How are tire sizes measured? I tried getting a good measurement with my caliper, but the flexibility of the tire doesn't help much. Let's say I have a 26&quot;x2.125&quot; tire. I know this fits on an ISO 559mm bead seat diameter rim, and 26&quot; = 660.4mm.
26&quot;:

The OD of the tire including the tread?
The OD of the tire excluding the tread?
When aired up or deflated?
Means it fits a 559mm ISO BSD
None of the above?

2.125&quot;:

Width of tread?
Width of sidewall?
Inflated or deflated?

Essentially, I'm converting the tire sizes to &quot;height off rim in mm&quot; and drawing them with a wheel in a web application, so I need to know how &quot;tire size&quot; translates to this measurement. I'm somewhat assuming that the fractional sizes can be converted to the same measurement, but I'm aware that they have certain differences - so any help there would be of value as well.
Sheldon Brown's Website has been helpful, but still leaves me with the questions above.
First Number = Nominal Outer Diameter of tire, including tread.

Second Number / Letter = Nominal width of tire at the start of the tread.

Tire size is determined by the outside diameter of the tire, as regards the 26 inch or 700c number. The 2.125 inch number is the nominal width of the tire at the point where the tread starts.

These are nominal measurements, though, and there is no universal standard applied consistently other than bead diameter.

700c x 23 tires from Schwalbe will measure 28mm in width, and about 5 mm extra in diameter. From Conti, its about 24mm and 1-2mm extra. There is no stadard for point of measurement. It is left to the manufacturer to decide, even before the marketing department gets involved.