What is HO scale? Read on.

Last Updated on April 15, 2023 by Dave Farquhar

Let’s play Jeopardy. Answer: Measuring at a 1:87 scale ratio relative to the real thing, this is the most common and popular scale of model railroad in the world outside of the United Kingdom. The question, of course, is, what is HO scale. It’s not a reference to Christmas or anything like that. HO is an acronym for “Half O,” a reference to its approximate size relative to the train scale it overtook in popularity around 60 years ago.

HO scale model trains, scaled at 1:87 scale, are the most popular size and scale of model railroad in most of the world since the 1950s. It uses realistic 2-rail track and DC power.

What is HO scale?

what is ho scale?
Everything you need to build an HO scale model train layout like this one is readily available commercially, and the techniques are well documented in popular books and magazines on the hobby. Image credit: Marshman at English Wikipedia [GFDL or CC-BY-SA-3.0], via Wikimedia Commons
HO scale model trains have their origins in 1920s efforts by the German toymaker Bing to make a model railroad smaller than O scale. By the mid 1930s, a nascent standard emerged. Eventually these gave rise to two scales, HO scale and OO scale. HO scale quickly built up popularity, but its popularity increased dramatically after World War II.

HO scale is 1:87 scale, where 3.5 millimeters in the model represents one foot in the real thing. The track gauge is 16.5 millimeters wide, or .65 inches.

The other standard to emerge from these early efforts, OO scale, is slightly larger, at 1:76 scale. It became and remains the most popular scale in the United Kingdom.

Due to its size, HO scale model trains caught on relatively quickly. By the late 1950s, HO scale model railroading was approaching or surpassing Lionel’s industry dominance. By the early 1960s, at the very latest, HO scale had dethroned Lionel. Today, it’s the most popular of model railroad scales.

In the United States and many other parts of the world, HO scale nomenclature is to use the two letters. In Germany, where numeric sizes for trains originated, they normally use the nomenclature H0, for “half zero.”

HO scale standards

In model railroads, HO is 1:87 with a track gauge of 16.5 millimeters. Standard sectional track has a radius of 18 inches. That means a circle of track is 36 inches in diameter. There is no technical reason why straight track has to be a certain length, but 9 inches is fairly standard. As a general rule, assume that a simple oval of HO scale track will occupy a space of 3×4 feet.

You may see track measured as code 100, code 83, or code 55, or somewhere in between. This refers to the height of the rail. Generally speaking, the larger numbers are more reliable and smaller numbers are more realistic. To use the smaller, more realistic track, you have to ensure the wheels on your locomotives and rolling stock can handle the shorter track.

Unlike earlier standards, HO generally uses DC power and two-rail track. The German manufacturer Marklin is the exception to this, using AC power and a hidden “stud rail” center rail.

What is HO/OO scale?

Here’s one thing that confuses people: In model railroads, HO scale is a pretty firm 1:87 or 1:87.1. Yet some items get sold as HO/OO scale. The difference between the two is small enough that a manufacturer can make things work for both scales, especially with items like trees, buildings, and people.

In HO scale, a six-foot-tall figure measures 21mm or .82 inches high. In OO scale, a six-foot-tall figure would be 24mm, or .94 inches high. A 21mm figure works out to 5’3″ in OO scale, which is a common height for women but below average for men. A 24mm figure works out to 6’10” in HO scale. That’s pro basketball player height. A good HO/OO figure would be somewhere in the middle. 23mm works out to 6’6″ in HO and 5’9″ in OO, so a reasonable compromise that works for both scales would be to make adult male figures about 23 mm tall, and adult female figures 20-21 mm tall.

Some budget model railroad suppliers will try to fudge vehicles as dual HO/OO scale. That doesn’t work as well, though modelers are often happy to use such vehicles to fill a parking lot, for example, if the price is low enough.

HO scale in other hobbies

If I haven’t confused you enough yet, in slot cars, HO scale is usually 1:64 scale, not 1:87. In model railroads, 1:64 is S scale. I don’t know of many people who mix slot cars with trains. But it sounds like fun, and if you plan to, keep in mind that HO scale slot cars will probably be oversized.


The number of listings on the Ebay auction site is a crude way to measure popularity, because you’re actually measuring how much product is available for sale, and there’s no good way to measure misidentified product. That said, it’s a better indicator than what we’ve had in the past, and anyone can examine it.

In mid January 2018, I counted 114,955 active Ebay listings for HO scale product. N scale, the second most popular scale, had 47,007 active listings. The third most popular scale, O scale, had 42,392 listings. Far behind were S scale and G scale, at 15,114 and 7,109 listing respectively.

Determining exactly how much more popular HO scale is than other scales would require collecting data over a larger period of time and pulling in additional data. But it doesn’t take a data scientist to look at those numbers and conclude that HO scale is extremely popular.

Why the infighting

diffusion of innovation
Diffusion of innovation states that products have to attract 15-18 percent market share in order to catch on and dominate.

The law of diffusion of innovation states that once a product achieves 15-18 percent market penetration, it is poised to dominate the market. But the first 16 percent of the market is made up of groups of people called innovators and early adopters–exactly the types of people non-academics call fanboys.

HO scale achieved a majority of the market, but two other standards also achieved more than the required 18 percent tipping point. And S and G scale achieved more than the 3.5 percent generally required to survive. So we ended up with HO scale with a majority of the market, and four other scales with cult followings.

With three standards at critical mass for dominance and two more at critical mass for survival, the result is a market that’s been pretty stable for about five decades now, but all five standards are just large enough that they think it’s still possible for theirs to dominate the market.

Industry coverage

There are hundreds of companies making items in HO scale, so regardless of your favorite type of train, chances are someone makes it in 1:87. The two largest model railroading magazines, Model Railroader and Railroad Model Craftsman, devote the overwhelming majority of their coverage to HO and N scale. When it comes to books for HO scale, Dave Frary is a good author to seek out.

There is no shortage of clubs and forums or other discussion groups for HO. Some are specific to HO scale and others cover model railroading in general. But HO scale, due to its immense popularity, will get much of the focus.


When you look at magazines for other scales, you start to notice a great deal of similarity in the layouts. Since there are only about a half dozen companies making buildings for O scale, you see the same buildings over and over again. Most of the modelers in those magazines seem to be modeling the same town.

That problem is less acute in HO scale. With a wider variety to choose from, you’re less likely to pick the same selection of buildings as everyone else does.

Also, since HO scale is so plentiful, there’s plenty of used and inexpensive product available.


If you want something a little bit different, HO’s ubiquity might or might not cause problems for you. If half the model railroaders in the world use it, you might want to find a minority scale if you like being different. Or you could choose a subject for your layout that’s something less common. If you choose to model the town you grew up in and its regional railroad, chances are few other people will have a layout like yours. And chances are no other scale will have the selection of items you need to model that time and place.

The main disadvantage to HO comes down to size. Larger-scale trains tend to be more forgiving when it comes to trackwork and how clean your track is. There is plenty of discussion online regarding how to deal with cleaning track, including using No-Ox ID on track, so plenty of people have overcome this.

All of that said, HO scale has plenty going for it, which explains its staying power over the years.

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