(H + number zero), often also written HO scale (H + capital letter O), is the most popular scale of model railway in most of the world (outside the United Kingdom, where the slightly larger 00 gauge is more common). The name is derived from the fact that its 1:87 scale is approximately half that of 0 (zero) scale, hence H0. 0 scale in turn was named following the older and larger 1, 2, and 3 scales. The term H0 is pronounced ("aitch-oh"), not "ho" nor "aitch-zero".

In H0 scale, 3.5 millimetres represents 1 real foot; this ratio works out to about 1:87.086. In H0, rails are usually spaced 16.5 millimeters apart which models the standard railroad gauge of 4' 8.5" or 1435 mm.

 

H0 scale's popularity lies somewhat in its middle-of-the-road status. It is large enough to accommodate a great deal of detail in finer models, more so than the smaller N and Z scales, and can also be easily handled by children without as much fear of swallowing small parts. Models are usually less expensive than the smaller scales because of more exacting manufacturing process in N and Z, and also less expensive than S, 0 and G scales because of the smaller amount of material; the larger audience and the resultant economy of scale also drives H0 prices down. The size lends itself to elaborate track plans in a reasonable amount of room space, not as much as N but considerably more than S or 0. In short, H0 scale provides the balance between the detail of larger scales and the lower space requirements of smaller scales.

 

H0 scale has several narrower gauges to represent narrow gauge railways in the same scale as their H0 counterparts.

Name	Gauge	Prototype	Notes
H0	16,5 mm	Standard gauge
H0m	12 mm	Meter gauge
H0n3-1/2	12 mm	3' 6" gauge	3' 6" is the "standard" gauge in much of Africa, Queensland (Aus), New Zealand and also non-Shinkansen JR lines in Japan
H0n3	10.5 mm	36" gauge
H0n30 and H0e	9 mm	30" gauge	Typically for lines in 24-30" gauge. Known as H0e in Europe
H0z and H0i	6,5 mm	15" gauge	Known as H0i (or H0f) in Europe