Most steel bicycle frames have tubing of standard outside diameter. Frames made to British or Italian standards will typically have 1 1/8" (28.6 mm) seat tubes. Older French bicycles used 28 mm. Old American style, commonly used on BMX bicycles and other bicycles that use one-piece-cranks, uses a 1" seat tube. Standard outside diameter accommodates standard lugs and clamp-on parts such as front derailers, pulleys and other cable guides, pump pegs, etc.

High-performance bicycles are made of thinner-walled tubing, so the hole will be larger if the outside diameter is kept the same.

As a result, a larger seatpost size is often a sign of a higher-quality bicycle. This has resulted in a de facto standard of 27.2mm for high-quality bicycles that have 1 1/8" O.D. seat tubes. Since high-quality seatposts are widely available in 27.2, most high-end aluminum and carbon fiber road bicycles also use 27.2.

A recent trend to oversized tubing has resulted in the introduction of oversized seatposts (anything over 27.4 would be considered oversized.) These are more commonly seen on mountain bikes, where the longer seatposts can use the extra strength.

Diameter is not the only factor affecting compatibility. Using a carbon-fiber seatpost in a metal frame, or vice versa, invites problems. Metal should be greased, but carbon fiber should not be. The seatpost clamps of metal frames easily damage carbon-fiber seatposts.

Most of the size information on the list below was provided by people like you. We have no way to check the accuracy of the data. It should be noted that frames can be reamed out to take a larger-sized seatpost, and sometimes a replacement seatpost may be a slightly different size than the original.

Measuring is better than guessing. Most better bicycle shops will have a set of seatpost-sizing rods like this:

Alternately, you could check the diameter of a seatpost that fits. The diameter is stamped on many seatposts (usually on the part that is fits inside the bicycle's seat tube and gets scratched up and covered with grease, *sigh*). You also could measure the diameter using a dial indicator caliper.