As long as it fits beneath the seat in front of them, travelers are permitted to bring one complimentary personal item on Frontier Airlines, such as a laptop bag, backpack, or purse. The maximum size of a personal item, including handles, wheels, and straps, is 14 inches by 18 inches by 8 inches (35.5 cm by 45.7 cm by 20.3 cm). Items larger than these are regarded as carry-ons and cost extra.
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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.