Model Railway Scales & Gauges
Model railway terminology can be confusing because scale (the size ratio) and gauge (the track width) do not always match perfectly. This reference covers all the popular railway modelling standards, with special attention to the UK's unique OO gauge and how it differs from the continental HO standard.
Railway Scales & Gauges at a Glance
| Name | Scale ratio | Track gauge (mm) | 1 m real = | Notes |
|---|---|---|---|---|
| Z | 1:220 | 6.5 | 4.5 mm | Smallest commercially available scale |
| N (UK) | 1:148 | 9 | 6.8 mm | UK N gauge; slightly larger than continental N |
| N (Continental/US) | 1:160 | 9 | 6.25 mm | International N gauge standard |
| TT | 1:120 | 12 | 8.3 mm | Table Top; popular in Germany and Eastern Europe |
| HO | 1:87 | 16.5 | 11.5 mm | International standard; world's most popular scale |
| OO (UK) | 1:76.2 | 16.5 | 13.1 mm | UK standard; larger models on HO track gauge |
| EM | 1:76.2 | 18.2 | 13.1 mm | More accurate gauge for OO-scale UK models |
| P4 / S4 | 1:76.2 | 18.83 | 13.1 mm | Exact-scale gauge for 4 mm scale (finescale standard) |
| S | 1:64 | 22.5 | 15.6 mm | Between OO and O; niche but growing |
| O (UK) | 1:43.5 | 32 | 23 mm | UK O gauge; 7 mm to the foot |
| O (Continental/US) | 1:48 | 32 | 20.8 mm | International O gauge; ¼ inch to the foot |
| 1 (Gauge 1) | 1:32 | 45 | 31.3 mm | Large indoor/outdoor; premium models |
| G (Garden) | 1:22.5 | 45 | 44.4 mm | Garden railways; LGB, Bachmann Garden |
OO vs HO — The UK Difference
The most common source of confusion for UK modellers. Both OO and HO use 16.5 mm track gauge, but OO models are built to 1:76.2 (4 mm to the foot) while HO is 1:87 (3.5 mm to the foot). This means OO models are about 13% larger than HO models but run on the same track.
This compromise was adopted in the 1930s because early UK manufacturers found 1:87 models too small to accommodate the electric motors of the time in the narrow British loading gauge. The tradition stuck, and OO is now firmly the UK standard. For scale-accurate gauge at 4 mm scale, modellers use EM (18.2 mm) or P4 (18.83 mm) track.
Railway Modelling Calculators
Use our free tools for railway modelling calculations:
- Track Length Calculator — work out how much track you need for your layout
- Curve Radius Calculator — calculate minimum curve radius for your rolling stock
Scale ratios are nominal standards. Individual manufacturers may vary slightly. Track gauges listed are for standard gauge prototypes (1,435 mm / 4' 8½″). Narrow gauge models use proportionally narrower track.