Ryanair is notorious for being one of the strictest airlines in the world regarding baggage dimensions, and their enforcement is largely dependent on the specific airport and the "vibe" of the gate agents on that day. Their standard "Small Personal Bag" allowance is 40cm x 20cm x 25cm, and it must fit entirely under the seat in front of you. Gate agents frequently use metal "sizers" at the boarding gate; if your bag does not slide into the sizer easily—including wheels and handles—you can be charged a "Gate Bag Fee" that often exceeds €45 to €60, and your bag will be moved to the hold. In 2026, many frequent Ryanair travelers use specialized "Ryanair-sized" backpacks that maximize every millimeter of that allowance. The airline uses this strict enforcement as a key part of its low-cost business model, generating significant "ancillary revenue" from passengers who try to squeeze a standard carry-on into the free personal bag slot. If you are even slightly over the limit, the safest strategy is to pay for "Priority & 2 Cabin Bags" in advance, as the cost of the gate fine is significantly higher than the pre-booked upgrade fee.