I'm a cycling beginner and am trying to purchase spare inner tubes from amazon. The tube sizes are usually given as:

20-622 - 25-630
27x3/4 - 27x1.00
700x20c - 700x25c

How should I understand these? What unit does 25c represent?

  • If you type "tyre sizes" or "tire sizes" into the search box on the top right of this page, or follow any of the suggested links given when you asked this question, you could read about it. Eventually you'll find a link to a fair exhaustive disccon from Sheld Brown. Reading that will help too: sheldonbrown.com/tire-sizing.html
    – Móż
    Jun 19, 2016 at 21:17
  • You have to understand that tire sizing nomenclatures, until the ISO/E.T.R.T.O. scheme was invented, were pretty idiotic. The top set of numbers above are the ISO ones and are what you should go by when selecting tires or tubes -- the others can lead you astray. (Eg, a "29-inch" wheel is smaller than an "27-inch" wheel.) See the Sheldon Brown reference above for a discussion of the various schemes and how they interconnect. Jun 19, 2016 at 21:52

1 Answer 1


Those are three different sizing standards

  • First one is 20mm inner tube diameter and 622-630mm inner wheel diameter
  • Second one is inner wheel diameter of 27inches and inner tube diameter of 3/4 - 1 inch
  • Third is rather more confusing. The 20 and 25 refer to the inner diameter of the tube in mm. The 700c is a measure of the diameter of the wheel but c is not a nit of measurement, its a standard that came about while tire standards were being written. 700c essentially means a diameter of 622mm or 27 inches.

Your best bet is too find the same tire you currently have


If you have any more doubts check out the website above

  • please don't copy comments on the question into an answer, especially on duplicate questions. Please take the tour and read the FAQ
    – Móż
    Jun 20, 2016 at 7:49

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