Understanding model railway scale and gauge

Today I’m looking into a topic that can be a bit of a head-scratcher for newcomers but is absolutely crucial to mastering the art of model railway building: understanding scale and gauge.

Whether you’re just starting out or you’re looking to refine your knowledge, this post aims to provide a clear and simple explanation that will help you navigate this fundamental aspect of model railroading.

What are scale and gauge in model railways?

OO gauge vs Z gauge locomotives

First things first, let’s clarify what we mean by “scale” and “gauge” because these two terms are often mixed up but mean different things.

  • Scale refers to the proportional size of the model compared to the real train. It answers the question: How much smaller is the model than the real thing. It’s typically expressed as a ratio or as a measurement scale such as 4mm to the foot in the case of OO scale (1:76.2). For example, HO scale at 1:87 means that one inch on the model represents 87 inches in real life.
  • Gauge, on the other hand, is the distance between the inner edges of the two rails on the track. It’s basically how wide the track is, common gauges include HO and OO gauge: 16.5mm wide rails; N gauge – 9mm wide;

For most, the track and trains are the most expensive aspect of a model railway and gauge, being the size of the track, determines how much railway can fit in a space chosen for a layout, so gauge is the usual term by which people describe their layout.

However, it’s important to realise that the gauge doesn’t always match to the corresponding scale.

Scale and gauge discrepancies: explained

While scale tells us the proportional size of the model, and gauge tells us the track width, they don’t always align perfectly in model railways.

For example, in OO model railways, the scale for models is 1:76.2 but the track width isn’t correct. In real-life standard gauge tracks, the track gauge is 4 feet 8.5 inches (1,435 mm).

If perfectly scaled down in OO scale (1:76.2), the track gauge should be about 18.83 mm. OO gauge however is the same as HO, 16.5mm.

This mismatch can be traced back to historical and practical reasons which we won’t go into here but it’s enough to say that once created, the incorrect sizing stuck and are now the norm for most people.

Unfortunately, this difference, while widely accepted, results in trains looking slightly out of scale for their track.

Purists wanting perfectly proportioned models and track, model in what is known as P4 which has a track gauge of 18.83 mm. There’s also EM sizing, which at 18mm is close to real-world proportions but not as precisely as P4.

While these are much more realistic, the track and rolling stock isn’t as easy to get and costs more so the majority stick to commonly available gauges.

Common Model Railway Track Gauges

Here’s a quick rundown of the most popular track gauges.:

  • Z Gauge: the smallest commercially available model railway size, with a track width of 6.5mm.
  • N Gauge: Known for its small size, N gauge has a track gauge of 9 mm. It’s perfect for those who don’t have a lot of space.
  • TT Gauge: Slightly larger, TT stands for “Table Top” with a 12 mm track gauge.
  • OO Gauge: Very popular in the UK, it’s the size Hornby have made famous, OO gauge features a 16.5 mm track gauge.
  • HO Gauge: This is the most common gauge globally, also measuring 16.5 mm, but scaled differently from OO.
  • 009 Gauge: A narrow gauge type within the OO scale, with a track gauge of 9 mm. These models narrower track than the standard gauge as used in mountainous regions, industrial sites, and areas where the construction of standard gauge railways was impractical or too costly.
  • O Gauge: Larger and more detailed, O gauge measures 32 mm.
  • S Gauge: Popularized by American Flyer, S gauge features a 22.5 mm track gauge.

In the next section, we’ll explore the scales that correspond to these gauges.

The most common scales used in model railways?

Matching the right scale to the gauge is key to realistic model railroading. Here are some of the most commonly used scales for the gauges above and the model scales you’ll need.

  • N Scale (1:160): Perfect for N gauge tracks.
  • TT Scale (1:120): Matches well with TT gauge.
  • OO Scale (1:76.2) and HO Scale (1:87): Both use the same gauge but differ in scale, with HO being more common globally.
  • 009 (1:76.2). This reflects narrow gauge railways so has a track width of 9mm but the models are the same as OO, 1:76.2 scale.

Dimensions for common objects on a model railway when scaled down

How tall should model figures be for major scales?

To enhance the realism of your model railway, you need properly scaled figures. Here are the approximate heights for figures in different scales:

  • N Scale: Figures should be about 10 mm tall.
  • TT Scale: Figures stand around 14 mm tall.
  • OO Scale: You’ll want figures about 20 mm in height.
  • HO Scale: Figures should be about 18 mm tall.

Common real-life distances in N, OO and HO scales.

Translating real-world distances into model sizes can be fascinating. Here’s how common distances scale down:

  • 1/2 mile translates to 53 feet in N scale and 33 feet in OO scale.
  • 1 mile is 106 feet in N scale and 66 feet in OO scale.
  • 10 miles would be 1,060 feet in N scale and 660 feet in OO scale.

How to calculate scale sizes

In the above sections, I’ve listed some common objects and their appropriate sizes in OO, N and other scales. if you want to check my maths or do your own calculations, here’s the calculations needed.

If you’re not good at maths or don’t have a calculator handy, I’ve created a simple online model railway scale calculator to help.

Founder of ModelRailwayEngineer, Andy Leaning

Andy is a lifelong modeler, writer, and founder of modelrailwayengineer.com. He has been building model railways, dioramas, and miniatures for over 20 years. His passion for model making and railways began when he was a child, building his first layout at the age of seven.
Andy’s particular passion is making scenery and structures in 4mm scale, which he sells commercially. He is particularly interested in modelling the railways of South West England during the late Victorian/early Edwardian era, although he also enjoys making sci-fi and fantasy figures and dioramas. His website has won several awards, and he is a member of MERG (Model Railway Electronics Group) and the 009 Society.
When not making models, Andy lives in Surrey with his wife and teenage son. Other interests include history, science fiction, photography, and programming. Read more about Andy.

Afflliate disclosure:The links on this page may take you to carefully selected businesses, such as Hornby, Amazon, eBay and Scale Model Scenery, where you can purchase the product under affiliate programmes. This means I receive a small commission on any orders placed although the price you pay does not change. You can read my full affiliate policy here. I also sell my my own ready to use, pre-made and painted buildings and terrain features. browse the range.

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