How a currency converter works with no internet connection

Why most converters stop working without a signal

Almost every converter, including search results, web pages and most apps, requests a fresh rate from a server each time you use it. With a connection that is the simplest design and the most accurate one. With no connection there is nothing to request, so the page fails to load or the app shows an error where the number should be.

This happens precisely where conversion is most needed: on a plane, underground, in a country where you have switched roaming off, in a hotel with a hostile wifi portal, or at a border where your phone has not yet found a network.

What a cached rate actually is

An offline converter downloads a full table of rates while it has a connection and writes it to the phone's storage with a timestamp. When you convert without a signal it reads that table instead of the network. Nothing is estimated or predicted: the arithmetic is exact, and it is exact as of the moment of the last sync.

Tables are normally stored against one base currency, and every other pair is derived from it. To convert yen into Swiss francs from a euro-based table, the app divides the francs-per-euro rate by the yen-per-euro rate. That derived figure is a cross rate, and it is why a single downloaded table covers every combination rather than only the pairs you have already looked at.

A converter that behaves well always shows the timestamp of the rates it is using. If you cannot see when the figures were last updated, you have no way to judge them.

How stale is a day-old rate

It depends entirely on the pair. Major floating currencies move constantly but not far in a day: euro against dollar, sterling against euro, dollar against yen typically drift by a fraction of a percent from one day to the next, with the larger moves clustered around central bank decisions, inflation releases and elections.

Other currencies barely move at all against their anchor. The West African and Central African CFA francs are fixed to the euro, and several Gulf currencies are pegged to the US dollar, so against those anchors a rate cached last month is still effectively correct. At the other extreme, a currency in a devaluation cycle or under capital controls can move several percent in a week, and any cached figure ages quickly.

• Pegged pair, such as the euro against the CFA franc: a cached rate holds for as long as the peg does.
• Major floating pair: a day-old rate is normally within a fraction of a percent.
• Emerging or thinly traded currency: expect meaningful drift within days.
• Currency in crisis: treat any cached figure as an order of magnitude rather than a price.

When a stale rate is not good enough

To decide whether a taxi fare is reasonable, yesterday's rate is fine, and the error is smaller than the rounding you would do in your head anyway. The cases where it is not fine all share one feature: somebody is about to act on the exact figure.

The distinction is between a rate you use to understand a price and a rate you use to settle one. Cached rates are excellent at the first and unsuitable for the second.

• Sending or receiving a large sum: take a live quote from whoever executes the transfer.
• Invoicing in a foreign currency, or converting for accounts and tax: use the official rate for the required date, published by your tax authority or central bank.
• Agreeing contractually to a fixed converted amount.
• Comparing two providers: both quotes have to be measured against the same moment, not against a stale reference.

Keeping the offline copy useful

All Currency Converter is built around this: it stores the last synced rates for all 148 currencies on the device, converts from them with no connection, and shows when they were last updated so you always know what you are reading. When the phone has a connection, rates refresh hourly.

Whatever converter you use, the habit is the same. The value of an offline converter is decided before you lose the signal, not after.

• Open it once on wifi before a flight or a border crossing, because that sync is what you will be using.
• Check the update timestamp before trusting a figure that matters.
• Refresh at any connection, however brief. An airport portal for one minute is enough.
• Set up the currencies you will need before you leave, so nothing depends on a search that needs data.