Knowledge > Scale Types > What Is The Difference Between OO And HO?

What is the difference between OO and HO?

Nothing as far as the gauge of the track is concerned, i.e. gauge refers to the distance apart of the rails and not the scale size. HO and OO gauge is 16.5mm between the rails.

Scale is a different matter altogether, OO scale is 1/76th of true size or 76 times the model length equals one life size item. Another way of referring to OO scale is 4mm to the foot. i.e. 4mm on the model equals a foot in real life so therefore a six foot tall person in model size is 6 times 4mm or 24mm tall. Funny that two different measuring systems are utilized but that’s the way it goes.

HO on the other hand is 1/87th of true size or 3.5mm to the foot. i.e. that six foot person is now 6 times 3.5mm or 21mm tall.

How come two different scales use the same track I hear you say?

The 2 scales are so close that many years ago the manufacturers opted for one gauge for compatibility.

Just as an aside, at standard gauge of 4’ 8 ½” HO scale is more correct being 4.71 times 3.5mm equalling 16.49mm.

OO is actually undersize and strictly speaking should be 4.71 times 4mm equalling 18.84mm.

This actual discrepancy is catered for by people modelling in EM or P4 gauge.

Most often OO scale is off