Model Rail Scales and Gauges

Newcomers to the hobby of railway modelling are often confused by the variety of scales and gauges in use. This is not helped by the fact that most British railway models are a slightly different scale to those used by most other countries! In this post we’ll cover the main scales and gauges in use in the UK.

First of all, let’s cover the difference: ‘Scale’ and ‘Gauge’ both tend to be used to describe a model (e.g. ‘O Scale’ or ‘O gauge’) but they are not the same thing! ‘Scale’ means the ratio between the model and the real thing, so 1:87 scale means that 1mm on the model represents 87mm (8.7cm) on the full-size original; whereas ‘gauge’ means the width of the track, i.e. the distance between the rails.

To understand the scales and gauges in use in model railways, we need to delve into history a bit, starting with the introduction of ‘O’ gauge:

O: Scale 1:43.5, track gauge 32mm

O gauge was originally introduced by the German manufacturer Märklin in about 1900, followed by other German manufacturers such as Bing. Hornby Railways (at that time part of Meccano Ltd) started their O gauge ‘tinplate’ models in 1920, originally in clockwork, later (in the 1930s) with a 20V AC electric system.

The 32 mm track gauge gives an almost exactly correct scale width for standard gauge track (4ft 8½in, or 1,435.1 mm).

(Incidentally, this was originally intended to be called ‘0’ (number zero), as being a step down from the larger scales numbered 1 – 6 (starting with ‘Gauge 1’: 1:32, 45 mm track gauge), but rapidly became known as ‘O’ (letter O). For a while the two were interchangeable but these days it is known as ‘O’ Gauge (letter ‘O’).

HO: Scale 1:87, track gauge 16.5mm

HO was introduced in the 1920s. (The name comes from ‘Half O’ (or ‘Halb O’ in German) because the scale is exactly half that of ‘O’). This is the most popular scale for models of European prototypes, with rolling stock available from numerous manufacturers such as Fleischmann, but very few ‘British outline’ models have ever been produced in HO.

(Hornby does not produce any HO models itself, but it does now own several European HO brands: Jouef (France), Lima (Italy), Electrotren (Spain) and Rivarossi (Italy) but so far it hasn’t show any inclination to rebrand these as Hornby; the only ‘co-branding’ is on the Hornby track packs which now carry the logos for Jouef, Lima, Electrotren and Rivarossi as well as Hornby.
OO: Scale 1:76, track gauge 16.5mm

Because the British loading gauge is smaller than the continental one and so British locomotives are smaller than their European counterparts, the electric motors available at the time were too large to fit into HO-scale-sized bodies, so a compromise was decided: the track gauge of 16.5 mm was retained, but the scale was increased slightly and named ‘OO’; hence the famous brand name ‘Hornby Dublo’ (‘Double O’).

This means that strictly speaking OO models are slightly ‘narrow gauge’ (the correct exact width for standard gauge in OO scale would be almost exactly 18mm), but this is not really obvious unless the model is viewed ‘head on’.

All Hornby train sets are ‘OO’. (The only exception is the ‘Arnold Starter Train Set’, which is based on a German prototype and is to ‘European N’ scale (see below). This is made by Arnold (a German manufacturer but now owned by Hornby) but sold and distributed in the UK by Hornby, and is listed on Hornby’s website. )
OO is the most popular scale in the UK, with models available from Bachmann, Heljan and numerous other smaller manufacturers as well as Hornby, and with the widest range of accessories (buildings, figures, cars etc.). Hornby produce the ‘Skaledale’ range of OO scale cast resin model buildings.

Note: Because the scales are quite close, some HO models can be used on OO layouts, with some careful placement. HO models of cars, people etc. are noticeably smaller if they are put right next to a similar OO model, but if they are situated on their own the difference is not so obvious. Many items where the exact dimensions are not so important, such as lineside accessories like fencing, are often described as “OO/HO”.

European N: Scale 1:160, track gauge 9mm

This was originally introduced by the German manufacturer Arnold, but many other manufacturers now make N scale models, including Fleischmann, Roco (now owned by Hornby) and Märklin.
There is also a wide range of accessories available in this scale.

British N (Scale 1:148, track gauge 9mm)

As with HO, when N gauge was first introduced the mechanisms available were too large to fit into N-scale-sized bodies of British prototypes, so a compromise of 1:148 scale on 9 mm track was introduced.

This is now almost as popular as OO. British N Gauge used to be dominated by Graham Farish (now part of Bachmann) and PECO, but a wide range of other manufacturers now produce models in this scale.

Hornby have dipped their toe in the water in this scale, with the introduction of the ‘Brighton Belle’ train, produced by Arnold, but so far this is their only item of British N Gauge rolling stock.

(Please note that this is a ‘Train Packnot a ‘Train Set’, i.e. it is sold as a complete 5-car train but without any track or a controller.)

Hornby also produce the ‘Lyddle End’ range of N scale cast resin model buildings, many of which are models of the same buildings as their ‘Skaledale’ OO equivalent.


Finally, it might be worth mentioning the ‘millimetres per foot’ ratios. At first glance this might seem a strange mixture of metric and imperial measurements, but it is actually quite useful, for example when trying to work out the dimensions of buildings. So in O Gauge, 7mm is (near enough) a scale foot (12 inches); in HO, 3.5mm; in 00, 4mm; and in N, 2mm; and these are often used as informal descriptions for the scale as well, i.e. O Gauge is called “7mm scale” and OO is called “4mm scale”.

(For details of other model rail scales not covered here, including those used in Europe, America and Japan, see the Wikipedia article ‘List of rail transport modelling scale standards’.)

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