Railway Pages Index

Encyclopedia Index: Definitions Standards  Scale/Gauge Tables >  Huge Large Medium Small Tiny

Crain's Scale and Gauge Encyclopedia

Scale and Gauge Definitions

The Scale/Gauge Encyclopia is my attempt to list all the model railway scales that have been used over the past 125+ years, and to list all the different names used to describe them. Further, a modeler can chose to create representations of railways that ran on different gauges of track in any one of these scales, leading to an explosion of scale and gauge combinations -- more than 1250 combinations are listed in this Encyclopedia.

To create the Encyclopedia, I read every issue of Model Railroader (and its predecessor), Narrow Gauge and Shortline Gazette (and its predecessors), Garden Railways, Finescale Railrtoader (and its predecessor), as well as a number of older hardcover books on model railroads, and the existing NMRA Standards. I kept a record of every scale and gauge combination mentioned in articles and advertisememts and built a spreadsheet to keep the list organied. Like most model railway projects, it just grew and grew, and continues to do so today.

There have been long standing differences between the practices in Britain, Europe, and North America that can cause major confusion in terminalogy, scale ratios, and usage of particular track gauges.  Over time, these have been clarified with the help of the Internet and Wikipedea.

I have modeled railways in several scale and gauge combinations across 70 years. I love model railways. I love the variety of styles, eras, and sizes that are modeled. I hate the commercial emphasis, especially by a few narrow-minded magazine editors, on just a few modeling scales (N, HO). There are magazines and websites that cater to many of the other available scales, so do search for what might appeal to you.

The Encyclopedia consists of 7 webpages containing Definitions (this page), Standards and  five Scale/Gauge Tables covering scales that can be generically described as  Huge, Large, Medium, Small, and Tiny. In all, there are 64 scales listed and more than 1250 scale and gauge combinations that have been or could be used to represent a model railway.


 A sample of the Encyclopedia page for S Scale (1:64 ratio)

Many of the scale/gauge combinations are merely different names for the same combination, but each is listed to make it easier to search for the obscure name or phrase. Many scales also have multiple names and these are listed as well.


Each Scale Table has a header that contains the primary Scale Name and the Scale Ratio, as well as Alternate Scale Names and sometimes a pronunciation guide, if the name is a series of individual letters.

The body of each table contains columns with the Table Number, Serial Number, Gauge Name, the Prototype Gauge represented by this Gauge Name (inches), Scale Ratio, Inches per foot and Millimetrs per foot for this Scale Ratio, followed by Minimum and Maximum model track gauge in inches and millimeters. If there is no maximum, it means that no standards have been published for this track gauge. This is followed by an Error  column and a commercial tarck gauge that could be used to represent this gauge.

The error column represents the percent difference between the correct gauge and the actual dimensions used to manufacture the track. A negatibe error means the track is too narrow; positive means the track is too wide.

Some of the words used in the foregoing may be unfamiliar. The following section provides the definitions you might need.

Scale and Gauge DEFINITIONS
Much confusion surrounds model railway scales, the names given to those scales, and the various track gauges that can be modeled in each scale. Let's review all the necessary definitions and possibly some of that confusion can be cleared up here.

PROTOTYPE is a word used to mean the original, full size item that is to be modeled.

SCALE or SCALE RATIO is the ratio in size between an original and a model of the original. A very popular scale ratio for model trains and model cars is 1:87, which means that the model's dimensions are 1/87th the size of the original. This translates to 3.5 millimeters equals 1 foot. The Scale Name given to this Scale Ratio is "HO", each letter pronounced separately as "aitch-oh".

This ismy 1:1 scale (full-size) model of Denver, South Park and Pacific Railway waycar #60. It is a model of a prototype that ran back in the 1880's. ==>

It's hard to image the size of the prototype compared to a model unless you put them side-by-side. Here's an example.

<== My full size caboose with a 1:20 or "F" scale train running along the bottom edge. The little caboose on thee left end of the short train is 1/20th the size of the full size caboose. An HO scale train would be about 4.5 times smaller again.





GAUGE or TRACK GAUGE is the distance between the rails of real or modeled railway tracks. On the illustration at the right, the TRACK GAUGE "G" is the distance between the inside edges of the rails. On a standard gauge railway in North America that distance is 56.5 inches. In HO Scale, the distance is 1/87th of that or 0.649 inches (16.5 millimeters). The gauge specified is usually the minimum allowed for safe operation and tolerances are gfiven to indicate the maximum distance allowed.

