Short answer: reasonably strict. United Airlines expects your in-cabin pet carrier to fit completely under the seat in front of you. While agents may not always pull out a tape measure, they will check for proper fit and overall compliance at check-in and the gate. If your carrier looks oversized or your pet can’t comfortably turn around and stand up (with a soft, compressible top), you may be denied boarding with your pet.
United’s published in-cabin size guidelines are typically up to 17.5 x 12 x 7.5 inches for hard-sided carriers and up to 18 x 11 x 11 inches for soft-sided carriers. Soft-sided options are preferred because they flex to slide under varying seat spaces, but they still must fit fully under the seat with your pet inside for the duration of the flight.
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Yes—United focuses on whether your carrier fits fully under the seat and keeps your pet safe and comfortable. Soft-sided carriers that meet the posted dimensions and compress slightly are your best bet.
United expects full compliance: approved carrier size, under-seat fit, and your pet remaining inside for the entire flight. If the carrier appears too large or your pet isn’t fully contained, boarding may be refused.
Airlines commonly verify carrier size and fit at check-in or the gate. They may not measure every bag, but they will check under-seat fit and overall safety.
