1 gauge is a toy train and model railroading standard, popular in the early 20th century, particularly with European manufacturers. Its track measured 1.75 inches (44.45 mm), making it larger than O gauge but slightly smaller than wide gauge, which came to be the dominant U.S. standard during the 1920s.

No 1 gauge was standardised, according to Model Railways and Locomotive magazine of August 1909 at 1 3/4" or 44 3/4 mm. The distance between the wheel tyres at 1 17/32" or 39mm and between the centre of the track 48mm (no inch equivalent suggesting it was metric users requirement only). The wheel width was set at 19/64" or 7 1/2mm.

Interestingly gauge, rather than scale, seems to be used more in the early days. For the record the four gauges for which standards were adopted were No. 0, No. 1, No. 2 and No. 3. This suggest the O gauge should more correctly be gauge zero!

1 gauge survives today as it is used by the modern G scale, a popular standard for outdoor model railroads.

Although initially popular in the United States, 1 gauge lost popularity due to World War I, which dramatically decreased foreign imports, allowing the U.S. wide gauge standard to gain traction. After the war, most surviving U.S. manufacturers switched to wide gauge.

Although vintage 1 gauge trains use the same track standard as modern G scale, scale modeling was not a primary design consideration in 1 gauge's heyday, so the actual size and scale of the locomotives and cars varied. Generally, 1 gauge equipment worked out to approximately 1:32 scale. 1:32 is the scale where the gauge 1 track represents standard gauge track. G scale at 1:22.5 scale means the gauge 1 track represents 1m narrow gauge track. 1m gauge railways are still to be found in Switzerland, the inspiration source for many commercial G gauge models.

By comparison, 1 gauge's 1:32 scale makes it nearly three times the dimensions (about 20 times the overall size) of modern HO scale, the most popular size of today.