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.
