OO gauge

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OO gauge
U.K. prototype model of a OO scale (1:76) British Rail Class 25 shown with five pence coin for scale
Scale	4 mm to 1 ft
Scale ratio	1:76.2
Model gauge	16.5 mm (0.65 in)
Prototype gauge	Standard gauge

OO gauge or OO scale (also, 00 gauge and 00 scale) is the most popular standard gauge model railway standard in the United Kingdom,^[1] outside of which it is virtually unknown. OO gauge is one of several 4 mm-scale standards (4 mm to 1 ft (304.8 mm), or 1:76.2), and the only one to be marketed by major manufacturers. The OO track gauge of 16.5 mm (0.65 in) (same as the 1:87 HO scale) corresponds to prototypical gauge of 4 ft 1+1⁄2 in, rather than 4 ft 8+1⁄2 in standard gauge. However, since the 1960s, other gauges in the same scale have arisen—18.2 mm (EM) and 18.83 mm (Scalefour)—to reflect the desire of some modellers for greater scale accuracy.