Which Exchange Rate Should You Use When Splitting a Trip?

By MarkAugust 5, 20265 min read

Nobody argues about this until the trip is over. Then one person points out that the yen moved 4% while you were away, and suddenly a settled bill is not settled.

The problem, stated plainly

On a week in Japan, three friends from Taiwan spend across two currencies. One paid for the hotel in yen on a card. One withdrew yen in cash at an ATM and paid for meals. One booked the flights months earlier in Taiwan dollars.

Every one of those transactions has a different exchange rate attached to it, and none of them is “the” rate. To settle up you have to choose one convention and apply it to everyone. The choice matters less than you think — but only if you make it deliberately and tell the group.

Four rates that all have a claim to being correct

1. The mid-market rate on the day of the expense

The midpoint between what banks buy and sell a currency at. It is the number you get from a search engine or a currency site, and it is the rate BillSmart uses, sourced from European Central Bank reference data.

It is neutral — nobody in the group can accuse it of favouring the person who happened to use a particular card. It is also the only one of the four that is easy for everyone to verify independently.

2. The rate your card actually charged

The one on your statement. It is the true cost to the person who paid, which is exactly why it feels fairest to them and unfairest to everyone else. Two people buying identical dinners on the same night with different cards will produce different “true” numbers, because one card carries a foreign transaction fee of a few percent and the other does not.

Using statement rates means the group is quietly subsidising whoever brought the worst card.

3. The rate on the day you settle up

Simple, and defensible if the group settles quickly. Its flaw shows up on long trips: if the currency moves significantly between the expense and the settlement, people who spent early and people who spent late are treated differently for no reason connected to what they bought.

4. One fixed rate for the whole trip, agreed in advance

Underrated. Before you leave, look up the rate and round it to something memorable — 1 TWD to 4.7 JPY, say. Everyone uses that number for everything.

It is not the most accurate method, and it does not need to be. Currency moves of one or two percent are noise next to the size of the errors people actually make, like forgetting a taxi. The advantage is that anyone at the table can do the maths on a napkin.

What to actually do

For most trips: use the mid-market rate, applied uniformly to everyone. Whoever pays foreign transaction fees absorbs their own fees, the same way they would if they had travelled alone. It is neutral, verifiable, and it takes no negotiation.

Two exceptions worth making consciously:

One person fronted a very large booking. If someone put NT$120,000 of flights on a card and ate a 1.5% fee doing it, that fee is NT$1,800 — no longer noise. Add it as its own line item shared by the group. Enter it as a separate expense rather than fudging the rate.
The currency moved sharply mid-trip. A move of five percent or more over a week is unusual, and it is worth agreeing on a single date for everything rather than letting the timing of each purchase decide who pays more.

Traps that cost more than the rate you picked

Dynamic currency conversion

The card terminal asks whether you would like to be charged in your home currency instead of the local one. Say no. Every time. That option lets the merchant’s payment processor set the exchange rate, and it is reliably worse than the one your card network would have used — often by several percent, which dwarfs any of the choices above.

The same prompt appears at ATMs abroad: “with conversion” or “without conversion”. Choose without.

Assuming the rate is fixed the moment you tap

It usually is not. Card networks convert when the transaction settles, which can be a day or several later. That is why the amount on your statement rarely matches the rate you looked up at dinner, even before fees.

Cash withdrawal fees hiding inside a good rate

An ATM can give a perfectly reasonable exchange rate and still cost you, because the withdrawal fee is charged separately — sometimes by both your bank and the machine’s operator. If one person is the group’s designated cash machine, those fees should be a shared expense, not a private tax on being helpful.

Agree on it in the group chat, before you fly

Two sentences is enough: “Everything settles in TWD at the mid-market rate on the day of each expense. Card fees are your own.” Nobody has to think about it again, and the person who eventually does the maths is not also negotiating the rules.

The BillSmart calculator applies this convention automatically — enter each payment in the currency it was charged in, and it converts to your chosen base currency and shows the rate it used for every entry, so the group can check the working rather than trust it.