As per the NMRA definition of Scale!  

For the NEM definitions, please don't click here.

Please note - the onus is on the purchaser to ensure the correct scale is selected at the time of purchase.

There are several scales of model railway equipment, the most popular are listed below:

G Scale, also referred to as 45mm gauge. (Ratio can be anything from 1:48 to 1:20.3)

O Scale, also known as 1/4 Inch scale (Ratio is 1:48)

OO Scale, also referred to as 4mm (Ratio is 1.76.2)

HO Scale, also called Half O (Ratio is 1:87.1)

TT Scale (Ratio is 1:120)

N Scale, also referred to as 9mm - more correct would be 8,97mm! (Ratio is 1:160 USA and elsewhere or 1:150 in Japan or 1:148 in Britain)

Z Scale (Ratio 1:220)