Why is the exchange rate at the airport worse than the one online?

Because the rate you see online is not a rate anyone offers you. It is the mid-market rate — the midpoint between what buyers are bidding and what sellers are asking on the wholesale currency market. It is a reference point, and no retail provider trades at it.

Every provider quotes a spread around that midpoint: they sell you currency slightly above it and buy it back slightly below. The spread is the margin, and it is usually the main cost even when a service advertises "0% commission" — which simply means the charge is inside the rate rather than added on top.

Airport bureaux have the widest spreads of anyone, often 8–12% away from mid-market, sometimes more. The reasons are structural: rent in an airport terminal is enormous, they hold physical cash in many currencies which ties up capital, they operate long hours, and above all their customers are captive. Someone who has just cleared security and needs local currency has no realistic alternative, and pricing reflects that.

Roughly how the options rank on cost, best to worst: specialist transfer services and multi-currency cards, then a debit card from a provider that does not add a foreign transaction fee, then high street banks, then airport bureaux, then in-terminal ATMs run by independent operators.

Two traps worth naming:

Dynamic currency conversion. When a foreign card machine or ATM offers to charge you in your home currency, decline it. The convenience of seeing a familiar currency costs a markup that is typically several percent. Always choose to be billed in the local currency.

"Commission-free" claims tell you nothing without the rate.

Compare providers by working out how much local currency you actually receive per unit, not by comparing advertised percentages.