Track Gauge & Scale


Gauge is the distance between the rails of the track in the scale chosen. Scale is the proportion of the model to the full-size item. Therefore, 4mm:1ft scale should be more correct in describing a layout, whereas OO gauge should really only refer to the track gauge. The UK has an odd combination of scales and gauges. Here in the Sawmill we have layouts in G Gauge, O Gauge, OO Gauge and N Gauge


In the early 20th century manufacturers decided to standardise on a numbering scheme for the various sizes of models being produced. Gauges 0, 1, 2 and 3 were created to cover the four popular scales at the time.

  • Basic O gauge is to a scale of ¼in to 1ft
  • Gauge 1 is 3/8in to 1ft
  • Gauge 2 is 7/16in to 1ft
  • Gauge 3 is 12/32in or 1/2in to 1ft

Gauges above these tend to use the measurement concerned compared to the foot, for instance 3.5in gauge, 5 1/2in gauge or 7 1/4in gauge – these tend to be the realm of live steam miniature locomotives and model engineering rather than model railways. Our miniature railway in the Park is 5 1/2in gauge

Before the second World War, German manufacturers began to produce railway models to the scale of 3.5mm to 1ft or half the size of O gauge. This scale was referred to as ‘half-O’ or HO scale. It remains the most popular railway modelling scale used throughout the world, with the exception of the UK. The UK uses HO gauge track, but with everything else modelled to a scale of 4mm:1ft because of the UK’s restricted loading gauge. Most British rolling stock in HO scale looked small in comparison to its continental counterparts and it was difficult to fit mechanisms into the smaller prototypes because of the motor and gearbox technology of the time.

The UK standard was set that rolling stock would be produced to a track gauge of 16.5mm (HO), but everything else would be increased by 0.5mm from 3.5mm:1ft to 4mm:1ft scale. This means that ‘OO’ is a ‘narrow gauge’ because the spacing between the track isn’t correct. Model railway manufacturer, Hornby, produced its ‘Dublo’ range, cementing the scale in the nation’s heart.

  • HO is to a scale of 3.5mm to 1ft (a ratio of 1:87)
  • OO is to a scale of 4mm to 1ft (a ratio of 1:76)
  • Both OO and HO use the same gauge of track (the distance between the two rails)

Those who model in OO gauge have the benefit of The Double O Gauge Association to help maintain standards within the manufacturing side of the hobby

Smaller still – N Scale

The scale of 2mm:1ft was tried out in the UK by Lone Star in the 1960s and branded OOO gauge. It was German manufacturer Arnold which first produced a full range in the scale and called it N after its nine-millimetre gauge. Changes occurred in Britain, possibly because of the problems in fitting suitable mechanisms into our stock. The UK and the rest of the world use the standard track gauge of 9mm, but the UK uses a scale of 2.06mm:1ft. Other countries use a scale of 1.91mm:1ft (ratios of 1:148 and 1:160 respectively).

Some modellers, dissatisfied with these standards in the UK and the over-tall appearance of the rails, introduced finer standards and thus the 2mm Scale Association was born. Working to a track gauge of 9.42mm (dead-scale for standard gauge at a ratio of 1:152), with a finer, more accurate track profile, the Association has set new standards in 2FS ‘finescale’ modelling.

Larger scales

Following a downturn, because of the popularity of OO gauge, O gauge has become more popular recently, due in no small part to the efforts of the Gauge O Guild and manufacturers old and new. As with most other scales, there is a difference between O gauge in the UK, mainland Europe and the USA. The scale in Europe has settled down over time to the unusual mixture of imperial and metric. Europe has a track gauge of 32mm, but the Continental ratio varies from country to country.

In the UK, we use a scale of 7mm:1ft (a ratio of 1:43.5). In the USA, O gauge is modelled to a scale of ¼in:1ft (or 6.4mm:1ft), with a track gauge of 1¼in (almost 32mm). Scale modellers in the UK tend to use a track gauge of 32mm, but there is a growing band of modellers working to the dead-scale gauge of 33mm as supported by the Scale 7 Society.

The larger scales haven’t been forgotten either, Gauge 1 has the Gauge 1 Model Railway Association who promote the standard scale of 10mm:1ft (a ratio of 1:30), but also the slightly smaller but more prototypical 3⁄8in, 1:32 or 9.5mm to 1ft (this is also more common in the US).

Gauge 3 has the Gauge 3 Society for support and is used as a scenic indoor scale, rather than its engineering cousin 21/2in gauge, used for ride-behind locomotives.

A scale which sits between the popular scales of OO and O scales is S scale. It’s still one of the true imperial scales, at 3/16 in:1ft (a ratio of 1:64) with a track gauge of 7/8in. Unlike the US, where the scale is used by a large manufacturer, it is mostly used by scratch-builders in the UK (those who build from scratch without using kits). The S Scale Society offers products to facilitate modellers.

An organisation supporting all of the scales between S and 3 is the Association of Larger Scale Railway Modellers.

