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Choosing a Scale, Ratio and Gauge

Page history last edited by Ian Stock 12 years, 5 months ago

Bill Winter

 A couple of words of caution before delving into what can be a contentious and confusing topic with many views and interpretations. 


Firstly I have tried to provide an unbiased summary of what is available and to offer further avenues for the reader to follow to obtain more detailed information on some of the options. There are a wide range of scales and gauges in use and it is well worth spending time considering which are the most appropriate for a particular builders situation.  Some are going to be better suited than others to meeting specific requirements, large and small spaces, limited time, retirement!, low or high budgets, manufactures support or scratch building and so on. 


Secondly, particularly if subscribing to the view that ‘the use of a consistent scale is important in achieving a sense of realism’, not everything is as simple and straight-forward as it might appear, or indeed should be.  Having worked through all the options and made a decision to adopt a particular gauge and scale it might be assumed that it would be a straight forward matter to purchase kits or ready to run items for the chosen scale.  Whilst it is possible to buy accurate scale specific items from suppliers they are less common than might be expected, particularly by those more used to indoor modelling. 


The constraints of manufacture and the need to operate the models with 12” to the foot fingers, particularly live steam locos, have some inevitable influence on scale accuracy.  These compromises are often acceptable or with very little effort concealed or changed.  However manufactures need for larger volumes than a single scale might support and wishing to make models of varying prototypes from different countries and loading gauges compatible in order to increase ranges can mean that products may have been ‘adjusted’ to suit.   Enter the ‘Elastic Rule Syndrome’. 


It is not unknown for different scales to be used on the same model for width, height and length!   It requires a certain amount of research on the part of the purchaser to ensure that a model is going to be suitable for the chosen scale.   For models of prototypes it is not too difficult to check at least the overall dimensions from published data.  When it comes to generic and freelance models the issues become rather more subjective.  The best that can be done is to find dimensions for prototypes that bear a close resemblance in terms of wheel arrangement, style, type and area of use and make comparisons with their dimensions.  With Generic narrow gauge models it may be worth considering models outside the chosen scale, they may prove suitable with some minor cosmetic changes or additions. Conversely models supposedly suitable may prove not to be if a consistency of scale is to be achieved. Time spent at this stage will go a long way to avoiding problems and possible dissatisfaction later when it might prove difficult and expensive to correct.  That is the end of the cautions down to the detail that is where the devil lies.


The reduction used to produce a model may be given in two different ways.  The ratio of the models size compared to the prototype may be given, 1 to 19 or 1:19 to denote that the model is one nineteenth the size of the original.  Alternatively a scale may be used, 10mm to the foot or 3/8” to the foot, sometimes abbreviated to 10mm scale or 3/8” scale, indicating that every foot on the prototype would measure 10mm or 3/8” respectively on the model. Before metrication scales were always imperial, (½” to the foot), in the US. which still uses imperial, and some of the scales used elsewhere this is still the case.  There has been some metrication of scales, though only in part, 12mm to the foot!  Old drawings are in imperial and modern material supplies often metric so it does make some sense as it automatically converts one to the other and there is also the benefit that the numbers are usually easier to work with.


To convert a ratio into a scale  Scale to the foot =     12”     or     305mm

                                                                            Ratio            Ratio


To convert a scale into a ratio  Ratio =           12       or       305mm

                                                                  Scale”            Scale mm


NB.  Modellers may mix imperial and metric units but mathematicians do not have that skill, the same units must be used top and bottom of the equation.


The scales used for garden railways range from 4mm to the foot to 7/8” to the foot.  The former is at the small end, common indoors but more exceptional out, and the latter is only used for small industrial narrow gauge otherwise it is getting into the ride on scales rather than the scenic railway.  In order to obtain some idea of the size of model resulting from the use of the various scales the following load gauges can be converted using the scale in question.

