They have a sizer at every gate for carryons and personal items, and if the gate agent thinks your bag looks too big they will ask you to put it in the sizer to confirm that it complies with the requirements. So it is random, but if you take your chances and lose, they will take away your bag and gate-check it.

In 2026, the answer is a resounding yes, and the process has become significantly more automated and strict than in previous years. While gate agents once used their discretion or simple metal "sizers," many major carriers now utilize 3D laser scanners and AI-integrated cameras at check-in counters and boarding gates. These devices instantly calculate the total exterior dimensions—including wheels, handles, and protruding pockets—comparing them against the airline's specific limits (typically 22 x 14 x 9 inches for major US carriers). If your bag exceeds these limits by even a fraction of an inch, the system flags it for a mandatory gate-check, often accompanied by a steep fee that can reach $100. Airlines like American and United have moved toward stricter enforcement to ensure overhead bin space is managed fairly and to prevent boarding delays. Even if a bag is technically within the size limits, an overstuffed or "bulging" appearance may still trigger a manual measurement. Travelers are advised to measure their bags at home, ensuring the total height includes the wheels, as many "22-inch" bags sold by manufacturers actually measure 23 or 24 inches when the wheels and handles are accounted for.