Popular Small Narrow Gauge Scales

 

  N scale: 2 1/16 mm to the foot (British)  or 1.9mm to the foot (American).  

Nn3, Nm or N-6.5

2mm scale running on Z gauge (6.5mm) track Can be used for modelling approximately 3ft gauge  or metre gauge prototypes.
  HO scale: 3.5mm to the foot  
HO-9, HOe, HOn2, HOn½ HO scale running on 9mm gauge track Suitable for European and American prototypes.
HOm HO scale running on 12mm gauge track Suitable for metre gauge lines.
HOn3 HO scale running on 10.5mm gauge track. Used for American 3ft gauge.
  OO scale: 4mm to the foot  
OO-9 OO scale running on 9mm gauge track. Suitable for 2ft to 2ft 6" gauge prototypes. 
OO-12 or OOn3 OO scale running on 12mm gauge track. For 3ft gauge prototypes.
  O Scale: 7mm to the foot (British) or ¼" to the foot (American)  

O-16.5, On30, Oe, On2 or On2½

O scale running on 16.5mm track.

For modelling 2ft - 2ft 6" gauge prototypes

Om O scale running on special 22.5mm track. For metre gauge prototypes.
  SM scale: 16mm to the foot  
SM-32 16mm scale running on 32mm gauge track. Correct for 2ft gauge lines.
SM-45 16mm scale running on 45mm gauge track. Ideal for modelling 3ft gauge prototypes.