Specifications – Scale and Gauge

This is a confusing subject for the newcomer to the hobby (and maybe for the long established modeller too!) because people talk about scale and ratio interchangeably. In addition note there are differences in standards from country to country. For our purposes scale is normally how many inches or mm represent one foot of the original whereas the ratio is the proportional relationship between the model and the prototype. For example, in the case of N (for Nine mm track) the scale is 1.9 mm to the foot and the ratio of scale to the prototype is 1:160 in Continental models but 2.0625 mm and 1:148 respectively with respect to British models. In an endeavour to clarify the situation the table below lists some scales and gauges with respect to models and the prototype.

Name Scale* Ratio Gauge  (mm) Gauge  (Imp) Comments
T 0.6773 1:450 3 0.1181 Introduced by Eishindo, Japan in 2006. Smallest commercial scale/gauge in world.
Z 1.39 1:220 6.5 0.256 Introduced by Marklin Germany in 1972.
OOO 2 1:152 8 0.315 Lone Star push along early form of N. Gauge widened to 9mm when Lone Star introduced motorised locos.
N 1.90 1:160 9 0.3543 Continental – Introduced by Arnold 1962; popular worldwide.
N 2.0625 1:148 9 0.3543 British N scale and ratio. Also popular.
TT 2.54   1.120 12 0.4724 TT (Table Top) originated in the US just after 2nd World War
TT3 3  1:1016 12 0.4724 Introduced in the UK by Tri-ang in 1957. Popular once but displaced by N scale.
HO 3.5 1:871 16.5 0.6494 Introduced in UK in 1920s but lost out to OO. Adopted by US post war. Widespread popularity.
HOn2½ 3.5 1:871 9 0.3543 3.5mm scale running on 9mm (N gauge) track. Used to replicate 2’ or 3’ narrow gauge prototypes as well as the correct 2’6”.
HOn3 3.5 1:871 10.5 0.4134 Popular with those modelling American 3’ NG prototypes such as Denver & Rio Grande Western; Colorado & Southern.
HOe 3.5 1:871 9 0.3543 Mainly Continental, modelling Narrow Gauge.
HOm 3.5 1:871 12 0.4724 Continental – modelling meter gauge. Runs on 12mm TT track. Closest track for modelling SAR in HO (should be 12.25mm).
OO 4 1:762 16.5 0.6496 Originated by Bing 1924. Popularised by Hornby Dublo/Tri-ang.
OO-9 4 1:762 9 0.6496 4mm scale narrow gauge models running on N gauge track.
EM 4 1:762 18.2 0.7165 Popularised by Pendon Museum UK. Closer to correct track width.
P4 4 1:762 18.83 0.7413 True OO (4mm) scale to gauge ratio.
S 4.76 1:64 22.43 0.8831 Long established scale (19th century!) Now mainly US.
Sn3 4.76 1:64 14.28 0.5622 3 foot gauge prototype modelled in S scale.
O 7  1:4354 32 1.25 UK scale and ratio. USA different – 6.35mm scale 1:48 ratio.
On3 6.35 1:48 16.5 0.65 Mainly US. O scale models of 3’ gauge prototypes on HO track.
1 9.525 1:32 44.45 1.75 Long history, now mainly live steam garden railways in UK.
16mm 16  1:1905 32 1.25 Most popular for live steam in the garden in the UK (SM32 track).
G 13.547 1:22.5 45 1.7717 Introduced by LGB. Always same gauge; many different scales. G tends to refer to ‘garden gauge’ these days.
3 12.7 1:226 63.5 2.5 Also referred to as 2½”, the width of the track. Can be used for pulling ride on trains but generally recognised as the largest model (as against model engineering) gauge. Scale 17/32” (13.65mm) Scale of 1/2” (12.7mm) to the foot also used.

*The mm to the foot scale of any ratio can be calculated by dividing the scale ratio e.g. 1:762 (.762) into 304.8 (the number of mm in a foot). Hence OO scale is 4mm to the foot.

The photo below give an idea of the relative sizes of locos in different scales. Gauge 1 would fit between the O and G locos.