You do not need to be a professional to enjoy cycling. Actually, most people ride a bike for fun or out of convenience. It is perfectly reasonable for them not to be overly familiar with various jargon terms or technical details. That said there are things that are worth knowing and that might help you enjoy your bike rides more. One of those things is bike tire sizes.
For a beginner cycler tire sizes can be overwhelmingly frustrating and confusing. A lot of misplaced terminology gets thrown in there and adds to the confusion. Reading a single tire product review can sometime leave you with an impression that a single tire is three different sizes all at once! This is because manufacturers stamp tires with markings in inch sizes, fractional sizes, decimal sizes and sometime even old French sizes.
It all started way back when different countries competing in cycling made their own tires. Since there were no standards and no uniformity they labeled their tires differently. And we got stuck with it today. 700C, 28 x 1 5/8 x 1 ¼, 29 inch are all different names for a single tire size! And this is the funny part; that tire actually measures 622 mm (bead seat diameter).
To avoid confusion the best thing you can do here is to always purchase your tires referring to their ISO number (or ETRTO number, ETRTO being the European Tire and Rim Technical Organization). This number gives you the bead seat diameter. Bead seat diameter measures the outside diameter of an inflated tire and you can’t go wrong with it.
Most road bikes use 622 mm wheels that are commonly referred to as 700C. This is pretty much the norm today. 700C is widely available all over the world and you won’t have any problems replacing your wheels or tires.
Casual cyclists usually buy preassembled road bikes and there are some great set-ups on the market today. There will come a time when you will have to replace the tire and, eventually, the whole wheel. There are two important considerations to keep in mind here. First, your new wheel size has to correspond to your bike size. If the bike came with 700C wheels stick to them. Otherwise you can end up with too narrow a rim for the brakes to be effective or too wide a rim so it does not even fit your bike.
Other thing you need to consider is the width of the tire opposed to the width of the rim. Using a narrow tire on a very wide rim will probably result in a pinched tire (and leave you stranded on the side of the road). Also, narrow tires require a lot more air pressure in them and overinflated tires result in a bumpier ride, especially on rough roads. On the other hand a wide tire on a narrow rim makes handling your bike extremely difficult and compromises tire sidewall. 13 mm interior rim width will combine well with 18 to 25 mm tires, and 25 mm interior rim width will be suited for tires wide anywhere between 44 – 57 mm. Measure your rim width and run some simple math to see what tire sizes it can accommodate.
The common misconception is that narrower tires will allow you to go faster. This is not the case at all except under perfect conditions. You have to be riding on glass for this to be true. For general and recreational purposes you will want 700C tires that are 28 – 30mm wide. At the same pressure level these will outperform narrow tires (20 mm) by far and offer a ride that is a lot more comfortable. If you ride your bike often and use it to commute you might even want to consider wider tires if your bike frame can accommodate them. Just make sure that in that case you replace the wheel as well if the rim is too narrow for the tire.