Large scale or G scale (45 mm or 13⁄4 inches, G gauge) is a track gauge for model railways which is often used outdoors because of its size and durability. These garden railways use a fixed track gauge of 45 millimetres (1.75 in) to accommodate a range of rail transport modelling scales between narrow gauge (~1:13‒1:19‒1:20), metre gauge (1:22.5), Playmobil trains (~1:24), and standard gauge (~1:29–1:32).[3][2]

G scale was introduced in 1968 by Ernst Paul Lehmann Patentwerk, a German firm that produced model railroad items under the brand name LGB, for Lehmann Groß Bahn—”Lehmann Big Train” in German. LGB products were intended for indoor and outdoor use; some people have come to interpret “G scale” as standing for “garden scale”.

Most track is made of brass which can remain outside in all weathers. Track can also be obtained in less expensive aluminium as well as oxidation-resistant, though more expensive, stainless steel.


A narrow gauge line (left) and standard gauge line (right) on Caroline Concrete Works.

A narrow gauge railway is a railway which uses a track gauge which is narrower than the standard track gauge. In the UK, the term is used to describe all railways which have a distance between rails of less than 4ft 8 1/2in. Most modelling scales have a narrow-gauge option, usually using the track gauge of one scale with the scale ratio of another. In the UK, the scale term is usually the main scale followed by either the track gauge or the secondary scale. This differs in the US by being the main scale plus the prototype’s track gauge.

  • In N gauge, to a scale of 2mm:1ft, Z gauge track is used to model metre or 3ft gauge lines and is known as N-Z (Nn3 in the US)
  • T scale track can be used with N gauge to produce 18ft gauge – known as N-T (Nn18).
  • N-5 (Nn30) uses 5mm gauge track to produce 2½in gauge
  • TT-Z is 3mm:1ft scale running on Z gauge track for a 2ft gauge prototype


Notation Ratio Scale Track gauge Society Support Comments
T 1:450 0.64mm 3mm Smallest commercially available RTR system
Z 1:220 1.4mm 6mm Small amount of trade support
N 1:160 1.9mm 9mm European N gauge RTR
2mm 1:152 2mm 9.42mm 2mm finescale
OOO 1:152 2mm 9.5mm Approximate N scale pioneered by Lone Star – obsolete
N 1:148 2.06mm 9mm UK N gauge – large range of RTR equipment
TT (or TT3) 1:120 2.5mm 12mm European 3mm scale RTR
TT 1:101.6 3mm 12mm UK 3mm scale started by Tri-ang Railways
3mm 1:101.6 3mm 13.5mm/14.2mm 3mm finescale
HO 1:87.1 3.5mm 16.5mm US/Continental outline – limited range of UK equipment
OO 1:76.2 4mm 16.5mm Most popular scale in the UK with huge range of RTR
EM 1:76.2 4mm 18mm/18.2mm Closer scale/gauge ratio for 4mm scale
P4 (or S4) 1:76.2 4mm 18.83mm Dead-scale track gauge for 4mm
S 1:64 3/16” 7/8” Imperial scale for scratch-builders, initiated in the US
O 1:48 ¼” 1 ¼” US O scale (NMRA standards) – see note 1
O 1:45 6.8mm 32mm European O gauge (MOROP standards) – see note 2
O 1:43.5 7mm 32mm UK O gauge – growing range of RTR, mostly kit or scratch-built
S7 1:43.5 7mm 33mm Dead-scale track gauge for 7mm scale
1 1:32 3/8” 45mm Closer-scale gauge 1, some RTR available including live steam
1 1:30.5 10mm 45mm Gauge 1, some RTR available including live steam
3 1:22.5 13.5mm 2 ½” Mainly live steam – some electric from fi rms such as GRS

*Scales/gauges above gauge 3 are considered model engineering. US Proto 48 standard uses 1.176in (29.87mm) gauge. 1:43.5 scale in common use in France and Germany.



Notation Ratio Scale Model Prototype Society Support
G 1:20-1:29 Various 45mm various
16mm 1:32 16mm 32mm 2ft
16mm 1:32 10mm 32mm 3ft
16mm 1:32 10mm 16.5mm circa 18in
O-16.5 1:43 7mm 16.5mm circa 2ft 3in
O9 1:43 7mm 9mm 15in
On30 1:48 ¼” 16.5mm 2ft 6in
5.5mm 1:55 5.5mm 12-16.5mm 2ft – 3ft
OOn3 1:76 4mm 12mm 3ft
009 1:76 4mm 9mm circa 2ft
P7.83 1:76 4mm 7.83mm 2ft



  • Gauge defines the distance between the tracks. Narrow-gauge is the modelling of tracks narrower than standard main line railways.
  • Scale defines the size of the model vs. reality. 4mm:1ft scale means every foot measured in real life equates to 4mm on the model.
  • Ratio is the difference between the size of the model and the real thing. 1:76, means that the model is 76 times smaller than the real thing.
  • Notations vary, and can often be confusing, but usually contain a combination of the scale or gauge.

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