Mr. Peanut's Platinum Series — 18 × 10.5 × 11 in, Double Expandable

Eighty-four dollars is the least you can pay for a Mr. Peanut's here, and it is the one that opens out on both sides. Under the seat it measures 18 × 10.5 × 11 in — the same footprint as the Gold Series. Once you have landed, two bays unzip and it stops being a box.

The measurements

18 in long × 10.5 in wide × 11 in high, closed. Clears the published under-seat footprint of five of the nine U.S. airlines in our guide: American, Delta, United, Frontier and Allegiant. It does not clear Alaska and Hawaiian, JetBlue, Southwest or Breeze. Southwest is the one that catches people out — it publishes 8.5 inches of height and this bag is 11. We would rather tell you now than at the gate.

• Two expansion bays that zip open on either side once you are off the plane.
• Recommended for pets up to 15 lb, and best for pets around 16 in long lying down.
• Weighs 2 lb empty — the lightest carrier in this range, so nearly all of your allowance is your pet.
• The interior is slightly smaller than the exterior because of fabric and padding. That is true of every soft carrier, and most listings do not say it.

Platinum or Gold?

Same size, same airlines, twenty-five dollars apart. The Gold Series is built in 600D nylon: stiffer, and it holds its shape under weight. The Platinum is 300D — lighter and wrinkle-free, and it packs down more easily. The manufacturer does not claim either one is better, and neither do we. If you want a bag that stands up on its own at the gate, buy the Gold Series. If you want the lightest bag that still locks shut, this is it.

What it does well

• Premium self-locking zippers. No fragile plastic clasps, and the bag cannot be pushed open from the inside.
• Faux fleece padding over a plywood base insert — it holds its shape under weight instead of sagging onto your pet.
• Top-loading luggage strap slides over a suitcase handle.
• Zippered mesh pocket for treats, medication and health papers.
• Padded shoulder strap with thick metal connectors, plus a hand carry handle.
• Wipe-down composite nylon.
• Name tag holder, leash tether and seat-belt attachments.
• Leak-proof design and materials. Includes a storage pouch.
• Snowflake gray.

Sizing: measure your pet lying down, nose to the base of the tail, and add 2 inches. Then measure standing, floor to the top of the head. Your pet must be able to stand up and turn around inside — that is the rule every airline writes, and the one gate agents actually check.

Before you book: airlines publish their own maximum under-seat dimensions and they are not the same. Compare the measurements above against your airline's published limit before you buy — our airline size guide has all nine side by side.