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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"x2.125" tire. I know this fits on an ISO 559mm bead seat diameter rim, and 26" = 660.4mm.


  • 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?


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

Essentially, I'm converting the tire sizes to "height off rim in mm" and drawing them with a wheel in a web application, so I need to know how "tire size" 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.

This Website has been helpful, but still leaves me with the questions above.

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I've never seen anything more complete than the Sheldon Brown website. If you read through it you'll see that there IS NO "SYSTEM" for the inch-dimensioned tires, but rather manufacturers kind of made it up as they went along. And, although "tire size is determined by the diameter of the tire", the real reference point is the diameter of the rim, since that determines what tires will fit. A "slick" and a heavily lugged off-road tire can fit the same rim, even though their actual outer diameters are significantly different. –  Daniel R Hicks May 2 '12 at 1:14
Got it. In this case, I don't much care for fitment, just how to draw it. Essentially I'm using if (tireSize > 70) tireOD = tireSize; else tireOD = tireSize * 25.4; to cover what I needed. For some bizarre reason I got to thinking the second number was height off the rim, and I was making it much harder for myself. –  Ehryk May 2 '12 at 1:45
Follow-up question: What is the 'C' in 700C? Is there also '700B' or '700D'? –  Popup Jun 12 at 11:59
Answering my own question - it looks like the 'C' is for 'clincher', as distinguished from earlier rim shapes. –  Popup Jun 12 at 12:09

1 Answer 1

up vote 4 down vote accepted

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.

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But the second number is width though? That explains some things. So I think the number I'm looking for is just the first one converted to MM - 660.4 for any 26" tire, 700 for any 700C tire. The 'height above rim' is thus variable based on the rim ISO bead seat diameter + 2 * (rim height above BSD (3-4mm or so)). –  Ehryk May 1 '12 at 18:43
At least as a nominal measurement, yes. A particular tire's measurement may vary. –  zenbike May 1 '12 at 18:53
If I was to guess based on your answer, you're saying: - 26" = OD including tread, inflated to recommended PSI - 2.125" = Width at tread, inflated to recommended PSI On a side note, how do you know where tread begins on a road tire? –  Ehryk May 1 '12 at 19:00
Road tire is measured to the highest point. But once again, these are marketing numbers, not physical measurements printed on the tire. They may or may not be more than passingly related. The bead diameter is the only one you can count on fro real accuracy. –  zenbike May 1 '12 at 21:54
The OD / Height is to the highest point, yes. How do you determine the start/end of the tread for the width though? Or is it just at the widest point, regardless of tread? Or expected width at rim? This would also apply for MTB / BMX slicks as well, where there isn't a clear transition from sidewall to tread. –  Ehryk May 2 '12 at 3:32

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