Scale & Gauge


This page contains information about the different scales and gauges used in model railways.

Definition of scale

At first brush, the definition of scale seems unambiguous. Scale is the ratio of the model size to the prototype size. This can be expressed as the ratio 1:n or as the length that specifies how long one foot on the prototype is on the model. The table below illustrates these two ways of specifying the scale for N, HO and G scale.

Scale 1:n Length
N 1:160 2mm
HO 1:87 4mm
G 1:22.5 13mm

As I hinted at above, there are ambiguities in the meaning of "scale". These ambiguities include the following:

  1. The same name is used for different scale ratios. For example, N scale can refer to a scale ratio of 1:160, 1:150 and 1:148. There is a similar ambiquity with G scale. I was mislead by the N ambiguity several months into the hobby. I bought one N scale steam locomotive and 10 N scale freight wagons. I did not realise until several weeks later that the locomotive was 1:160 and the wagons 1:148.
  2. The words scale and gauge (the distance between the inner faces of the two rails) are often used as synonyms. For instance, many modellers use the expressions "HO scale" and "HO gauge" as synonyms. This may seem strange because scale and gauge are clearly different concepts. The interchanging often works because a particular scale is usually run on just one scale. An ambiguity can arise because some modellers, in an effort to improve the accuracy of the modelling, use a gauge that is slightly different from the norm. This is illustrated by O scale and P48 scale. Both have a scale ratio of 1:48. O scale uses a gauge of 31.75 mm and P48 a gauge of 29.9 mm. If the distinction between O  and P48 is not made clearly, an ambiguity can arise.

Scales and gauges for standard gauge

The table below lists combinations of scale and gauge for standard gauge. Some combinations have more than one name. I have listed what I believe is the mostly commonly-used name first and the alterative names in parenthesis . I have given more information about the combinations after the table.

Name 1:n Gauge
T 1:450 3mm
Z 1:220 6.5mm
N 1:160 9mm
  1:150 9mm
  1:148 9mm
2mm 1:152 9.42mm
TT 1:120 12mm
  1:130 12mm
3mm 1:101.6 12mm
  1:101.6 13.5mm
  1:101.6 14.2mm
Proto:87 1:87 16.5mm
HO 1:87 16.5mm
  1:87.1 16.5mm
P4 1:76.2 18.83mm
S4 1:76.2 18.83mm
EM 1:76.2 18.2mm
OO 1:76.2 16.5mm
American OO 1:76.2 19mm
S 1:64 22.42mm
5.5mm 1:55 26mm (need confirmation of gauge)
6mm 1:50  28mm (need confirmation of scale ratio and gauge)
Proto:48 1:48 29.90mm
ScaleSeven 1:43.5 33mm
O 1:48 32mm
  1:45 32mm
  1:43.5 32mm
Gauge 1 (1 gauge, 3/8") 1:32 45mm (44.45mm)
Gauge 3 1:22.5 63.5mm
F scale 1:20.32 70.64mm


T scale

T scale is the smallest commerically available model railway scale. Items went on sale 2007. Not surprisingly given the size of the rolling stock and scenery items the scale has a limited following. Nevertheless, the number of commercial items available is increasing and people who have a limited amount of space or enjoy working with very small scale objects could will find T scale a rewarding challenge.

Z scale

Z scale was introduced 1972 and is now well established. There is a good choice for most standard items except steam locomotives with DCC and to a lesser extent diesel locomotives with DCC. Unlike HO scale, there are now cheap, low precision locomotives available. The smallness of z scale items means locomotives must be of high precision to be reliable.

N scale

N scale is reportedly the second to most popular scale behind HO scale and many commercial products are available. As noted earlier in the article, the term N scale can refer to three different scale ratios: 1:160, 1:150 and 1:148. The main ratios in the US and the UK are 1:160 and 1:148 respectively. The ratio 1:160 is also used in Europe and Japan. The ratio 1:150 is used in Japan. Why was the scale called 'N'?

TT scale

The 'TT' in 'TT scale' denotes table top, the suggestion being that the scale is sufficiently small layouts will fit on the top of a table. The scale was invented in the US and the first commercial parts appeared 1946. Two scale ratios are used: 1:120 and 1:130. The ratio 1:120 is the more common; 1:130 is used in Russia and the former USSR.

