This question came into the PIC recently via email. Although the writer knew that the answer was 8 nautical miles, he was asking where that is stated. He noted that although he remembered this from primary training, as do most pilots, he could find no official reference to that fact anywhere.
The AIM defines airways, and federal airways, but it really doesn't cite the dimensions of them. Instead, one has to look at FAA Order 8260.3E, the TERPS manual, for an official description of an airway's dimensions.
Chapter 14, sections 14-1-2 and 14-1-3 give the official dimensions as 4 nautical miles on either side of the primary radial, or 8 nautical miles total. In fact, if the airway is longer than 102 nautical miles, a little extra width is provided. See the link below for the specifics.
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