EM gauge (named after the track gauge of a nominal Eighteen Millimetres) is a variant of 4 mm to a foot (1:76) scale used in model railways.

EM was developed because OO gauge, favoured by manufacturers of British prototype models, utilised track that was too narrow. OO was developed in the UK in the 1930s as a response to manufacturers finding they were unable to fit the motors of the time into British prototype small boilered locomotives when scaled at the globally popular HO scale's 3.5 mm to a foot (1:87). As the scale was increased to 4 mm to the foot to make the locomotives larger, the track gauge was left at , and hence is too narrow (by a scale ) to correctly depict the prototype's track gauge of .

EM gauge was founded in the 1950s, originally with gauge track and rolling stock wheelsets based upon the crude and massively out-of-scale products of the contemporary OO model manufacturers.

18 mm gauge was still undersize by almost a millimetre. With the limitations of modelling at this time, particularly the width of tyres, the largest gauge that could fit within the outline of a scale model would be 18.5 mm, no larger.

  • EM Gauge Society