Every airline picks a rule set for what you can bring on board and put in the hold, and that rule set changes with the ticket you buy, the route you fly and any status you hold with the carrier. Two figures do most of the work: size, measured as length plus width plus height in centimetres or a maximum linear dimension in inches, and weight, in kilograms or pounds per bag.
Weight concept vs. piece concept
Most of the world runs on the weight concept: economy passengers get a total allowance, commonly 20 to 30 kg, split across one or two bags with a per-bag cap of 23 to 32 kg. Business and first cabins get 40 to 50 kg. The piece concept is common on flights to and from the Americas: instead of a total weight, you get a set number of bags (usually 1 or 2 in economy, 2 or 3 in premium) each with its own weight limit.
Cabin and personal item
Nearly every full-service airline lets you carry a wheeled cabin bag around 55 x 40 x 20 cm at 7 to 10 kg, plus a smaller personal item that fits under the seat in front of you: a laptop bag, handbag or small backpack. Low-cost carriers vary widely. Some (Ryanair, Wizz Air, easyJet, Spirit) charge extra for the wheeled cabin bag while keeping the personal item free. Others (Southwest, JetBlue, AirAsia) treat the cabin bag as included.
Excess baggage and how to avoid it
Overweight bags are almost always cheaper to buy in advance. Prepaying extra weight online typically costs 20 to 40 percent less than paying at the airport counter. A luggage scale at home usually pays for itself on the first trip. Frequent-flyer status or a co-branded credit card can add 10 to 20 kg to your allowance on many airlines. Skywards Gold on Emirates or Star Alliance Gold on Lufthansa, for example, adds a full extra bag on piece-concept routes.
Where to check before you pack
Every airline page on this site gives you the current cabin and checked limits by fare family, the fee tables and a comparator across cabins, and where the carrier publishes its rates, a live calculator that estimates what you'll pay if you go over. Rules and fees drift each year, so the update date at the bottom of each page tells you how fresh the data is.