3.5mm ft HO scale

This is the major gauge used outside the UK. At 3.5mm to 1ft, the track gauge at 16.5mm is virtually exact to scale for the standard gauge. When using this gauge it must not be confused with 00 gauge, HO gauge is almost 15% smaller. One can run HO gauge rolling stock on 00 gauge layouts, the track gauges both being 16.5mm, but the difference in scale will be very obvious.