Frontier Airlines allows each traveler to bring one personal item on board. The maximum dimensions of this item, including handles, wheels, and straps, are 14 inches in height, 18 inches in width, and 8 inches in depth. Small backpacks, briefcases, diaper bags, computer bags, handbags, and totes are all acceptable personal belongings.
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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.