Paying abroad: the conversion the terminal offers you
Abroad, at the moment of paying by card, the terminal often asks a question: do you want to be charged in the local currency or in your own? The offer looks like a courtesy and it is a commercial one. This page says who sets the rate behind it and what the rules require that party to tell you.
What the terminal is actually offering
The technique has a name, dynamic currency conversion, and a simple mechanism: the amount is converted before it ever reaches your bank. What travels through the card network is then a sum in your own currency, already converted, and your bank passes it on as it stands. It is abbreviated DCC.
The conversion is therefore performed by the merchant's side of the transaction, not yours. The party doing it is the terminal's operator or the service it uses, and the rate applied is the one that party has chosen.
Who sets the rate, and what the rules require
Declining leaves the conversion to your own card network and your own bank, at their rate and their fee. Accepting hands it to a company selected by the merchant, whose margin is part of what it earns from the service.
In the European Economic Area this is not left in the dark. Since April 2020, Regulation 2019/518 requires the total currency conversion charge to be shown as a percentage markup over the latest euro reference rate published by the European Central Bank, which is the same rate this site uses. The figure has to be visible before you choose.
The same offer, in four other places
A cash machine abroad makes the identical offer, worded as a guarantee of the amount. An online checkout can make it too, sometimes without a visible question, by pricing the whole site in the currency it has guessed from your address.
Airline and hotel booking pages are the third case, and the fourth is the paper receipt: when the slip shows a total in your home currency for a purchase made abroad, the conversion has already happened, and the receipt is where it becomes visible.
How the choice is declined
By choosing the local currency, which is the country's own money: euros in Portugal, yen in Japan, zloty in Poland. That single choice sends the amount unconverted through the network, and the conversion happens at your bank instead.
Terminals word the two buttons differently and some present the home-currency option as the safe one. The wording is not a reliable guide; the currency name is. Reading which currency each button names takes two seconds and it is the whole decision.
What this site can and cannot tell you
It can give you the reference rate with the date it was published, which is what the European rules use as the yardstick for the markup. That number is a starting point for reading an offer, not a price you can obtain.
It cannot tell you what you will actually be charged, and no converter can, because that figure is set by whoever handles your payment and is not published anywhere in advance. This page describes a mechanism; it gives no financial advice and recommends no course of action.