Frontier Airlines allows passengers to bring one free personal item onboard, which must not exceed the dimensions of 14 x 18 x 8 inches (35.5 x 45.7 x 20.3 cm), including handles, wheels, and straps. This item should fit under the seat in front of you and can include items like a backpack, laptop bag, or purse.
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Frontier Airlines Personal Item Size: What You Need to Know
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Frontier Airlines Personal Item Size: What You Need to Know

This blog post explains Frontier Airlines personal item size limits, including the rules, sizes, and what you can bring for free.