Wogster

You can not buy a bicycle on specifications, you need a test ride of a moderate distance, say 6-10km (4-6 miles) to see if it's reasonably comfortable.

Seat tube lengths come in 3-4 different formats, some bikes have sloping top tubes, so the seat tube length may be an actual or theoretical measurement (theoretical in that the size is based on where it would be with a non sloping top tube, this is more common), often specification sheets will list both actual and theoretical measurements. . Mountain bikes which are an American design are usually measured in inches, road bikes which are a European design, usually use centimetres (usually abbreviated cm there are 2.54cm to 1 inch). You can not directly convert from one to the other, in that mountain bikes need a crotch clearance of 2-3 inches, where as road bikes only need a clearance of 2-3cm. Cyclocross tend to use cm, hybrids sometimes use inches, sometimes cm. Top tubes can vary in length, between manufacturers for the same seat tube length, this is often a more critical measurement, in that 1cm can be the difference between being too cramped and too stretched out. Although a different stem can often fix this. You will find that mountain bikes will use a longer top tube for a given seat tube, to account for the crotch clearance issue,

There are a couple of online sites for bike measurements, one that includes both mountain and road sizes is Wrench Science The sites will ask for a number of measurements then spit out a size, this is only a starting point though, in that different bicycle manufacturers will size a little differently. For road bikes a 54 in one, a 56 in another and a 58 in still another may all fit.

The best way to do it, is to pick a bike, that when you test ride it feels comfortable, over 6-10km (~4-6 miles), then get a professional bike fitting to dial it in perfectly. Wheel sizes for adult bicycles come in 3 flavours, folders and some recumbents will use a 20" wheel, mountain bikes tend to use 26" or 29", road bikes will use 26" or 700C sometimes you will see old road bikes with the now obsolete 27" size. While you can still get 27" tires and rims, the size has not been used on new bikes in about 20 years, so don't expect them to be around forever. One thing to watch with 27" wheels, many old ones are chromed steel straight sided rims, when wet braking is well nigh impossible, and they can't take as high pressure as modern aluminum alloy hooked rims.

Now when dealing with CL or fleabay, you will sometimes see road bikes with inch measurements, because someone does not know the size, and simply measured it, but you then need to know from where to where they measured it. Often such measurements are from the top of the bottom bracket to some point on the top tube, usually the top or bottom, meaning it can be an inch or two off from the manufacturers measurements, which tend to be from the centre of the bottom bracket to the centre of the top tube, as per the design drawings from engineering. Again the only true way to know it fits, is to take a test ride......