SCALE/GAUGE COMBINATION is a track gauge used with a particular model scale. The same gauge of model track can be used in several scales to represent different gauges in these various scales. For example, 1-3/4 inch (45 mm) gauge track is used to portray many gauges in many scales.

SCALE NAME is the word or abbreviation used as a shorthand label to identify a model's scale. A scale of 1:87 is called HO Scale. Some scales have many names; some have more than one scale ratio.

GAUGE NAME is the word or abbreviation used as a shorthand label to identify a model track gauge. For example, a track gauge of 1-3/4 inches (45mm) is traditionally called Gauge 1, but could be called Fn3 if it was used to represent 3 foot narrow gauge in F Scale. There is a phenomenal variety of names used for the same gauge.

A railway rolling stock model is not fully described unless both SCALE and GAUGE are specified. A railroad structure like a station or a water tank is fully described by its scale only, as there is no tracck gauge involved.

Unfortunately, some SCALE NAMES are used to represent more than one SCALE RATIO. For example, O Scale can mean any one of four scale ratios. This is really confusing, even to experts.

To make matters difficult, there are more than 60 different scales in use today for model railways. Not all scales are equally popular, and some are more popular in Europe or Britain than in North America. Some are exceedingly rare and never seen except in old magazine stories.

Even more confusion comes from mixing the words SCALE and GAUGE. For example HO is the name of both a scale (HO Scale) and a gauge (HO Gauge) of model train track. Some writers use the word GAUGE when they mean SCALE, and vice-versa.

A name like HOn3 is often called a SCALE but is, in fact, a GAUGE of track, namely 3 foot NARROW GAUGE, modeled in HO SCALE. Even the National Model Railroad Association (NMRA) fails to make the distinction in their Standard S-1 and related documents.

Manufacturers, advertizing copywriters, and editors have a collective amnesia about perfectly good names that have been used in the past, and insist on inventing new names. This is usually done without regard to any established conventions or naming rules.

Scales that have small ratios are called LARGE SCALES (eg. 1:20), because the models are quite large, and SMALL SCALES have large ratios (eg, 1:160). The breakpoint between large and small is usually at about 1:40 scale.

The most common commercially available scales for model trains in North America are named
Z (1:220 ratio), N (1:160), HO (1:87), S (1:64), O (1:48), and G (1:22.6+/-) scales. G Scale is only one of seven so-called “Large Scales” that have scale ratios running between 1:13.5 and 1:32. The illustration below, showing the head-on view of a modern diesel, illustrates the relative sizes of these scales. Note that the illustration on the screen is about one-half actual size.


The HO locomotive ahown above would be a little more than 1 inch wide and the G scale locomotive would be about 4-1/4 inches wide.

In the background, a 1:20 scale locomotive (22.5 inches long), in front of it is a 1:24 scale model of the same locomotive (18.75 inches long), and at the bottom right, an HO scale model of the same locomotive (5.0 inches long), all of them a DSP&P 2-6-6T Mason Bogie locomotive of the 1880's. These are 3-foot narrow gauge steamers, so they could be called Fn3, Hn3, and HOn3 gauge locomotives.

All four of these models are 28 foot 3-foot narrow gauge boxcars. At the rear is a 1:22.5 "G" Scale model. It can be used in 1:20 and 1:24 scales and no one would notice the fact that the car was not exactly 28 feet long. The Gorre and Daphetid car represents a 1:64 "S" Scale version. In front of that is a 1:87 HO Scale version and a 1:160 "N" scale car. If the G Scale car was painted for a Standard Gauge railroad, it would represent a 36 foot old-time boxcar in 1:32 "1" Scale. The G&D boxcar is actually a 36 foot HO standard gauge car, "standing-in" as an S Scale narrow gauge car for this photo.

MIXING SCALES and GAUGES can be used in model railroading for several reasons. A mix of standard and narrow gauges using a common scale is the usual situation. It mimics real life railroad scenes where two railways of different gauges meet and interchange shipments. It can lead to DUAL GAUGE TRACKWORK, which can be complicated and interesting to viewers and operators of the model trains.

Using a much smaller scale for track, trains, and buildings can simulate an amusement park ride for children and adults. I used an N Scale train on my indoor G Scale railway for this purpose. A real life garden railway built at 1:20 scale could use a Z Scale model train to represent a garden railway in the back yard of a model home.

