There's a very reliable way to ensure that you always buy the correct tire size and that's to reference the ISO (ERTRO) dimension. This is expressed as the bead diameter, followed by the width - both in mm. A typical 29' tire will be a 622-50. The 622 is exact and there's no fudge room, but you have lattitude of 20% or more on the width value for any given rim.
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