British Standard Gauge 9’ 0” wide x 13’ 6” high

American Standard Gauge 10’ 6” wide x 15’ 0” high

Narrow gauge is not so easy as every line had its own loading gauge, the Festiniog being small and the Irish lines, Welshpool & Llanfair and others quite large, a Talyllyn bogie coach 5’ 8” wide x 8’ 3” tall might be a reasonable average.  As a very approximate guide the diagram is based on the British loading gauge, a Talyllyn coach for UK narrow gauge and a typical continental metre gauge bogie coach for G scale.





There is a chicken and egg situation with many of the scale gauge combinations, particularly those used for narrow gauge.  For convenience and availability of track an existing track gauge from one of the standard gauge scales was often adopted and a scale chosen that would enable it to be used to represent the required narrow gauge rather than choosing a scale and creating a new gauge for the models to run on. 


Track gauge is measured between the inside edges of the rails.  Depending on the type of track that is being represented it may have a designation rather than just stating the gauge, in some instances the designation refers to both the gauge and the scale used as a combined standard.  There are a considerable number of different standards for both gauge and scale/gauge combinations that have been adopted by various bodies and groups.  Just to add to the confusion the terms used in various countries are often different for what is basically the same standard.   


Track standards relating to rail sections, profile, clearances sleepers etc. are beyond the scope of this already complex set of notes, reference to the appropriate Associations, Societies and Guilds should yield further guidance. 


In some cases the track gauge used many not correspond to the prototype gauge scaled down precisely, for example 16.6 mm track is used for anything from 2’ 0” to 2’ 6” in 7mm and ¼” scale narrow gauge and 45mm can be used for 2’ 6” and 3’ 0” in 16mm scale.  There appear to be too many different prototype gauges to get them all into the existing gauges and scales.  To check the actual gauge being represented given the gauge being used for the model and the scale;


Actual  =  Model Gauge



Working in all metric or all imperial units the answer will be in feet and a decimal of one foot.  To convert the decimal foot into inches multiply 12” by the decimal to get inches and decimal inches.  Use one of the tables to convert the decimal inch to a fraction if required.  For a metric gauge 1 inch = 25.41mm


What follows is a broad outline of the gauges and scales in common use it is not a definitive list.


The information follows the format:-




Gauge represented

Actual representation at the scale, Red, Amber, Green gives an indication of accuracy

Group that uses the scale gauge combination


The first group are predominantly indoor electric scales using track power but are being used in the garden. Hornby produce live steam locos in ‘00’ and others have been scratch built in some of the other scales.  (0 was originally a numerical zero, half of gauge 1, and 00  zero, zero as half of 0,  it is now common to use the letter O,  O gauge and Double O gauge.)


Originally 5/8” now 16.5mm gauge


3.5mm scale 1:87.14 as 4  8 ½” gauge 4’ 8 ½”       (HO)
4mm scale  1 :76.2  as 4’ 8 ½” gauge  4’ 1 ½”        (00)
7mm scale  1:43.5  as 2’ 0” to 2’ 6” gauge  2’ 4 ½”  (016.5)
¼” scale  1:48  as 2’ 0” to 3’ 0” gauge  2’ 7”  (0n30
3/16” scale  1:64  as 3’ 6” gauge  3’ 5 ½”    (Sn3.5) 



¾” gauge


¼” scale

1:48 as 3’ 0”gauge    3’ 0”               


1 ¼”  (32mm) gauge


7mm scale   1:43.5        as 4’ 8 ½” gauge                 4’ 7”                         (0)


This section covers the common garden scales. (SM32 and SM45 were originally a manufacturers code for it’s narrow gauge track and are sometimes used to denote the respective gauges)


Originally 1 ¼” now 32mm gauge


9mm scale 1:32 as 3’ 6” gauge 3’ 6”    
½” scale 1:24 as 2’6”gauge 2’ 6”  
12.8mm 1:23.8 as 2’6” gauge 2’ 6”  
16mm scale 1:19 as 1’ 10 ½”, 600mm, 2’ 0”gauge 2’ 0”  (Sm32) (16mmng)
7/8” scale 1:13.7 as 18” gauge estate railway 17 ½”   


