The most important part of buying a tube is getting the size right. It must fit correcting inside your tyre when it is full of air. Otherwise, you can get pinch punctures, where the tube is caught by the rim of your bike, or over stretching which will again cause puncturing. Your wheel size is found on the sidewall of your tyre…see How to Buy a Bike Tube for more info as there is a huge range of wheel sizes (kids, road, touring, mountain bike).
Not only do you have a wheel diameter size to find but a width of your tyre size as well. Tyre widths are usually expressed in a range e.g. 1.95/2.125 means the tube will fit tyres between 1.95 and 2.125 inches. Again the width is found on the sidewall of your tyre and will come after the wheel size. Look for something like 26″ x 1.95/2.125″
Valve Type – Presta vs Schrader
Buy a replacement tube that has the same valve that is currently on your bike. Generally Schrader valves (the fat ones) are used for lower pressure types of riders e.g. kids & mountain biking. Presta valves (the skinny ones) are therefore high pressure e.g. road riding, road bike touring but more and more mountain bikes are now being sold with Presta valves. For more of an explanation see How to Buy a Bike Tube
Not all tubes are created equally. You can invest in tubes that will help prevent punctures from happening (thorn resistant tubes) or will instantly fix a small (less than 3mm) hole (self-seal tubes). Thorn resistant tubes are 3 times the thickness of a standard tube (much heavier and bigger than standard tubes so not designed to be carried). The other option is self-sealing tubes that have sealant inside them. Puncture protection is popular with mountain bikers, for bike touring, with regular commuters or for anyone that wants to avoid spending time on the side of the road/trail repairing a flat.
Special Performance Tubes
Downhill tubes are specially designed to take the force of slamming into rocks and roots on the trail. Lightweight tubes take into account weight so for performance athletes looking for an another way to minimise weight & go faster these are a good option. Tubeless tyres have no tubes in them but it is always a good idea to carry a spare tube incase your tyre is slashed rather than punctured.
Make your purchase a success…
Carry your tube
This is an essential part of the process, make sure it is where it needs to be so that when you have a puncture it can be easily fixed. Carry it in a protected way so that the valve doesn’t get damaged before you have even used it.