A week in Japan with friends. The flights were paid in euros three months ago, the hotel in yen on arrival, dinners in cash from three different wallets, and half the card payments went through at a conversion rate nobody can name. On the last evening, someone opens the notes app and asks the question that ruins the trip’s final dinner: so, who owes what?

Splitting expenses in one currency is arithmetic. Splitting them across two or three currencies is arithmetic plus exchange rates, and that second part is where group accounts quietly go wrong. Here is the method that keeps them right.

Why does converting by hand never add up?

Because there is no such thing as “the” exchange rate: every expense has its own. The European Central Bank publishes euro reference rates once every working day, and they move: between the day you booked the hotel and the day you settle up, a currency can shift by several percent. Add your bank’s own card rate, the cash you exchanged at the airport at a worse one, and a friend rounding 1,000 yen to “about six euros”, and you end up with balances that three people computed three different ways.

The result is predictable: small errors pile up in someone’s favour, nobody can rebuild the total, and the group settles on a number that feels fair rather than one that is right.

Pick the group’s home currency before you leave

The fix starts with one decision: choose a single reference currency for the group, almost always the one you all live in. Every balance will be expressed in it, whatever currency each expense was paid in.

This is the same principle as choosing one recording system for travel expenses: what breaks group accounts is rarely the arithmetic, it is three people using three systems. One group currency means one answer to “how much do I owe?”, not one answer per wallet.

Log every expense in the currency you actually paid

At the till, never convert in your head. A 15,000 yen dinner is logged as 15,000 yen, a £40 taxi as £40. The conversion is the app’s job, not yours: Banana Split lets you mix currencies within a single group, and each expense is converted into the group currency at the daily rate. Balances stay live for everyone, in one currency, while every expense keeps its original amount, so a receipt can always be checked against what was logged.

This is also where card and cash stop being different problems. Log the amount and currency that actually left the wallet, and the tally takes care of the rest.

The group currency is euros. In Tokyo, Léa pays a 15,000 yen dinner for herself and two friends, at 150 yen to the euro. How much do the other two owe her in total?

How do you settle up across currencies?

Back home, the app holds one balance per person, in the group currency, and computes the minimum number of transfers to bring everyone back to zero.

One case remains: the friend who lives in London and would rather pay you back in pounds than in euros. Done by hand, that means one more conversion and one more debate. Banana Split handles repayments made in a different currency: the debt is in euros, the repayment arrives in pounds, and the balance closes correctly. It is one of the features neither Splitwise nor Tricount offers, as our honest comparison details.

One group, every currency

The method fits in three lines: pick the group’s home currency before leaving, log every expense in the currency it was actually paid, and let the app convert at the daily rate and work out the transfers.

Banana Split is free, works offline in places where roaming does not, and handles all major currencies. Download it and create your trip group before the first airport coffee: it will be the last currency conversion anyone on the trip does by hand.