how bicycle tire tubes are sized?
 Inspect the sidewall of the bicycle tire that needs a new tube. The sidewall is the area of the tire next to the rim on either side that does not touch the ground when the bike is in motion. The tire size will be printed there in raised numbers along with the recommended tire pressure and possibly other information. The numbers on the sidewall of you tyre are the best place to look for your tyre and tube size information. Most tyres will show both an ETRTO size and a Metric or Imperial size. 