​Frontier Airlines permits one free personal item per passenger, which must not exceed dimensions of 14 inches in height, 18 inches in width, and 8 inches in depth, including handles, wheels, and straps. This item should fit entirely under the seat in front of you. Common examples include small backpacks, laptop bags, purses, or briefcases.
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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.