Card machines and cash dispensers abroad frequently ask whether to charge in the local currency or in the cardholder's own. The choice determines who performs the conversion and who keeps the margin on it.
Two different parties can do the conversion
If the transaction is processed in the local currency, the amount travels through the card network and is converted by the network at its own rate, with any charge applied by the issuing bank.
If it is processed in the cardholder's currency, conversion happens at the terminal, performed by a provider contracted by the merchant or the machine's operator.
Both routes reach the cardholder's account, but the exchange rate applied and the party earning the spread are different.
The offer is framed as certainty
The screen typically shows an exact amount in the home currency, presented as a guarantee against surprises on the statement.
That certainty is real, but it is delivered by fixing a rate chosen by the terminal's provider rather than by using the rate the network would apply later.
Because the provider sets that rate and competes for the merchant's business rather than the cardholder's, there is little pressure holding the margin down.
Disclosure requirements make comparison possible
Rules in several jurisdictions require the terminal to display the exchange rate being offered, sometimes alongside a comparison with a reference rate.
Reading that screen rather than pressing the highlighted button is the whole of the decision, and it takes a few seconds.
The comparison figure is the useful part, since an absolute rate means little without knowing what the alternative would have been.
Cash dispensers apply the same logic
Independent operators of cash machines in tourist areas frequently offer conversion at the point of withdrawal, using the same mechanism as a shop terminal.
Those machines may also charge their own access fee, which is separate from the conversion and separate again from anything the card issuer applies.
Machines operated by established banks tend to present fewer of these layers, though this varies by country and cannot be assumed.
Knowing your own card's terms matters most
Card products differ substantially in what they charge for foreign transactions, and the terms are set out in the account documentation rather than at the terminal.
Declining terminal conversion is only advantageous if the issuer's own arrangement is reasonable, which is something to establish before travelling rather than at a till.
This describes how the mechanism works and is not advice about which product to hold, which depends on individual circumstances.