Most baggage tools restate a rule you could have read on the airline’s own page. These do arithmetic you cannot do in your head, on data we have verified ourselves. Every one of them is free, works in your browser, and never asks for your email.
They all read the same dataset: the carry-on box, the personal item box, the checked allowance and the fee table for every airline on the site, each stamped with the date it was last checked against the carrier. Change an airline’s rule in the database and every tool changes with it, which is why none of them can drift out of step with the airline pages.
Start with the size checker if you own the bag and want to know where it fails, the bag finder if you are still shopping, and the true cost calculator if the question is money rather than inches. The trip planner handles itineraries that mix airlines, where the most significant carrier rule decides whose allowance applies. The liquids packer, the rebalancer and the packing list screener answer the narrower questions that come up the night before a flight.