Finally, using the next smaller scale on trains running in the baclground can be used to enhance the sense of distance by tricking the eye with forced perspective. I did this by using an O Scale train on a "distant" mountain on my G scale layout. You can translate these concepts easily to smaller scale model railways in HO, S, and O Scales. Smaller scale buildings, vehicles, and trees in the background can be used in the same way to expamd the horizon.

The yellow train in the upper background is O Scale (1:48) while the yellow train in the middle foreground is G Scale (1:22.5). The two trains are only 12 feet apart, but the sense of distance is much larger.

A 1:22.5 Scale passenger train pauses at Tiny Town to drop off children to ride on the 1/8 scale
"live-steam" train (N Scale) that circles the 1/8 scale grain elevator and station.

Mixing rolloing stock from different Scales in the same train can look strange when  the sizes are too different. This happens most often in the large scale environment where 1:32 and 1:29 Scale standard guage trains can run on the same physical track as 1:20 or 1:22.5 Scale narrow gauge trains. When an old-time steam engine is ohysically bigger than a modern diesel, the sense of realism is broken.

SCALE LENGTH is shorter than prototype length by a factor equal to the SCALE RATIO. The equation is:

Scale length (feet) = Prototype length (feet) divided by SCALE RATIO
   For example: a 40 foot boxcar in O scale would be 40 feet times 12 inches / foot / 48 = 10 inches.

Scale track gauge (inches) = Prototype track gauge (inches) divided by SCALE RATIO
   For example: standard gauge track (56.5 inches) in O scale would be 56.5 inches / 48 = 1.77 inches.

Scale track gauge (millimeters) = Prototype track gauge (mm) divided by SCALE RATIO
   For example: standard gauge track (56.5 inches) in HO scale would be 56.5 inches times 25.4 mm / inch / 87
   =  16.5 mm.

SCALE AREA is smaller by a factor equal to the SCALE RATIO squared, and SCALE VOLUME decreases by a factor of the SCALE RATIO cubed.

Thus a mile of track at a scale of 1:87 is 5280 feet divided by 87, which equals 60.68 feet. A scale square mile of land would be about 61 by 61 feet, which is much larger than most model railways. That's why we use SELECTIVE COMPRESSION to pack a meaningful scene into a small space on a model. For example, a typical paved 2-lane highway is 100 feet wide between fence lines. This is 13-3/4 inches wide in HO Scale. We can't afford to give up over a foot of space for a highway on a model, so we selectively compress it to less than 6 inches. The eye usually doesn't mind.

SCALE WEIGHT is proportional to volume, so the weight of a 100 ton locomotive, at 1:87 scale, would be: (100 tons x 2000 lb/ton x 16 oz/lb) divided by (87 x 87 x 87) = 4.86 ounces. This would be far too light to operate; the average model locomotive at this scale weighs 10 to 20 ounces. Unfortunately, we can't model the pull of gravity.

SCALE SPEED equals actual speed multiplied by the SCALE RATIO. A model traveling 20 feet per minute is moving at an actual speed of 0.227 miles per hour, equivalent to almost 20 scale miles per hour in HO Scale (1:87). Most models travel too fast; the worst being run at over 300 scale miles per hour. The conversion equation is:

Scale Speed (mph) = 1/88 times Speed (feet/minute) times SCALE RATIO

Many people use a SCALE MILE (often called a Smile) which is shorter than a real scale mile, and others use SCALE TIME, usually 5 to 10 times faster than real time, to account for selective compression of model railways and the high speeds of model trains. Here, Scale speed (sph) = Smiles divided by Scale time.


The tables were created on a spreadsheet, using these conversion factors:

1 inch = 25.4 mm exactly

1 meter = 3.281 feet exactly

Ratio = 12 / (inches/foot)

inches/foot = 12 / Ratio

Ratio = 12 / (mm/foot) / 25.4

mm/foot = 12 * 25.4 / Ratio

Ratio = 1000 / (mm/meter)

mm/meter = 1000 / Ratio

Model track gauge (inches) = Prototype gauge (inches) / Ratio

Model track gauge (mm) = 25.4 * Prototype (inches) / Ratio
% Error To Prototype = ((Model track gauge * Ratio / Prototype gauge) - 1) * 100
(+) = model tracks are too wide,
(-) = model tracks too narrow.