.5mm
OM 1:48 22.5mm
ON3 1:48 19mm
ON 2-1/2 1:48 16.5mm
OE 1:48 16.5mm
O16.5 1:43.5 16.5mm
On2 1:48 14mm
Sn3-1/2 1:64 16.5mm
Sn3 1:64 14mm
Sn2 1:64 9mm
OOn3 or OO12 1:76.2 12mm
HOM 1:87.1 12mm
HON3 1:87.1 10.5mm
HoE 1:87.1 9mm
HON2-1/2 or HO30 1:87.1 9mm
HON2 1:87.1 6mm
TTN3 1:101.1 9mm
NM or NN3 1:160 6mm

The size of models varies from around 700 mm (or 28 inches) tall for the largest ‘usable’ size models of a 1:8 size, down to matchbox size in Z -scale. Typical HO engines are about 50 mm (2") tall, by 100 mm to 300 mm (4" to 12") in length.

The five most popular scales are: G, O, HO (also known in Britain as OO), TT and N. The Z scale is also growing in popularity and the Nn3 scale and Hon3 Scale are popular ‘narrow’ gauge scales.


{ 2 comments… read them below or add one }

John Stewart May 21, 2010 at 1:06 pm

Good to see the list of many scales and gauges.

In New Zealand, to model the local prototype (3′ 6″) there are three scales commonly used. Most popular is obviously Sn3-1/2. Smaller is N120 using N scale track and mechanisms. In a larger scale is 9mm with a scale ratio of 1:34, using 32mm gauge to represent the prototypes 3′ 6″. There are some excellent models and layouts in this large scale, sometimes referred to as New Zealand O Scale.

I find your comment about HO being known as OO in Britain strange. These are two different scales, HO being 3.5mm to the foot and OO being 4mm to the foot, although in most cases OO is used with HO gauge track (16.5mm), and this represents a slightly narrower than prototype track gauge. There is some true 4mm scale/gauge modelling done and you have correctly listed this under the P4/EM area.

It annoys me to see manufacturers (mainly from Britain) list a product as being OO/HO scale. They can’t be both, although if it is track they can justifiably use OO/HO gauge, as this is assumed to be 16.5mm gauge track.

John Stickler February 26, 2011 at 7:54 am

What scale is O27?

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