Model Railway Scales and gauges explained

A simple guide to understand the difference between scale vs. gauge

One of the most confusing aspects for a new railway modeller to understand in the hobby is the difference between the terms ‘scale’ and ‘gauge’. In the UK, ‘gauge’ is used more often than the word ‘scale’ - a term more popular around the rest of the world.

From left to right, three different model railway scales: N gauge (2mm:1ft scale), OO gauge (4mm:1ft scale) and G scale.

GAUGE VERSUS 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. Read on to discover why…

BACK STORY

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.

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.

Manufactured in 1938 for export only, this Marklin HO three-rail E800 4-4-0 'Compound' locomotive and tender sold for £21,000 at the Vectis train and toy auction on January 23, 2010.

SETTING OO STANDARDS

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.

EM AND P4

Some modellers wanted to remain true to scale and couldn’t cope with an inaccurate scale/gauge ratio in OO. A group of modellers decided to continue to model in 4mm:1ft scale, widening the track gauge to suit. Real railway tracks in the UK have a gauge of 4ft 8½in. In the scale of 4mm:1ft, this equates to approximately 18mm. The EM (Eighteen Millimetre) Gauge Society was born.

Later, it was considered by some that 18mm was too approximate, so the Protofour (P4) Society was formed, working to the exact scale track gauge of 18.83mm. The P4 Society was superceeded by The Scalefour Society who took over the reins of exact scale 4mm modelling.

GETTING SMALLER: TT

As things changed post-World War II, the amount of space available in the average house for model railway layouts was getting smaller. Mainland Europe had responded by creating a scale of 2.54mm:1ft (a ratio of 1:120). This was observed by the Managing Director of Tri-ang Railways, then one of Hornby’s competitors, who saw the possibilities that this scale could provide. A range was started in the UK, to a scale of 3mm:1ft (a ratio of 1:101.6) because of the usual problems with fitting mechanisms inside the smaller bodies. Tri-ang called this ‘Table Top’, or TT gauge. The track gauge was maintained at 12mm, causing a similar arrangement to OO.

From its launch in 1957, TT wasn’t as popular as hoped, probably because of the sheer dominance of OO gauge and the rising popularity of N gauge. When Tri-ang decided to drop the range in the late-1960s, a group of modellers in the scale formed The 3mm Society. The accepted track gauge was 12mm, but by then there were some already working (as with EM gauge vs. OO gauge) to the closer-to-scale track gauge of 13.5mm or the dead-scale (as with P4 vs. OO gauge) 14.2mm gauge.

SMALLER STILL: N

Loch Tat is an N gauge model railway set in the Highlands of Scotland. The smaller size of N gauge makes it easier to create greater landscapes for a given space than OO or O gauges. Read more about this layout here.

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.

Z AND T SCALES

The problems affecting the other scales in the UK haven’t affected the smaller scales of Z and T, probably because the former isn’t as commonplace and the latter is comparatively new! Z scale is almost entirely the province of German manufacturer Märklin as its ‘Mini-Club’ range, with a track gauge of 6mm and a scale of 1.4mm:1ft (a ratio of 1:220). A range of UK-based layouts have been built in Z, even one in ‘finescale’ Z, or Proto-Z, with a track gauge of 6.5mm.

The newest of the scales is T, produced in Japan, with the remarkably small gauge of 3mm between the rails, which scales to 0.61mm:1ft or a ratio of 1:450. You can now achieve your model of the terminus at Waterloo in a small room!

BACK TO THE BEGINNING

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.

Need more information - watch Phil Parker explain the key differences between the scales and gauges in this useful video.

NARROW-GAUGE EXPLAINED

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

Options increase as we head towards the 4mm:1ft scales...

• By far the most popular narrow-gauge is OO9, supported by the OO9 Society, where 4mm:1ft scale is used with N gauge (9mm) track to produce a 2½ft gauge prototype. This has the benefit of many kits and adapting OO gauge models.
• OOn3 (or OOn12) is the modelling of 3ft gauge railways using 4mm:1ft scale models on TT or 12mm gauge track.
• 5.5mm scale enables the correct scale modelling of Talyllyn, Ffestiniog or other 2ft to 2ft 6in gauge rolling stock on commercial TT gauge track. It also extends to modelling 3ft gauge prototypes on OO (or HO0 scale track).

In the larger scales...

• O-16.5 is the modelling of approximately 2ft 3in gauge prototypes in 7mm scale using OO gauge track (16.5mm).
• On30, is ¼in :1ft scale, using OO gauge track, therefore producing close to prototype 2ft 6in gauge.
• 16mm narrow-gauge uses 16mm:1ft models, running on 32mm gauge track to produce models of 2ft narrow-gauge prototypes.
• The little-known 10mm scale reproduces approximately both 3ft and 18in prototypes depending on the use of O or OO gauge track.

One of the mainstream narrow-gauge ranges in the larger scales is G. Originally started by Lehmann in Germany in the 1960s, using the trade name LGB (translated as Lehmann’s Garden Railway), G scale is the modelling of narrow-gauge railways (to various scales ranging from 1:20 to 1:29) on track 45mm wide - the same as gauge 1, but having a heavier profile track suitable for use in the garden.

SCALE/GAUGE COMBINATIONS - STANDARD GAUGE

Notation	Ratio	Scale	Track gauge	Society Support	Comments
T	1:450	0.64mm	3mm	www.tgauge.co.uk	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	www.2mm.org.uk	2mm finescale
OOO	1:152	2mm	9.5mm	www.ngaugesociety.com	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	www.3mmsociety.org.uk	European 3mm scale RTR
TT	1:101.6	3mm	12mm	www.3mmsociety.org.uk	UK 3mm scale started by Tri-ang Railways
3mm	1:101.6	3mm	13.5mm/14.2mm	www.3mmsociety.org.uk	3mm finescale
HO	1:87.1	3.5mm	16.5mm	www.british-ho.com	US/Continental outline - limited range of UK equipment
OO	1:76.2	4mm	16.5mm	www.doubleogauge.com	Most popular scale in the UK with huge range of RTR
EM	1:76.2	4mm	18mm/18.2mm	www.emgs.org	Closer scale/gauge ratio for 4mm scale
P4 (or S4)	1:76.2	4mm	18.83mm	www.scalefour.org	Dead-scale track gauge for 4mm
S	1:64	3/16”	7/8”	www.s-scale.org.uk	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	www.gauge0guild.com	UK O gauge - growing range of RTR, mostly kit or scratch-built
S7	1:43.5	7mm	33mm	www.scaleseven.org.uk	Dead-scale track gauge for 7mm scale
1	1:32	3/8”	45mm	www.gaugeone.org	Closer-scale gauge 1, some RTR available including live steam
1	1:30.5	10mm	45mm	www.gaugeone.org	Gauge 1, some RTR available including live steam
3	1:22.5	13.5mm	2 ½”	www.gauge3.co.uk	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.

SCALE/GAUGE COMBINATIONS – NARROW GAUGE

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

SUMMARY

• 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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