SWA737-200 to -800, Top of arm rest maybe 18.5". But the important part is bumper to bumper measurement at the lower part of the seat. That maybe reducing the width to 17.5". As a Mechanic that made step stools 15" wide to fit between the seats to work the bins & ceiling area. For the most part Rows 3- aft entry the step stool fits just fine. But rows 1-2 where the plane narrows the stool needs to be carried over those seats. https://www.highskyflying.com/how-wide-are-airplane-aisles/
20/More Seat Passenger Airplanes
[Big airliners like the ‘Boeings’ and ‘Airbuses’ are found within this category of airplanes. The FAA Seat and Aisle Regulations require that the aisle width of these airplanes must be 15 inches from the floor to a height of 25 inches and 20 inches above a 25-inch height.
Therefore, it is safe to say that the aisle width of the last Boeing or Airbus flight that you took was between 15 to 20 or more inches.]
Note: B707, 727, 737 all have the same BS 480 to nose section reduction in width and from BS 480 on back width. BS 480 is the split in the seat track to where it goes narrow.
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