HO scale is 1/87th the size of the real thing or 3.5 mm to every 1 foot. So a HO scale locomotive is going to be 1/87th the size of an actual locomotive, and a HO scale railway is going to be 1/87th the size of the actual railway. The minimum requirement for a HO scale turning radius is 15 inches, HO scale rails are 16.5 mm apart, and the average HO scale train is about 15 to 22 inches long.