The FM broadcast in the United States starts at 88.0 MHz and ends at 108.0 MHz. The band is divided into 100 channels, each 200 kHz (0.2 MHz) wide. The center frequency is located at 1/2 the bandwidth of the FM Channel, or 100 kHz (0.1 MHz) up from the lower end of the channel. For example, the center frequency for Channel 201 (the first FM channel) is 88.0 MHz + 0.1 MHz = 88.1 MHz.
So the FM center frequencies are determined as follows:
88.0 MHz + 0.1 MHz = 88.1 MHz 88.2 MHz + 0.1 MHz = 88.3 MHz 88.4 MHz + 0.1 MHz = 88.5 MHz up to 107.8 MHz + 0.1 MHz = 107.9 MHz
Every FM center frequency ends with a decimal extension of .1, .3, .5, .7, or .9.
To convert FM channel numbers to/from a corresponding frequency, use the table in Section 73.201 of the FCC's Rules, or a conversion tool.
In the AM band, each AM station has a maximum bandwidth of 10 kHz, extending 5 kHz above and 5 kHz below the assigned center frequency. The AM band in the United States covers frequencies from 540 kHz up to 1700 kHz, in 10 kHz steps (540, 550, 560 ... 1680, 1690, 1700). 530 kHz in the United States is not available for broadcast use, but is reserved for the use of very low powered Travelers' Information Stations. AM band stations do not have assigned channel numbers, they are referenced by the frequency in kilohertz only.
AM and FM station assignments in other countries may not be made according these procedures. In some countries, an FM station may be assigned a frequency with an even decimal such as 106.2 MHz. In many places, AM broadcast stations are assigned on frequencies with a 9 kHz bandwidth (531 kHz, 540 kHz, 549 kHz, etc.). There are a few AM stations in the United States assigned in this manner, in Guam.
Information about AM and FM broadcast radio stations is available at the Audio Division on the FCC's website, and at Broadcast Radio Links.
FCC > Media Bureau > Audio Division