3mm scale

The 3mm scale is used with gauges of 12mm, 13.5mm and 14.2mm for standard gauge. The combination of 3mm scale and 12mm gauge is often called British TT scale because this combination started from TT scale - the British prototypes were smaller than American protoypes and scale ratio was increased to 1:101.6. The gauge 14.2mm is a finescale form.

HO and Proto:87

HO scale, the 'HO' denotes half O, is the most popular scale in most countries and regions except the UK where OO scale is the most popular. Commerical parts for HO first appeared in the 1920s. By the 1960s, HO scale was rivaling O scale in the US. There is now a very large range of rolling stock and scenery items.

There are two standards for HO scale. The National Model Railway Association (NMRA) defines the scale as 87.1 round to one decimal place. This scale is that typically used in the US. The MOROP defines the scale as 1:87 exactly. This scale is used mostly in Europe. The difference between 1:87 and 1:87.1 is insignificant for most modellers.

Proto:87 is a finescale alternative to the NMRA's HO scale.

OO, EM, P4, S4 and American OO

The first commerical items for OO appeared in 1922. OO is now the most popular scale in the UK. The scales EM, P4 and S4 are refinements that make OO scale more prototypical. EM scale, more correctly EM gauge, was introduced in the 1950s with a gauge of 18mm. This was later increased to the current 18.2mm. P4 is a refinement of EM. The standard for P4 was published in 1966. The main difference between EM and P4 is the increase of gauge from 18.2mm to the prototypical 18.83mm. S4 is a refinement of P4. It removes an allowance in the P4 standards for the tight curves used on model railways.

As the name implies American OO is an American variant of OO. The gauge is 19mm. American OO was introduced in the 1930s as one of two ways of making OO scale more prototypical by increasing the gauge (the other way was reducing the scale to 1:87). American OO retains a small but loyal following today.

S scale

Commerical items for S scale first appeared in 1896 making the scale one of the oldest. Althought S scale is nowhere as popular as HO and N scale, there is a good range of rolling stock and scenery items commercially available.

O, ScaleSeven and Proto:48

The name O scale is used with three scale ratios. In the US, O scale usually means 1:48. This is the scale ratio used in the NMRA standards. In the UK, O scale means 1:43.5 which is 7.007 millimetres to the foot. In Europe O scale usually means 1:45 which is the scale ratio used in the MOROP standards. The scale ratios 1:48 and 1:43.5 are also used in Europe.

ScaleSeven is a finescale version of the UK's 1:43, and Proto:48 is finescale version of the US's 1:48.

Gauge 1 

Gauge 1 was popular in the US and UK before World War I. The popularity declined after the war. Fortunately the scale was kept alive by enthusiasts and the scale saw a resurgence in the 1950s and 1960s. The scale is now well established and there is a reasonable selection of commerical items available.

Scales and gauges for narrow gauge

The expression 'narrow-gauge' has more than one meaning. I use it to mean any gauge smaller than 4 foot 8 1/2 inches. Although there is no standardized nomenclature for narrow gauges there are some general rules.

  1. The standards issued by MOROP denote a narrow gauge by appending a lowercase letter to the name for the scale. The possible letters and there meaning are
    Gauge (metric) Gauge (imperial) Description Letter
    850-1250 mm 33-49 in metre gauge m
    650-850 mm 26-33 in narrow gauge e
    400-650 mm 16-26 in industrial i
        Feldbahn (field railway) f
    300-400 mm 12-16 in park p
  2. Narrow gauges in the US are usually denoted by appending the letter 'n' followed by size of the gauge in either inches or feet to the name of the scale. For example 'HOn3' means HO scale with a gauge of three feet, and 'HOn30' means HO scale with a gauge of 30 inches.
  3. Some narrow gauges were define independently of standard gauge.

I have listed some combinations of scale and narrow gauge. The list is ordered by descreasing size of gauge.

