HO vs OO: what's the difference?

Step by step

1. 1

   Step 1: Know the two numbers

   HO (written H0 in Europe) is 1:87 — 3.5 mm to the foot. The European NEM standards define it as exactly 1:87 and the American NMRA as 1:87.1. Standard-gauge HO track is 16.5 mm between the rails.

   OO is 4 mm to the foot (1:76.2) and runs on the same 16.5 mm track.

   Next: step 2
2. 2

   Step 2: Understand why Britain went its own way

   British trains are built to a smaller loading gauge (the outline that trains must fit inside) than European or American ones. Early clockwork and electric motors would not fit inside HO-sized British models, so makers enlarged the bodies to 4 mm scale but kept the 16.5 mm track. Hornby Dublo launched OO in 1938 and it has been the UK's main scale ever since.

   Next: step 3
3. 3

   Step 3: See what the compromise costs

   At 4 mm to the foot, 16.5 mm represents about 4 ft 1½ in instead of the real 4 ft 8½ in, so OO track is too narrow for the models on it. Finescale modellers use EM (18.2 mm) or P4 (18.83 mm) to fix this. HO models are in proportion with their track.

   Next: step 4
4. 4

   Step 4: Expect a 14% size difference

   An OO model is roughly 14% bigger than an HO model of the same train (87 ÷ 76.2 ≈ 1.14). They will run on the same track, but side by side the difference is obvious, and OO and HO buildings, figures and vehicles don't mix well either.

   Next: step 5
5. 5

   Step 5: Check the label on track and accessories

   Much UK track is sold as "OO/HO" — Peco's Streamline and Setrack, for example — because the gauge is the same. Figures and kits labelled "OO/HO" may be one or the other, or somewhere in between, so look for the actual ratio. Preiser, for example, sells separate 1:87 and 1:72–1:76 figure ranges.

   Next: step 6
6. 6

   Step 6: Decide which suits you

   Choose OO if you want British trains ready to run in the widest choice. Choose HO if you want continental European, North American or Japanese trains — HO is the main scale in Europe and North America. A small group models British trains in HO; the British 1:87 Scale Society was formed for them in 1994.