with these two Airlines, you have to be careful in using the designation 'carry on.' Both Frontier & Allegiant allow one "personal item" at no charge, which must fit under the seat in front of you. Allowable dimensions are 8” D x 18” W x 14” H (Frontier) or 7” D x 15” W x 16” H (Allegiant). Conversely, carry-on items are chargeable at various rates. As John suggests, you'll want to check their websites for details on sizes and fees.
Go to allegiant or frontier website they both give dimensions of carry on.
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