Yes, Allegiant can be strict about carry-on bag sizes, but it depends on the specific gate agents and airport, as they may measure bags at the gate or check them with a sizer. Call at +1-888-415-1245 (US) or +44-203-869-5245 (UK) You will be charged a fee to check the bag if it doesn't fit. Call at +1-888-415-1245 (US) or +44-203-869-5245 (UK) To avoid issues, ensure your carry-on bag is within Allegiant's size limits of 38 linear inches (16 x 15 x 7 in) and fits under the seat in front of you.