  • 45 mm gauge
    • SE45. Scale ratio of 1:13.7 (7/8", SE scale). Used to model two foot gauge.
    • SM45. 1:19 (16 mm scale). Two foot nine inch gauge.
    • Fn3. 1:20.3 (F scale). Three foot gauge.
    • G. 1:22.5. One metre gauge.
  • 32 mm gauge
    • SE32. 1:13.7 (7/8"). Eighteen inch gauge.
    • SM32. 1:19. Two foot gauge.
    • Fn2. 1:20.3. Two foot gauge.
    • P34. 1.34 (9mm). Three foot six inch gauge.
  • 22.2 mm gauge
    • On42. 1:48 (1/4", O scale). Three foot six inch gauge.
  • 21 mm gauge
    • O21. 1:43.5 (7mm, O scale). Three foot gauge.
  • 19 mm gauge
    • On3. 1:48 (1/4", O scale). Three foot gauge.
    • ???. 1:55 (5.5mm). Three foot six gauge.
  • 18.2 mm gauge (EM gauge)
    • Pempoul. 1:50. One metre gauge.
  • 16.5 mm gauge (HO / OO gauge)
    • Gn15. 1:22.5 (G scale). Fifteen inch gauge
    • 3/8n20. 1:32 (3/8"). Twenty inch gauge.
    • O16.5. 1:43.5 (7mm, O scale). Two foot four inch gauge.
    • Oe. 1:45 (O scale). 750 millimetre gauge
    • On30. 1:48 (1/4", O scale). Two foot six inch gauge.
    • On2.5. The same as the previous.
    • ???. 1:55 (5.5mm). Three foot gauge. The obvious name for this combination of scale and gauge is 5.5mmn3 but that is clumsy nomenclature.
    • Sm. 1:64 (3/16", S scale). European metre gauge.
    • Sn3.5. 1:64 (3/16", S scale). Three foot six inch gauge.
  • 14.3 mm gauge.
    • Sn3. 1:64 (3/16", S scale). Three foot gauge.
  • 14 mm gauge.
    • O14. 1:43.5 (7mm, O scale). Two foot gauge.
  • 12.7 mm gauge
    • On2. 1:48 (1/4", O scale). Two foot gauge.
  • 12 mm gauge (TT gauge)
    • Towy Valley Tramway. 6mm. Two foot gauge.
    • ???. 1:55 (5.5mm). Two foot gauge. The obvious name for this combination of scale and gauge is 5.5mmn2 but that is clumsy nomenclature.
    • OOn3. 1:76.2 (4mm, OO scale). Three foot gauge.
    • HOm. 1:87 (3.5mm, HO scale). One metre gauge.
    • HOn3.5. 1:87 (3.5mm, HO scale). Three foot six gauge.
  • 10.5 mm gauge.
    • On20. 1:48 (1/4", O scale). Twenty inch gauge.
    • HOn3. 1:87 (3.5mm, HO scale). Three foot gauge.
  • 9 mm gauge.
    • Gnine. 1:22.5 (G scale). Eight inch gauge.
    • O9. 1:43.5 (7mm, O scale). Fifteen inch gauge.
    • On15. The same as the previous.
    • Op. 1:45 (O scale). Used to model 400 mm gauge.
    • On18. 1;48 (1/4", O scale). Eighteen inch gauge.
    • Of. 1:48 (1/4", O scale). 450 mm gauge.
    • Sn2. 1:64 (3/16", S scale). Two foot gauge.
    • OO-9. 1:76.2. (4mm, OO scale). Two foot three inch gauge.
    • HOe. 1:87 (HO scale). 750 mm gauge.
    • HOn30. 1:87 (HO scale). Two foot six inch gauge.
    • HOn2.5. The same as the previous.
    • TTn3. 1:120 (3mm, TT scale). Three foot gauge.
    • NZ120. 1:120 (3mm, TT scale). Three foot six gauge.
    • ???. 1:55 (5.5mm). Fifteen inch gauge.
    • ???. 1:55 (5.5mm). Eighteen inch gauge.
  • 6.5 mm (Z gauge).
    • ???. 1:76.2 (4mm, OO scale). Used to model 500 mm gauge. The combiniation of scale and gauge could be called OOn500 or OOi (or OOf).
    • ???. 1:76.2 (4mm, OO scale). Used to model 495 mm gauge. The combiniation of scale and gauge could be called OOn500 or OOi (or OOf).
    • HOf. 1:87 (HO scale). 600 mm gauge.
    • HOi. The same as the previous.
    • Nn3. 1:160 (2mm, N scale). Three foot gauge.