What are you owed if your flight is cancelled?

The firm US rule is the refund. If your flight is cancelled, or significantly changed, and you choose not to travel on the alternative offered, the airline owes you a refund to your original form of payment, not a voucher, regardless of whether the ticket was nonrefundable. That applies to any airline flying to, from, or within the United States. Airlines will offer rebooking first, which is often what you actually want, but the refund is your right if you decline it, and Department of Transportation rules also require prompt refunds of bag fees and paid extras you did not receive.

What US law does not guarantee is compensation for the disruption itself, meals, hotels, or cash for your ruined day. Those come from each airline's own customer service commitments, which vary and which the DOT now publishes in a comparison dashboard, and airlines generally distinguish between controllable cancellations (mechanical, crew, scheduling) where they provide more, and weather or air traffic events where they provide less. Europe is stricter: EU rules require set cash compensation for cancellations and long delays within their scope, on top of care and rebooking, which is why the same disruption pays out very differently depending on where you are flying. Practical steps: get in the rebooking line and call the airline simultaneously, check the airline's own commitment page, save receipts, and know that credit card travel protections may cover costs your airline will not.