how long is a scale mile? - Model Railroader Magazine

HO track gauge is 16.5mm

If this is the definition of HO (I don't know what the real definition is, frankly), then the scale of HO would be found by comparing the prototype gauge to the model gauge.

Prototype Gauge: 4ft-8.5in = 56.5in = 1435.1mm

Scale Gauge: 16.5mm

Scale of HO = 1435.1 / 16.5 = 86.98

Alright - forget it. I just demonstrated that the "definintion" of HO is not the 16.5mm track gauge.

What is the definition of HO? I guess it's 3.5mm to the foot, as Ironrooster said.

So, let's see:

1 Prototype foot is 304.8mm
1 HO scale foot is 3.5mm

304.8mm / 3.5mm = 87.08571

So I guess that's the exact answer - with no rounding: HO scale is 1:87.08571

So, one scale mile would be 5280ft / 87.08571 = 60.62992ft.

There you have it - one HO scale mile is 60.62992ft.

Please make sure your layout is scale properly. That last 2 after 5 decimal places is extremely important :)