Railway scales and gauges
In model railways, scale is the size ratio and gauge is the distance between the rails. The two usually go together, but not always, and that catches people out.
Common combinations
| Name | Scale | Track gauge | Real 1,435 mm gauge at this scale |
|---|---|---|---|
| Z | 1:220 | 6.5 mm | 6.52 mm |
| N (most of the world) | 1:160 | 9 mm | 8.97 mm |
| N (UK) | 1:148 | 9 mm | 9.70 mm |
| TT | 1:120 | 12 mm | 11.96 mm |
| HO | 1:87 | 16.5 mm | 16.49 mm |
| OO (UK) | 1:76.2 | 16.5 mm | 18.83 mm |
| O (US) | 1:48 | 32 mm (1¼ in) | 29.9 mm |
| O (UK) | 1:43.5 | 32 mm | 33.0 mm |
Last column: 1,435 mm (standard gauge) ÷ N, calculated here. Names and nominal values follow the NMRA and MOROP NEM standards as summarised in rail transport modelling scale standards. Check the NMRA standards or NEM standards for exact tolerances.
The OO compromise
OO models are built at 4 mm to the foot (1:76.2) but run on HO's 16.5 mm track. At 1:76.2, standard-gauge track should be 18.8 mm apart, so OO track is about 12% too narrow for its models. Finescale standards such as EM and P4 exist to fix this. For a modeller, the practical lesson is that OO buildings and figures are larger than HO even though the trains share track. 1:76 and 1:87 accessories don't mix unnoticed.
Converting a real station or building
- Find the real dimensions. Drawings, maps with a scale bar or a site visit with a tape all work. Treat dimensions you estimate from photos as rough.
- Enter them in the converter at your layout's scale. Add them to a project so every part uses the same scale.
- Print a 1:87 or 1:76.2 ruler to measure plans and mark card directly.
Many layouts use selective compression, shortening buildings and platforms on purpose because real lengths are huge at scale. A 200 m platform is 2.3 m long in HO. That's an artistic choice. Record the compressed sizes in your project so they stay consistent.