How to convert currency on your phone when you have no data

Where the iPhone keeps its converter

Apple's instructions are short. Open Calculator, and while you are in the Basic or Scientific calculator, tap the calculator button at the bottom left and turn on Convert. Then tap the unit or currency shown and choose the pair you want. Swipe through the categories at the top of the list to reach others, such as Speed, Time and Weight. Type a value in either field and the other one changes as you type.

The same conversions work in Math Notes, inside the Notes app. Apple's example is writing or typing "50 m in feet =" in a note, and the answer appears after the equals sign. That is handy for a shopping list: write the prices in the local currency once and see them converted next to each line.

What needs a connection and what does not

Units are the easy half. A kilometre is always 1,000 metres and a mile is always the same length, so a converter only needs a fixed factor that never changes. Temperatures, weights and volumes work the same way.

Currencies are different. A rate is a number that changes during the day, so any converter has to fetch it from somewhere. Apple does not publish where the Calculator's rates come from, how often they refresh or what the app shows when it cannot reach the internet. The practical answer is to find out on your own phone: the night before you leave, open Calculator on Wi-Fi with your currency pair selected, then switch on airplane mode and try again. Whatever it shows offline is, at best, the last rate the phone received. That is fine for a menu or a taxi meter, not for deciding on a large purchase.

On Android: three different tools

Google's Calculator app for Android describes itself as basic and scientific calculations, with nothing about units or currencies. Samsung's Calculator on Galaxy phones goes further: Samsung says it helps with converting measurements. Neither replaces a currency converter offline.

The tool most Android users reach for is Google Search. Type a conversion such as 100 EUR in CHF and a currency converter card appears with the result, the date and time of the rate and a link to a disclaimer. The same search box has a unit converter for things like Celsius to Fahrenheit or cups to litres, according to Google. All of it is a web search, so it needs data. If you want a currency app that works offline, choose one whose description says it stores rates on the phone, and open it on Wi-Fi before the trip so it has something to store.

Where in Europe you will actually need it

The euro covers most of the EU, and Bulgaria has joined it too. The other EU members keep their own money: the Czech koruna, Polish złoty, Hungarian forint, Romanian leu, Danish krone and Swedish krona. Outside the EU you meet the pound in the United Kingdom, the Swiss franc in Switzerland and Liechtenstein, and the Norwegian and Icelandic krónur.

Some of these are where a converter earns its place. Forint prices often run to four digits for everyday things, and in Prague or Kraków a quick mental division is easy to get wrong by a factor of ten. The UK adds a second conversion: road signs give distances in miles and speed limits in miles per hour, which is worth setting up in the Length and Speed categories before you pick up a rental car.

Converting is not the same as what your bank charges

The converter tells you roughly what something costs. What you actually pay depends on your card: the rate your bank uses, any fee for paying in another currency and, at some terminals and cash machines, the offer to charge you in your home currency instead of the local one. EU consumer rules require the fee for that currency conversion to be shown before you agree.

Our guide on paying with your phone abroad goes through that choice at the terminal. For the converter itself, the habit that helps is simple: compare the total on the screen with what the converter says. If the two are far apart, ask before you tap.

A routine that holds up without signal

• At home, on Wi-Fi, open Calculator, turn on Convert and select your currency pair and the units you will need.
• Switch on airplane mode and check what still works.
• Note a rounded reference rate in Notes, so you can multiply in your head if the converter fails.
• Once you land and have data, open the converter again to refresh the rate.

With data on the phone, the live rate is a second away. If you travel with a Snoozim eSIM for Switzerland, Czechia, Poland, Hungary or the United Kingdom, it only starts counting at the first connection in that country, so you can install it at home and refresh the rate the moment you land.