How to understand tyre sizes? 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?
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


  http://www.sheldonbrown.com/tire-sizing.html


If you have any more doubts check out the website above