Originally 1 ¾” now also 45mm


3/8” & 9.5mm scales 1:32 as 4’ 8 ½” gauge 4’ 8 ½”  (G1)
10mm scale 1:30 as 4’ 8 ½”gauge 4’ 8 ½”  (G1)
10.5mm scale 1:29 as 4’ 8 ½” gauge 4’ 3 7/16” (G1)
12.7mm scale 1:24 as 3’ 6” gauge 3’ 6”  (G1)
13.5mm scale 1:22.5 as metre gauge 1 metre (G scale) (G1)
15mm scale 1:20.3 as 3ft gauge 3’ 0” (G1)(Fn3)(16mmng)
16mm scale 1:19 as 2’ 6”, to 2’ 9” gauge 2’ 9 ¾”  (SM45) (16mmng)
20mm scale 1:15 as 2’ 3”gauge 2’ 3”  
3/4” scale 1:16 as 2’ 3”gauge 2’ 4”  
7/8” scale 1:13.7 as 2’ 0” gauge 2’ 0”  


2 ½” or 63.5mm gauge


17/32” &13.5mm at 1:22.5 as 4’ 8 ½”gauge 4’ 8 ½” (Gauge 3)


70.64mm gauge


15mm scale 1:20.3 4’ 8 ½”gauge 4’ 8 ½” (F)


3 ½” gauge


¾” scale 1:16 as 4’ 8 ½”gauge 4’ 7 3/16”     


F and Gauge 3 are usually considered the largest of the practical Garden gauges.  F standard gauge matches Fn3 narrow gauge and 3 ½” standard gauge is only used for scenic details such as transfer sidings and for mainline wagons to use on transporter wagons at ¾”, 20mm or 7/8” scale.  (There are scenic ground level lines on 3 ½” and 5 inch gauges but some find the full-sized driver dents the illusion a little!)


Three broad groupings cover the common garden scales and are each represented by their own Societies or Associations.  The following brief summary covers the areas of interest covered by each group and provides links to websites providing further information. 


Gauge 1 Initially British standard gauge (4’ 8 ½”) using 1 ¾” (45mm) Gauge 1 track. Models are usually self contained accurate scale models, steam locomotives traditionally spirit fired and manually controlled though coal and gas firing is used.  Trains being of ‘mainline’ types are often long and travel at scale speeds requiring sweeping curves and good access.  The need for access often results in raised level tracks and limits on the landscaping. The Gauge 1 Model Railway Association was formed in 1947 and now caters for all scales using Gauge 1 track and all gauges using Scale 1 (i.e. The range around 9mm scale to 10mm scale).  In practice the Association promotes, and local groups accommodate, whichever stock their members have.  (The further from London the greater the variety, Australia and Californian loco groups going up to 7/8” scale.) 


16 Millimetre Narrow Gauge (16mmng) originally used the then common ‘0’ gauge (32mm) track to represent 2ft narrow gauge using a scale of 16mm to the foot.  More recently models from gauges of 2’3”, 2’ 6”, 3’0” and metre have appeared and 45mm track has been adopted for some of these gauges.  The original definition of 16mmng has now become a broad term for narrow gauge railways in the garden and is no longer a precise definition of scale.  There are some still using 16mm to the foot but there are a large proportion of models that fall outside the original definition.  Locomotives are self contained electric or live steam.  Spirit firing for steamers has largely been superseded by gas, coal firing is in the minority but becoming increasingly popular.  Manual and radio control of locomotives each have their respective following. Railways using the smaller track gauges were often industrial or operating in difficult terrain and used tighter curves so for confined spaces they can offer an invaluable alternative.  The Association of 16mm Narrow Gauge Modellers now encompasses narrow gauge in the garden at scales around 16mm to the foot with models ranging from accurate scale models of a prototype through those based on prototypes to generic and totally freelance narrow gauge.  


