Scale vs gauge
Two terms come up in every discussion of model train sizes, and they mean different things.
- Scale is the ratio between the model and the real thing (the prototype). In HO scale, at roughly 1:87, one unit on the model stands for about 87 units on the real locomotive.
- Gauge is the distance between the inside faces of the two running rails. It is usually given in millimeters or inches.
Scale and gauge are usually chosen together, but they are not the same. Some scales share track, and some track widths serve more than one scale. Model railway names are also inconsistent: “O gauge,” “HO,” and “G” can refer to a scale, a track width, or both, depending on who is talking.
Model train scales at a glance
Figures below are the commonly cited values. Exact ratios and track widths can vary a little by region and standard.
| Scale | Typical ratio | Typical track gauge | Best known for |
|---|---|---|---|
| Z | 1:220 | 6.5 mm | Very small layouts |
| N | 1:160 | 9 mm | Long trains in limited space |
| TT | 1:120 | 12 mm | Popular in parts of Europe |
| HO | 1:87 | 16.5 mm | Widest selection |
| OO | 1:76.2 | 16.5 mm | Britain |
| S | 1:64 | about 22.4 mm | American Flyer heritage, scale modelers |
| O | 1:48 (US) | 32 mm | Large, sturdy models, Lionel-style trains |
| G | 1:22.5 to 1:32 (varies) | 45 mm | Outdoor garden railways |
The main scales
Z scale
At about 1:220, Z is the smallest of the scales with a broad commercial range. A layout can fit in a very small footprint, but the models are delicate and small details are hard to see and handle.
N scale
N (1:160, 9 mm track) is the usual choice when you want long trains, broad curves or sweeping scenery in a modest space; see N scale model railroad layouts for ideas. The range of locomotives and cars is wide, and the models have become much more detailed over the years. Handling and wiring are fiddlier than in larger scales.
HO scale
HO (about 1:87 on 16.5 mm track) is generally considered the most popular scale, especially in North America. It balances detail, size and choice of products. The name comes from “half O,” since it is roughly half the size of O scale. The NMRA gives the ratio as 1:87.1 (3.5 mm to the foot). For how it got there, see the history of HO scale model trains.
OO scale
OO is mainly a British scale. It uses 1:76.2 models on the same 16.5 mm track as HO. Because the models are larger than HO but run on HO-width track, the track is slightly too narrow for the scale. Modelers who want more accuracy use finer standards with wider track.
S scale
S (1:64) sits between HO and O. It has a loyal following, helped by the American Flyer trains of the past, and a smaller range of products than HO or N.
O scale
O (1:48 in the US, 32 mm track) is one of the oldest model train sizes. Many toy-style trains from makers such as Lionel use three-rail track and are sometimes sized more loosely than strict scale (the O27 line is one example). Two-rail O scale modeling is a separate, more scale-accurate branch of the hobby.
G scale
G, or “garden” scale, uses 45 mm track and is built to withstand outdoor weather. Models can use ratios from about 1:22.5 to 1:32, which is why fine detail and sizing can differ between makers. See the largest model train gauge for what lies beyond G.
Smaller than Z: T gauge and TT
People often ask which is smaller, T gauge or Z gauge. T gauge is smaller, at about 1:450 on 3 mm track. It is a niche area. TT, at 1:120, is actually between N and HO in size, not smaller than Z.
What scale is 00 gauge?
“00” (or OO) is 1:76.2 in the British system, running on 16.5 mm track. It is very close in size to HO but not interchangeable for detailed scenery, buildings and vehicles.
Why is 4 ft 8.5 in the standard railroad gauge?
Real standard gauge is 4 feet 8.5 inches (1,435 mm). It is commonly traced to early British railways, where George Stephenson’s work spread the width, and it later became widespread in many countries. Stories linking it to Roman chariots are popular but should be treated as folklore. Models use scale versions of this width: HO’s 16.5 mm is a scaled-down standard gauge.
Which scale is most popular?
HO is generally cited as the most popular, with N as a strong second in many places. Popularity matters in practice: more people means more locomotives, cars, kits, scenery and used equipment to choose from.
How to choose a scale
- Space: Smaller scales let you fit more track, longer trains and gentler curves in the same room. A larger scale takes more space for the same layout.
- Volume: An O scale structure takes up roughly six times the volume of an HO one, since every dimension is nearly twice as large. Layout space needs grow quickly.
- Detail and handling: Larger scales are easier to work on and wire. Smaller ones are harder on fingers and eyes.
- Availability and budget: HO and N have the widest choice. Check what is sold for the railroad and era you want before committing.
- Purpose: Operating layouts with switching often suit HO. Scenic or long-train layouts often suit N. Garden railways suit G.
Track plans are not interchangeable between scales, so design for the scale you choose.