When flying Southwest you can have one carry-on bag and one personal item. The personal item is typically described as a purse, briefcase, camera, food container, or laptop. The carry-on bag would go in the overhead compartment and the personal item would go under the seat in front of you. The offical size limitation for these items are as follows:
Carryon size limit: 10 x 16 x 24 inches.
Personal item size limit: 18.5 x 8.5 x 13.5 inch
More information about carry-on bags and personal items can be found at:
In your example, the item is 1" too large on one dimension to technically be considered a personal item, BUT assuming you have some ability to squish the tote a bit you won't have a problem. I've seen people cram much larger items into both the overhead bins and under the seat. Chances are that you'll also be able to put it in the overhead, even with a carry-on bag (if you have one) unless you are among the last to board and overhead space is full or limited.
Have a great trip!
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