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A few Words about Sn3½ Scale & Gauge

For the uninitiated, "Scale" reflects the proportion of reduction in relation to the prototype, ie HO Scale is modeled at 3.5mm= 1 foot (304.8mm) and its reduction ratio is 1:87
S Scale is 3/16in = 1ft and is a reduction ratio of 1:64

Where as, "Gauge" represents the track width eg HO Scale trains run on 16.5mm track, representing the "Uniform" or Standard Gauge (4'8½") This being the standard "HO" originating from USA and "00" in the UK

In Queensland we have a prototype gauge of 3'6", but if we model in HO SCALE, (1:87) which to be correct, means that we should use 12mm track to accurately (within the limits of what track is commercially available) represent the 3'6" gauge (in HO Scale).

However there are some modellers who do not use the 12mm track, as determined by the above, as they use the common 16.5mm track, as in most cases, they already have such, and possibly have other trains of other systems or countries which they can run without encountering gauge problems!


in a larger SCALE, namely S Scale, we can model in a different reduction ratio of 1:64, which produces a larger model, but runs on the same HO track that most modellers already have, due to the fact that 3' 6" scales back (almost) to 16.5mm, which is commercially available, HO Scale track.

The 16.5mm proportions out to 3'6" in S Scale

The modelling scale is known as Sn3½

The breakdown of the digits means:

  • S.....The Scale reduction.....1:64
  • n.....narrow gauge is being modeled
  • 3½.....indicates the prototype track gauge being modelled

(in this case it is in "feet", but has also been written in places as Sn1067, other varieties are Sn2, On2 or HOn3½)
In essence, Sn3½ is 1:64, using 16.5mm track, and HOn3½ is 1:87, using 12mm track. In both cases, the relationship of scale to gauge is correct for the prototype being modelled

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