Model railway scales and gauges explained: OO, HO, N, TT, O and Z
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Before you buy a single locomotive, you need to choose a scale. It decides how much room a layout takes, which models you can buy, how much detail you can see and how fiddly the work will be. The terms are confusing at first, mainly because people use scale and gauge as if they meant the same thing. They do not.
Scale versus gauge
- Scale is the size of the model compared with the real thing, written as a ratio such as 1:76 or as millimetres per foot, such as 4mm to the foot.
- Gauge is the distance between the rails of the model track, measured in millimetres.
- Two systems can share the same gauge but use different scales. That is exactly what happens with OO and HO, the most common source of confusion for beginners.
OO: the British standard
OO models are built to about 1:76, which is 4mm to the foot, and run on track 16.5mm apart. It is by far the most popular choice in the UK, so British outline locomotives, coaches, wagons, buildings and accessories are easiest to find in OO. The compromise is that 16.5mm is slightly too narrow for the scale, which purists notice and most people do not.
HO: the world standard
HO is about 1:87 on the same 16.5mm track. It is the dominant scale in continental Europe, North America and much of the rest of the world. Because HO and OO share a gauge, the trains will run on each other's track, but HO models look noticeably smaller beside OO ones, so most people stick to one.
N: more railway in less space
N gauge runs on 9mm track. The scale varies by market: British N is usually about 1:148, while European and American N is generally 1:160 and Japanese N commonly about 1:150. N lets you fit long trains and sweeping curves into a small room, and a whole layout can sit on a bookshelf or in a coffee table. The cost is size: couplings, detailing and track work are small enough to test eyesight and patience.
TT: the in-between option
TT, short for table top, is 1:120 on 12mm track. It sits between HO and N in size. It has long been established in parts of continental Europe, and in recent years British outline TT:120 models have arrived from UK manufacturers, so it is worth looking at if OO feels too big and N too small.
O and larger
- O gauge in the UK is 7mm to the foot, about 1:43.5, usually on 32mm track. American O uses a slightly different scale on similar track.
- O models are heavy, detailed and impressive to watch, but a layout needs a lot of space, and many O gauge modellers build a small station or yard rather than a full circuit.
- Larger garden scales exist too, often running outdoors.
Z: the smallest common scale
Z is about 1:220 on track roughly 6.5mm apart. Complete layouts fit inside a briefcase. It is a niche choice with a narrower range of models.
How to choose
- Measure your space first. Curves take room, and the larger the scale, the wider the minimum curve.
- Decide what railway you want to model. If you want British trains, OO has the widest choice; for continental or American trains, HO usually does.
- Think about your hands and eyes. Smaller scales demand finer work.
- Visit an exhibition or club. Seeing scales side by side in real life tells you more than any chart.
- Check what is actually available. Browse current catalogues for the prototype you love before committing.
A note on narrow gauge
You will also see names such as OO9 or HOe. These model narrow gauge railways by running a larger scale on smaller track, for example 4mm scale models on 9mm N gauge track. It is a charming corner of the hobby that suits small layouts.
Whichever scale you choose, the best one is the one you will actually enjoy building in. Which scale did you choose and why? Share your thoughts or questions in the comments.