page_title: OO gauge
section_title: Scaling and accuracy
hierarchical_section_title: OO gauge / Scaling and accuracy
caption_attribution_description: Parkside Dundas Diagram 100 hopper in 4mm (OO) scale.
context_page_description: OO gauge or OO scale is the most popular standard-gauge model railway standard in the United Kingdom despite being virtually unknown in other countries. This track gauge is one of several 4 mm-scale standards used, but it is the only one to be served by the major manufacturers. Unfortunately, the OO track gauge of 16.5 mm is actually incorrect for 4 mm scale, giving a track gauge equivalent of 4 ft 1½ in instead of 4 ft 8½ in. However, since the 1960s, other gauges in the same scale have arisen to reflect the desire of some modellers for greater scale accuracy.
context_section_description: 16.5 mm (0.65 in) gauge at 4 mm:1 foot means that the scale gauge represents 4 ft 1 ¹⁄₂ in (1,257 mm), 7 inches (178 mm) narrower than the prototype 4 ft 8 ¹⁄₂ in (1,435 mm). This difference is particularly noticeable when looking along the track. As the market for proprietary track is mostly for HO scale, sleeper size and spacing are designed for HO and are therefore underscale.
  OO is also used to represent the 5 ft 3 in (1,600 mm) Irish gauge, where it is a scale 13 ¹⁄₂ inches (343 mm) too narrow.
  Though they run on the same track, OO gauge and HO gauge models of the same prototype do not sit well together since the OO models are larger than the HO equivalent.
  These differences have led to the development of the finescale standards of EM gauge and P4 standards. Nevertheless, it is possible to model using OO to standards that fall just short of finescale.
  In common with most practical model railways of any scale (and not related to the OO gauge inaccuracy) the following compromises are made: Curves are often sharper than the prototype, and often not transitioned, particularly when using "set-track" systems (radius 1 = 371 mm, 2 = 438 mm, 3 = 505 mm, 4 = 571.5 mm). Overhang from long vehicles means that track centres are overscale to prevent collisions on curves between stock on adjacent lines, at up to 65 mm (for set-track (reduced down to 50 mm for Peco Streamline)). Overscale wheel width and deep wheel flanges are used on typical models (but particularly older models), and these require overscale rail profile and much larger clearances on pointwork than is prototypical. Pointwork is often compressed in length to save space.
caption: OO scale permits a level of detail including weathering and individual numbering.
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