CO2 cartridges come in two basic sizes: 12g and 16g. At least those are the sizes for road bike tires. There are larger sizes as well for mountain bikes, such as 20g and 24g. The other distinguishing characteristic to choose from is threaded or non-threaded. Your choice will depend on what type of CO2 tire inflation device you have. Bear in mind that threaded CO2 cartridges are always more expensive than non-threaded ones. In fact, did you know? 12g non-threaded CO2 cartridges are more conventionally known as BB gun cartridges. That's right – that size CO2 cartridge is what powers firing BB gun ammunition. And you can find a 24 pack of 12 g non-threaded CO2 cartridges for about eight dollars in regular stores, meaning not bicycle stores. In general, prices on CO2 cartridges in the biking world are notably higher than what you can find for non-biking specified purposes. But that's just the way it goes. Your Best Buy for CO2 cartridges is on either eBay or Amazon, where you can get a large order including free shipping for about $1.50 for each unit. Granted, you sure hope not to flat that much even in the course of bike riding for two or more years! When it's all said and done, choose a CO2 pump as threaded or noncredit first, factoring in these two things. First, as said before, thready cartridges will cost more. Second, though, is that threaded cartridges only need a small amount of space to carry in your bicycle bag. The non-threaded version requires a CO2 canister contraption that is much larger, taking up much more space in your bike bag, your bike jersey pocket, for your Bento box or similar storage item. So the final trade-off in choosing between threaded and non-threaded CO2 cartridges is size and weight versus cost.