SCALE AND GAUGE STANDARDS                       Railway Pages Index

The National Model Railroad Association (NMRA) has established wheel and track standards or recommended practices for some scale and gauge combinations used in North America. Similar organizations in Britain and Europe have also set standards (MOROP and NEMA), some of which conflict with NMRA standards. Some small, and not so small, special interest groups (SIGs) have also set their own standards to permit equipment interchange between modelers who follow these, often more rigourous, standards. You may find minor incompatibility in wheels and track from different countries or different manufacturers.

Click here to see NMRA Standards web page.

The purpose of standards, of course, is to assist in creating at least some minimum compatibility between models, of nominally the same scale, made by different manufacturers. Unfortunately, standardization has not been achieved in any of the larger scales, and has been fragmented in the smaller scales, by separating fine scale and high rail standards from normal, or coarse scale, standards.

There are at least 60 scales in use today; the most common 10 basic scales are listed below:

Note that most scales are approximately 3/4, or 0.750, times the next scale in the list. I call this happy coincidence the 3/4 RULE.

The 3/4 RULE makes it easy to use track and wheel sets from a smaller scale as narrow gauge components in a larger scale, because 3 foot and meter gauge railway equipment is usually constructed to be about 3/4 the size of standard gauge equipment. For example, the average older standard gauge boxcar is 10 feet wide and 40 feet long. Many 3 foot gauge boxcars are 7 to 8 feet wide and 30 to 32 feet long.

In the USA, the NMRA has traditionally recognized three additional scales, namely OO Scale (1:76.2), O17 Scale (1:45.2), and G Scale (1:22.5). These scales are close to HO, O, and 1/2" respectively. Very recently, NMRA has proposed some additional scales as standards. The additions to the NMRA list are 1-1/2 inch Scale (1:8), M Scale (1:13.5), F Scale (1:20.3), and A Scale (1:29).

In Britain and Europe, additional standard scales are defined as:

Most of these are considered fine scales, but normal or coarse scale versions also exist. The 3/4 RULE is not as neat for continental scales. Some North American scales. such as Z, N, HO, O, and G Scales, are also common in Britain and Europe, as are a number of lesser used scales not listed above.

The STANDARD GAUGE for North America, Britain, and parts of Europe, Asia, Africa, and Australia is 4 feet 8-1/2 inches (1435 mm). This strange dimension may go back to Greek and Roman chariots, which were designed to fit a standard stone road or bridge. Many early steam locomotives in Britain were made to the same size.

Standard gauge was adopted by law in the USA in 1886. At that time a census of railways showed 25 different gauges in use across the country. Many railways were built, and some still operate, with wider gauges. One of the widest gauges, 7 feet 1/4 inch (2.14 meters), was used in the early days in Britain. These are usually called WIDE GAUGE or BROAD GAUGE. Some logging railways, especially those with horses for dragging logs, used gauges of from 6 to 9 feet.

In mountainous regions, and on construction or mine sites, standard gauge was too expensive, so NARROW GAUGE railways were built, often 24, 30. 36 or 42 inches (or equivalent metric gauges).

Although the plural of foot is obviously feet, the gauges are traditionally named with the word foot, or centimeter, not pluralized. Note that some European standard gauge is 1440 mm instead of 1435 mm.

To model one of these track gauges, we would take the original TRACK GAUGE and divide by the SCALE RATIO. For example, at 1:87 scale, standard gauge is 56.5 inches divided by 87, which equals 0.649 inches. As it happens, this is the current NMRA standard for this scale and gauge combination. A 3 foot narrow gauge in 1:87 scale would be 0.413 inches and at 1:22.5 it would be 1.600 inches. The first is an NMRA standard, the second is not.

The narrow gauges for which NMRA provides standards are:

NMRA has proposed adding the following standards

Whether any or all of these proposals are adopted or modified, only time will tell.

NMRA Standard S-1 and other NMRA documents incorrectly refer to theae narrow gauge names as SCALES, instead of GAUGES. Additional standards are specified in NMRA recommended practices for fine scale and high rail versions of some gauges.

Many model track gauges were chosen arbitrarily to be easy to use and remember, either in inches or millimeters. Thus the chosen model gauge may not match the true gauge. The percent error is shown in the tables for all cases. Some early models adopted track gauges that were 10 to 15 percent too wide or too narrow for the scale, making some models look clumsy and out of proportion. Most modern models are within 5% of the correct track gauge.

As mentioned above, due to the 3/4 RULE, a standard track gauge from one scale can be used to represent another gauge in a different scale. This gives rise to an almost infinite variety of scale and gauge combinations, some using nearly exact standards and others using "near-enough" track and wheel standards from a different scale. Where such inter-scale swapping is (or could be) done, they are listed in the tables.

For example, Z scale (standard gauge) track dimensions adequately represent 30 inch narrow gauge track in HO scale. Note that it is only the dimensions of the track gauge that match. The size and spacing of the ties on the track would probably need to be changed to reflect actual dimensions in HO scale.

Where the 3/4 RULE holds, such as for locomotive bodies and rolling stock, some models survive inter-scale mixing with few problems. In more extreme cases, the expensive and difficult to build locomotive drive mechanism can be used with a scratch built body. Trucks, couplers, or track components also may be useful.

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

In the tables that follow, track gauge dimensions are given in inches and millimeters. Where two dimensions are given, these are the minimum and maximum dimensions given in NMRA Standards or Recommended Practices (either official or proposed). Where a single value is shown, a non-NMRA source was used.

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."