Why OO track is the wrong width, and what that means when you are choosing where to start
Published 22 April 2026
Two words get used as if they mean the same thing, and they really do not. Scale is how much smaller the model is than the real thing, expressed as a ratio or as millimetres to the foot. Gauge is one measurement only: the distance between the inside faces of the rails. In a perfect world the second would follow from the first. In Britain it famously does not, and once you understand why, half the confusing jargon in the hobby falls into place.
Almost every main line in Britain, most of Europe and North America is built to standard gauge, which is 4ft 8½in, or 1,435mm. Divide that by your scale ratio and you get the track gauge you ought to be using. Divide it by 87 and you get 16.5mm, which is where HO comes from. Divide it by 76.2 and you get 18.83mm, which is not what OO track measures. Hold that thought.
| Scale | Ratio | Scale to the foot | Track gauge |
|---|---|---|---|
| Z | 1:220 | 6.5mm | |
| N (British) | 1:148 | 2mm | 9mm |
| N (Europe and USA) | 1:160 | 9mm | |
| TT:120 | 1:120 | 2.54mm | 12mm |
| HO | 1:87 | 3.5mm | 16.5mm |
| OO (British) | 1:76.2 | 4mm | 16.5mm |
| S | 1:64 | 3/16in | 22.42mm |
| O (British) | 1:43.5 | 7mm | 32mm |
| O (USA) | 1:48 | 1/4in | 31.75mm |
| Gauge 1 | 1:32 | 3/8in | 45mm |
| G | 1:22.5 to 1:29 | 45mm |
Look at that table again. OO and HO run on identical 16.5mm track, but OO models are built to 4mm to the foot while HO models are built to 3.5mm. That is a 14% difference in body size on the same rails.
Sixteen and a half millimetres at 4mm to the foot works out at a prototype gauge of about 4ft 1½in, roughly seven scale inches narrower than the real thing. Every British OO locomotive you have ever seen is standing on track that is too close together, and almost nobody notices.
The reason is historical and entirely practical. British locomotives are physically smaller than their American and continental equivalents because our loading gauge is tighter. When manufacturers tried to fit the motors and mechanisms of the 1920s and 1930s into British outline bodies at 3.5mm to the foot, they simply would not go. So the bodies were enlarged to 4mm while the track stayed at 16.5mm, and Britain has been quietly out of proportion ever since.
Some modellers do care, and there are two established answers. EM gauge widens the track to 18.2mm, which is very close to correct for 4mm scale and requires re-gauging your wheelsets but is otherwise a manageable step. P4, sometimes called Scalefour, goes all the way to 18.83mm with correct wheel and track standards throughout.
What you gain is track that looks unmistakably right, particularly through pointwork. What you give up is ready-to-run convenience: everything needs converting, tolerances are unforgiving, and a badly built P4 turnout will derail things that would have sailed through a Peco point. It is a superb way to model and a poor way to start.
OO is the default in Britain for a reason. The ready-to-run range from Hornby, Bachmann, Dapol, Heljan, Rapido and others is enormous, second-hand stock is everywhere, and every model shop stocks the track. The cost is space: an OO oval needs a metre and a half of width before it looks like anything.
N at 1:148 halves your space requirement and lets you run a proper-length train through actual scenery rather than a train that fills the whole layout. The range is smaller but has grown enormously, and Farish and Dapol cover most British subjects. The catch is size: small hands, small couplings, and a dropped coupling under a viaduct is a genuinely annoying afternoon.
TT:120 is the newcomer in Britain. Peco introduced TT:120 track and accessories in June 2022 and Hornby launched its range in October 2022, reaching general retail from October 2023. It sits neatly between N and OO: roughly two thirds the size of OO, big enough to handle comfortably, small enough to fit a decent layout in a spare room. The range is young, which is either an opportunity or a frustration depending on what you want to model.
O gauge at 7mm to the foot is glorious and demanding. The detail is superb, the weight of the models is satisfying, and a modest tank engine and three wagons will occupy the same space as an entire OO branch line. Garage or garden, generally.
G scale and Gauge 1 on 45mm track are outdoor propositions. Weatherproof, chunky, often battery or live steam powered, and a completely different hobby in practice.
This is where the scale and gauge distinction earns its keep. Narrow gauge prototypes ran on track much closer together than 4ft 8½in, so modellers use a smaller scale's track under a larger scale's stock.
The appeal is obvious once you see it. Narrow gauge lines had tight curves, short trains and small locomotives in real life, so a convincing layout fits in the space where a standard gauge one would look absurd.
Measure your space first and choose the scale second, not the other way round. Then check three things before you commit any money: whether the locomotives you actually want to run exist ready-to-run in that scale, whether your local shop stocks the track, and whether you can comfortably see and handle a coupling in that size. That last one catches more people than they expect, and it does not get easier.
If you want to sketch out a layout before you commit to anything at all, the virtual trainset on this site lets you try track arrangements in a browser and see how a plan flows before you buy a single length of rail.