There are 3 numbers you are concerned with when you look for a spare road bike tube, and they tend to come in this sort of combination: 700 x 23, 46 mm.
The first number, for a road bike, would tend to be 700 as in 700 mm (or 70 cm). This is the standard road bike wheel size. If you are less than 5'2" tall or so and riding a bike that fits you, though, you might be running a smaller set of wheels, probably a 650 mm, because your bike frame would be too small to run the standard 700 mm wheels without you ending up hitting the front one with your foot when turning the bike. Simply park your wheels next to a full size bike and you should be able to see if your wheels are the same size or smaller.
The second number is the tire width. For road bikes these days it's usually 23 mm, though 25 mm tires are becoming more popular now (this is the tires I'm using, btw). This number can vary a lot, the bigger the number, the bigger the tire, though the width of the tire on your bike is limited by how much clearing you have on your frame and brakes (I wish I can use 28 mm tires, but they are too big to clear my brakes, so I run a set of 25 mm).
Your tires' size is usually printed on the side of the tires... like in the photo above. On the inner tube boxes, this is usually given in a range instead of a single number. If your size is inside the range given (in the photos, the ranges are 20-28 for the Specialized, and 20-25 for the Giant), then you are good to go. If you are stuck in the middle of nowhere and only have a tube that is a bit smaller than the tire (like, a tube with range of 20-25 but your tire size is 28), you can get away with using the smaller tube. It'll just be stretched thinner than usual when inflated, though, and a bit more vulnerable to puncturing.
The third number is the length of the valve stem. As you can see, most road bikes come with the long and spindly Presta valve instead of the usual Schrader valve that is prevalent on car tires. My bike wheel in the photo has the standard size aluminum rim (about 25 mm). I need the valve to be at least 15 mm longer than the rim (or the pump or CO2 nozzle wouldn't fit over the cap when I reinflate), so I need 40 mm or longer stem.
|If you use deep rimmed aero wheels like this guy does, you need tube with 60 mm or longer valve stem.|
Mind, I've been able to use tubes with 38 mm stem, but I have to use the stem nut to secure the tube, and it's always harder to keep the pump nozzle from slipping off when reinflating. It also takes longer to change flats since I always have to take out the nut, and is just an extra aggravation that I don't need. So, usually I go with the Giant tube in the 1st photo, with 48 mm stem. Some people use deeper (more aero) rim on their wheels, and they need tube with longer valve stem, like the Specialized tube with 60 mm one.
If you aren't so weight conscious and don't mind a few extra ounces on the bike, I'd recommend using the cheaper (thicker and heavier) inner tubes rather than the more expensive (thinner and lighter) ones simply because they are a bit more flat resistant. And, unless you are a super avid or pro cyclist you probably won't even notice any weight difference anyway.
So, you now know how to choose the right inner tubes for your bike... But do you know how to prep them before going out for a ride? It's pretty easy, and it could make flat-changing a lot more pleasant (if pleasant can be used to describe any such repair!).
Most tubes come unpowdered. Why do we powder our tubes? To make it less sticky, of course! Sticky tubes are uncooperative tubes and like to stick to things, and this can sometimes cause an extra flat when it pulls the rim tape, that rubberly tape that shields the tube from the spoke nipples, out of alignment and exposes the tube to the metal. You can buy pre-powdered tubes, of course, but they typically costs a couple of dollars more. It's way cheaper to prep your own!
Just take the fresh new tube out of the box. Take the valve stem nut out now, if you don't plan on using it. Then powder the whole thing. Re-fold it, with a CO2 cartridge if that's your method of re-inflation. Put the whole thing in a plastic bag and then wrap it up with a rubber band (usually comes with the new tube). Be sure to rubber band the plastic bag rather than the tube. Sometimes, if you rubber band the tube itself and then go a long time before using it, the rubber band works itself into the tube and can actually tear it.