SCALE AND GAUGE DEFINITIONS                        Railway Pages Index

This work is a labour of love. 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 magazine editors, on just a few modeling scales (N, HO). To see what can be done in railway modeling, read on.

Much confusion surrounds model railway scales, the names given to those scales, and the various track gauges that can be modeled in each scale. We'll review all the necessary definitions, then we'll present tables listing all known model railway scales and gauges.

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 translates to 3.5 millimeters equals 1 foot.

GAUGE or TRACK GAUGE is the distance between the rails of real or modeled railway tracks. The standard track gauge on most North American railways is 56.5 inches, but many other gauges exist.

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. All variations are listed in the tables below.

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.

A railway model is not fully described unless both SCALE and GAUGE are specified. Unfortunately, some SCALE NAMES are used to represent more than oneSCALE 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, 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.

BELIEVE IT OR NOT, there are more than 1250 different SCALE/GAUGE NAME COMBINATIONS listed here from 64 different SCALES.

NOTE THAT "3 Inch Scale" infers that 3 inches = 1 FOOT. This is true even for millimeter scales, such as 7 mm Scale, which means 7 mm = 1 FOOT. For pure metric scales, such as 10 mm = 1 METER, the "meter" is always mentioned.

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

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

Scale track gauge (inches) = Prototype track gauge (inches) divided by SCALE RATIO

Scale track gauge (millimeters) = Prototype track gauge (mm) divided by SCALE RATIO

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 a pound or two. 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.

E. R. (Ross) Crain, P.Eng. is a Consulting Petrophysicist and a Professional Engineer with over 40 years of experience in reservoir description, petrophysical analysis, and management. He has been a specialist in the integration of well log analysis and petrophysics with geophysical, geological, engineering, and simulation phases of oil and gas exploration and exploitation, with widespread Canadian and Overseas experience.

"I am a life-long model railroader and have modeled in O27, HO, HOn3, and N Scales. Failing eyesight brought me to G Scale. My father started me in model railroading as a tiny tot in 1944 - he scratch built his first locomotive in 1940, the year I was born, and I still have this loco on my mantle-piece. I am a Life Member (#517) of NMRA, a member of the Rocky Mountain Garden Railroaders (Calgary, Alberta), and have toured a lot of model railways, railway shows, and garden railways. I have never seen a model railway I didn’t like. An extensive library of railway magazines and books, covering topics that appeal to me, sit behind my office desk, ready to be put to use at a moments notice. I hope these pages can communicate to you some of my accumulated experience, my successes and failures, and my love of model railways."