What's the difference between OO and HO scale or gauge?

When browsing a model shop either online or in person you'll no doubt have seen models marked as OO scale or HO scale, and invariably some marked OO/HO. But what does it all mean? In this post, we're going to explain the subtle differences between the two and what to look out for when shopping for model railway items in either scale.

The key difference between OO scale and HO scale

While both are popular model railway scales, they differ in size and proportions. HO scale is popular in Europe and the USA, while OO scale is the most popular scale here in the UK, Australia & New Zealand.

OO scale is the most popular scale for British railway models and has a scale ratio of 1:76.2, which means that the models are 76.2 times smaller than their real-life counterparts. For simplicity, this is normally abbreviated to 1:76 scale. The track gauge for OO scale is 16.5 mm and models made for this scale are approximately 1:76 in size. You may also see it referred to as 4mm scale too, as 1 foot measures 4mm when scaled down.

HO scale, on the other hand, is a European and American scale and has a scale ratio of 1:87, which means that the models are 87 times smaller than their real-life prototypes. The track gauge for HO scale is 16.5 mm, which interestingly is the same as OO scale. HO scale models are approximately 1:87 in size, and they are commonly used in North America and Europe. In HO, 1 imperial foot measures 3.5mm. The sheer range of models available in this scale makes it a very popular choice.

So are HO scale models smaller than OO scale?

Absolutely, yes! In general, HO scale models are slightly smaller than OO scale models. In some cases, the difference is quite noticeable, and in others, you'd never know. Over the next few posts, we'll take a look at when you can and can't mix and match HO & OO scale models.

What about models marked at OO/HO?

This happens most commonly with model building kits. When it comes to architecture, the size of buildings varies hugely in real life. Houses, shops, offices etc vary hugely and while they all mostly use the same size bricks, it's much easier to get away with using an HO-scale building on a OO scale layout and vice versa than it is with other models. 

Can you mix OO & HO scale models on the same layout?

We'll take a look at this in more detail in another post, but the short answer is yes… some of the time. However, you will need to be careful. Looking at the height of model figure is probably the easiest way to get an idea of the differences between the two scales. Humans come in different sizes, there's no doubt about that, and while we could say the average height of a man is 6ft, not every one is 6ft tall; some people are a lot taller, and some a lot smaller.  Let's take a look at each scale…

From the above table, we can quite safely say that HO figures are probably acceptable for use on a OO scale layout, but maybe not vice versa. It's advisable though to take care when positioning the figures on a layout. Mixing 1:87 & 1:76 scale figures in the same group is probably not advisable as the difference in height would be very noticeable. Populating a busy pedestrian area entirely with 1:87 scale figures is however likely to be fine. Just don't place an HO scale figure too close to something that's obviously 1:76 scale. Walls and steps should be fine. But a 1:76 scale telephone box may look a little too large against a figure from the smaller of the two scales.

In what cases is it most noticeable?

Rolling stock is probably where you'll notice the biggest difference. Whilst the track gauges for both are the same: 16.5mm, HO scale rolling stock will be shorter than the OO scale equivalent. So if you mix and match rolling stock from the two gauges behind the same locomotive, the differences would be quite noticeable. However, a rake of HO wagons carrying shipping containers being hauled by a OO scale locomotive probably wouldn't look out of place.

Vehicles will also need to be carefully placed too. The sheer range of 1:87 scale diecast vehicles available makes them very attractive to a 1:76 scale modeller. But the same logic needs to be applied as with figures. If you position items in two scale side by side, you'll immediately be able to tell the difference. But with plenty of space between them, there shouldn't be a problem.

Tricks of the trade

A super easy way to use HO scale figures and vehicles on a OO scale layout is to make sure all the larger OO gauge items are positioned at the front of the layout and in immediate view. Then use the HO scale items to detail areas further away at the back of the layout. This also helps to make the layout look a little deeper too through the effect of forced perspective.

In Conclusion…

Taking all that into account then, there are some key differences between OO scale and HO scale, but with both scales using the same track gauge of 16.5mm, the temptation to mix and match items is overwhelming. And with careful consideration, the results can be very effective indeed. Just space your figures and vehicles out far enough so that the difference isn't noticeable, or group items of the same scale together and you should be just fine.

We'll cover forced perspective along with mixing and matching scales in future posts.