G scale was a term coined by the German firm of Lehmann in the 1960’s to describe their Continental narrow gauge models marketed under the trade name LGB (Lehmann Big Railway).  Models were indeed big as they were predominantly based on ‘mainline’ metre gauge at 1:22.5 and ran on 45mm gauge track of heavier proportions than Gauge 1.  Today there are other manufacturers including those for the American market producing models based on their 3ft gauge and standard gauge railroads rather than the original Continental metre gauge.  The availability of set track, the smallest having a radius of only 600mm, has not had a beneficial effect on achieving a sense of realism. The track allows for unrealistically high track density in confined spaces and, although designed to traverse it, long bogie stock and large locos can look less than real.  Locomotives are usually electric track powered and often fitted with sound cards and smoke generators. The G Scale Society was founded in 1987 and whilst proclaiming the scale to still be 1:22.5 models from different manufactures may be at variance with this.


The above all offer support and advice through websites, national exhibitions and local groups who host open days, and it would be well worth seeking out one of these to get an impression of any chosen scale before committing to action. There are also scales that fall outside the groupings given but they do not have any formal societies or Associations.  The initially American 7/8” scale for small 2ft gauge prototypes on 45mm gauge is gaining popularity, some of the smaller scales are only represented by groups that work indoors. A search on the Internet using any of the scales or bold headings above or the web sites below should throw up plenty of further information on specific scales.


Double O Gauge Association (www.doubleogauge.com)

Gauge 0 Guild (www.gauge0guild.com)

Association of 16mm Narrow gauge Modellers (www.16mm.org.uk)

Gauge 1 Model Railway Association  (www.gaugeone.org)

G Scale Society (www.g-scale-society.co.uk)

Association of Large Scale Modellers (www.alsrm.org.uk)

Gauge 3 Society  (www.gauge3.co.uk)


Constructing a garden railway with the intention of giving an overall impression of realism in all aspects is not entirely a new idea but it is not common practice either.

Unlike the indoor scales where there is a distinct divide between a train set and a model railway, where model railways are an attempt to create a sense of a real scene, are be it using synthetic materials, and operating in line with prototype practices there is far less of a clear-cut division outdoors.  


Aspects that create a sense of realism are demonstrated on many garden railways, fine scale locos, many live steam, well developed landscapes, accurately modelled buildings, prototype running etc. however they are far less frequently found together on any one railway.  It should be possible, having given due consideration to the site, scale and type of railway that interests, to produce a true sense of realism in all aspects of the railway in any of the scales available.  They may offer different challenges.  The smaller scales requiring great care in construction to get smooth running and probably increased maintenance to keep it.  The larger scales may be easier in some respects but getting the sweeping curves and open spaces on a mainline or a good representation of narrow gauge track in a quarry create there own challenges.  Track power also has its issues. 


All this before giving consideration to the other aspects of landscape, infrastructure, planting etc., the good news is that it can be done and offers great satisfaction in both appearance and operation to those who achieve it.



Bill Winter  Jan 09  (technical consultant D Halfpenny) 


Comments (1)

George said

at 4:10 pm on Mar 19, 2009

In the scales / gauges list SM32 is lumped with 2 others and it is implied that 16mm:1' - 2'0" gauge is thus innaccurate:
16mm scale 1:19 as 1’ 10 ½”, 600mm, 2’ 0”gauge 2’ 0” (Sm32) (16mmng)
Should this not be separated out so that SM32 is given the credit of true accuracy?
Also should the scale not be 1:19.05 if we are being realistic as over larger vehicles an error of measurement will soon creep in.
Finally I have always been led to believe that the only imperial to metric conversion factor is 1" = 25.4mm which means 1' = 304.8mm
or are we not going to that level of realism?

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