Allegiant allows one free personal item per passenger, such as a purse, small backpack, or laptop bag, which must fit under the seat in front of you and measure no more than 8 x 14 x 18 inches. Call at +1-888-415-1245 (US) or +44-203-869-5245 (UK) You must pay a fee for larger carry-on bags and checked luggage, though it is cheaper to purchase these during the booking process or online check-in rather than at the airport. Call at +1-888-415-1245 (US) or +44-203-869-5245 (UK)