Reference
Model railway scales chart
From Z at 1:220 to Gauge 1 at 1:32, every popular railway scale in one table — the ratio, the track gauge it runs on, and what modellers use it for. Building a layout or scratch converting a wagon? The scale calculator turns any prototype dimension into its model size.
The quick-reference chart
Ordered by reduction ratio, largest model first. Track gauge is the model track the scale runs on; ratios marked * run on a track narrower than their ratio implies.
| Scale | Ratio | Track gauge | Typical use |
|---|---|---|---|
| Full size | 1:1 | — (1,435 mm standard gauge) | Real trains — the prototype every scale below reduces. |
| Gauge 1 (No. 1) | 1:32 | 45 mm | Garden railways and premium tinplate; the classic large indoor/outdoor scale. |
| O scale (US) | 1:48 | 32 mm | American O — 1/4 inch to the foot; toy-train heritage meets modern finescale. |
| O scale (UK) | 1:43.5 | 32 mm | British O — 7 mm scale, the UK finescale standard. |
| OO scale (UK) | 1:76 | 16.5 mm* | The mainstream British scale — 4 mm scale running on HO-width track (*slightly narrow for the ratio). |
| HO scale | 1:87 | 16.5 mm | The world's most popular railway scale — 3.5 mm scale, half of O. |
| TT scale | 1:120 | 12 mm | Table Top — the mid-size comeback scale; British TT:120 runs at 1:101.6 (3 mm scale). |
| N scale (UK) | 1:148 | 9 mm* | British N — 2 mm scale on standard N track (*slightly narrow for the ratio). |
| N scale (global) | 1:160 | 9 mm | The world's most popular small scale — whole layouts on a coffee table. |
| Z scale | 1:220 | 6.5 mm | The smallest widely available commercial scale — maximum railway in minimum space. |
Continental (German) O gauge is nominally 1:45 — the ratio the shared 32 mm track actually represents for standard-gauge prototypes — sitting between the UK (1:43.5) and US (1:48) conventions.
Worked conversions
Two everyday examples of turning prototype dimensions into model sizes — both take seconds with a calculator:
- OO (1:76), a 60 ft express locomotive. Enter 60 ft with a custom ratio of 1:76.2 and you get roughly 240 mm — about 9½ in of shelf-hugging mainline.
- HO (1:87), a 40 ft boxcar. Scale 12.2 m by 1:87 and the boxcar comes out around 140 mm — 5½ in — the classic starter freight car.
The same arithmetic works for anything on the layout: platform lengths, engine shed footprints, headshunt runs. Set the ratio once and read off every dimension.
Frequently asked questions
What is the difference between OO and HO?
They share the same 16.5 mm track, but the model size differs: OO is 1:76 (4 mm scale) while HO is 1:87 (3.5 mm scale). HO gives a track gauge that is true to the ratio; OO track is slightly narrow for 4 mm scale — a compromise kept because British prototypes are small and detailed, and the shared track made manufacturing economical. Visually, an OO locomotive is noticeably chunkier than an HO model of the same prototype.
Why does N scale have two ratios, 1:148 and 1:160?
Both run on 9 mm track. The global standard N scale is 1:160, where 9 mm track is essentially correct for standard-gauge prototypes. British N is 2 mm scale — 1:148 — because UK trains and loading gauges are smaller than American or European ones: the slightly larger ratio gives better-running, better-detailed models of British stock, at the cost of an undersize track gauge (the same trade-off OO makes).
What is the difference between scale and gauge?
Scale is the reduction ratio applied to every dimension — 1:87 means the model is 1/87th the size of the prototype. Gauge is the distance between the running rails of the track, measured on the model (16.5 mm, 9 mm, and so on). The two are only loosely coupled, which is why 'OO gauge' — a common colloquialism — is technically a contradiction: OO is a scale (1:76) running on HO gauge (16.5 mm) track.
What does "4 mm scale" mean?
British modellers traditionally name scales by how many millimetres represent one real foot. 4 mm to the foot gives 1:76 (OO), 7 mm gives 1:43.5 (O), 3.5 mm gives 1:87 (HO), and 2 mm gives 1:148 (N). That convention is why UK scale ratios look like odd numbers — they come straight from a mm-per-foot rule rather than a round reduction.
Can I use a scale calculator for model railway dimensions?
Yes — enter the prototype dimension, pick a custom ratio such as 1:76.2 or 1:87, and read the model size in mm or inches. The batch mode is handy for a whole consist: paste in the length of every wagon and get the full model-length list in one pass.
Planning a build? Open the free scale calculator →