Every passenger on Frontier Airlines is allowed one complimentary personal item, which must not be larger than 14 inches high by 18 inches wide by 8 inches deep, including handles, wheels, and any outside pockets. The airline uses metal sizer frames at boarding gates to strictly enforce these size restrictions, even though there is no weight limit.
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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.