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spirit airlines backpack size classifies backpacks larger than 18 x 14 x 8 inches as carry-on luggage. These must be no larger than 22 x 18 x 10 inches and fit inside the overhead bin. Fees for carry-on backpacks apply and differ depending on where and when the allowance is purchased. Selecting a bag that fits within Spirit's carry-on guidelines guarantees that it satisfies airline regulations and provides enough room for necessary travel items. https://airlinesofficedesk.com/blog/spirit-airlines-carry-on-size/