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Gauge and Scale

To newcomers to the hobby, the wide range of scales available can be a bit daunting, added to which are the various track gauges, often several associated with one scale and often several scales associated with one track gauge.

We have produced a summary, highlighting the common gauge/scale combinations, those where there is significant specialist support and those where you will really need to do pretty much everything yourself from scratch.


The scale defines how big the model is relative to the original. Using OO scale it is 1:76 scale. So the model is 76 times smaller than the original. To make life easier for modellers they also use the number of millimetres to the foot. So OO is 4mm to the foot, often shortened to 4mm scale. So, the same scale has, typically three names, in this case OO, 4mm & 1:76. Narrow gauge only adds to the confusion but an overview is available.

An additional confusion is that Britain, typically, uses a different scale to the USA and Continental Europe (and much of the rest of the world). It's easier in OO in some respects as the equivalent to OO is HO, so a different name. Note that these are 4mm/3.5mm and 1:72/1:87 respectively, so not identical but occupy the same space in the modelling world. O gauge and N gauge are, however, more confusing as they use different scales but are called the same thing.

If you want to understand a bit of the background to the different scales you can find a potted history of each of the scales and why, if there is any rationale at all, they are called what they are.

It's not as bad as it seems because there are only a few scales that are widely used and you will soon get used to them.


The gauge is more straightforward, mostly. In the UK you will hear the terms Standard Gauge, Narrow Gauge and, possibly, Broad Gauge. For the UK standard gauge is 4' 8.5“. Anything less than this is “Narrow Gauge” and anything bigger, which in the UK is almost certainly historical, is “Broad Gauge”. However, the terms are more complex when considering other countries. Ireland for instance uses 5' 3” and Russia 5' which, to us, are broad gauge but to them “Standard Gauge” so you do need to beware these terms. Narrow gauge too covers a very wide range, typically from 1' 11“ to 3' 6”. There is more on Narrow Gauge available.

This diagram helps show the relative differences between the scales, it may not display on screen at actual size. We haven't covered scales larger than O Gauge here but you can find more information in the potted histories. You will also note that the smallest scale here is Z Gauge, remember that T Gauge is approximately half this size.


So how does this affect us in model form. 4' 8.5“ equates, in 4mm scale, to 18.83mm. As you can see from the table below the popular OO scale has a gauge of 16.5mm which, in scale terms, is 4' 1.5”, so somewhat narrower than the prototype. This is largely for historical reasons and the potted histories here explain how this came about. 16.5mm is, however, dead scale in HO. Similar anomalies exist in O scale and N scale, although to a lesser extent but for similar reasons. Addressing this issue has led to modellers developing specifications that provide for a more accurate track gauge. These specifications also come with more accurate wheel profiles when compared to the prototype. Typically, although there is specialist trade support, you would wouldn't be able to use the widely available commercial track and proprietary rolling stock would need to be re-wheeled. On the other hand all other aspects of these mainstream scales are still available to use. The table below summarises the gauge issues with the more common British scales.

Scale Gauge Prototype Equivalent
N 9mm 4' 6“
2mm FS 9.42mm 4' 8.5”
HO 16.5mm 4' 8.5“
OO 16.5mm 4' 1.5”
EM 18.2mm 4' 6.5“
P4 18.83mm 4' 8.5”
O 32mm 4' 7“
S7 33mm 4' 8.5”

The following tables break the common British scales down into three groups:

  1. Those widely supported by the mainstream commercial manufactures.
  2. Specialist versions of those mainstream scales that address the gauge issues.
  3. Other scales available to a greater or lesser degree in the UK.

Mainstream scales, widely supported with off-the-shelf products by Hornby, Bachmann, Dapol, Peco and many others as well as having an extensive range of specialist suppliers:

Scale Ratio mm/foot Gauge Benefits
N 1:148 - 1:160 2mm 9mm Perfect for smaller spaces or for long trains, well supported by the trade
HO 1:87 3.5mm 16.5mm Popular in Europe, the USA and elsewhere with extensive international support. Minimal support for UK prototypes
OO 1:76 4mm 16.5mm The most common model scale with huge support from the mainstream trade as well as extensive specialist trade support
O 1:43.5 - 1:48 7mm 32mm In recent years primarily a kit building scale, with very strong specialist trade support, but increasing ready-to-run support from the mainstream trade. Ideal for gardens

The first figure in the ratio column is for the UK, the second in N gauge is for the USA and Continental Europe, in O gauge most of Europe uses the same as the UK, some using 1:45, the second figure is for the USA.

Specialist British scales which all of which have good specialist trade support:

Scale Ratio Benefits
2mm FS 1:152 Dead scale version of N gauge
EM 1:76 The first near dead scale version of OO
P4 1:76 Dead scale version of OO
S7 1:43.5 Dead scale version of O

Other scales:

Scale Ratio Benefits
T gauge 1:450 Very, very small models. There is some trade support but a very limited range of British prototypes, most are Japanese
Z gauge 1:220 Limited trade support. No British ready-to-run prototypes, mostly German and a few from the USA
TT 1:100 Largely for the scratch builder with limited trade support but a strong society providing some parts and support
S scale 1:64 A scratch builders scale. Very limited support but a good society to help. Wide support in the USA
G scale 1:22.5 Almost exclusively narrow gauge prototypes, Ideal for garden railways. Gauge 3 scale on Gauge 1 track
Gauge 1 1:32 Almost certainly for the garden, ideal if you want live steam
Gauge 3 1:22 2.5“ track gauge gives you large models ideal for garden use and live steam
newcomers/scales.txt · Last modified: 2021/05/26 22:53 by timpeacock

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