EUR or USD: why your return changes with the currency you measure it in
Switching the display currency doesn't just relabel the figures — it changes them, because every holding priced in another currency carries the exchange rate's move on top of its own.
One holding, two returns
You buy 10 shares of a US company at $100 each, $1,000 in all, on a day when one euro buys $1.10. In euros the purchase costs 1,000 ÷ 1.10 = €909.09. A year later the shares are at $110 — up 10% — but the euro has strengthened and now buys $1.20. Your $1,100 is worth 1,100 ÷ 1.20 = €916.67.
In dollars the holding returned 10%. In euros it returned 916.67 ÷ 909.09 − 1 = 0.8%. Both are correct; they are answers for two different people. An investor whose bills are in dollars made 10%. You, with bills in euros, made less than 1%, because the dollars you own lost about 8% of their value in euros over the same year.
The two effects multiply rather than add: (1 + the return in the share's own currency) × (1 + the change in that currency measured in yours) − 1. Here that is 1.10 × (1.10 ÷ 1.20) − 1 = 0.8%. When both move the same way the effects compound; when they move in opposite directions, as here, one eats the other.
Reading an exchange rate the right way round
Most of the confusion about currency comes from reading the rate upside down. EUR/USD at 1.10 means one euro buys 1.10 dollars. When that number rises, the euro is getting stronger — and everything you own in dollars is worth fewer euros. A rising EUR/USD is bad news for a euro investor's US holdings, even though “the number went up”.
The value of one dollar in euros is the reciprocal: 1 ÷ 1.10 = €0.909 at the start of the example, 1 ÷ 1.20 = €0.833 at the end, a fall of 8.3%. That 8.3% fall is the currency term in the formula above.
Why the display currency changes every figure
To show a portfolio in euros, every amount has to be in euros on the date it happened: a deposit at that day's rate, a purchase in dollars at the rate on the day of the purchase, the value of a US holding at each day's rate. That is how this tracker converts — each row at its own date, never everything at today's rate — and it is why switching between EUR and USD changes the returns, not just the currency symbol.
Converting everything at today's rate would be simpler and wrong. It would apply this year's exchange rate to a deposit made five years ago, and the currency effect would vanish from the history — every past value would move together with today's rate, and a holding bought when the dollar was strong would look exactly as profitable as one bought when it was weak.
The same logic applies to cost and profit per position. The euro cost of a US share is what you actually paid in euros on the day you bought it; its euro value is today's price at today's rate. The profit between the two includes the currency move, because you were exposed to it from the day you bought.
Where currency shows up in a DeGiro account
DeGiro keeps a cash balance per currency, which is why the account statement's Balance column carries a currency of its own. Buying a dollar-priced share from euros normally converts the money for you at the time of the trade, and a dividend paid in dollars lands in your dollar balance — or is converted back, depending on your settings. The account statement guide describes how those conversions appear, and the fees guide covers what they cost.
Every one of those balances is converted into your display currency at each day's rate when the account is valued, so cash left in dollars carries a currency effect too, even though it isn't invested.
Which currency to judge your portfolio in
The one you will eventually spend. If your life is in euros, the euro figures are your return, currency effect included — that effect is real money, gained or lost. The dollar view is useful for a different question: how did the investments themselves do, before the exchange rate had its say?
Whatever you pick, compare like with like. A euro return set against the S&P 500's dollar headline mixes two currencies and shows the exchange rate as if it were your skill or your mistake. The tracker's benchmark is converted into the same display currency, day by day, so both lines carry the same currency effect. And if you would rather not carry the effect at all, currency-hedged ETF share classes remove most of it, at a cost that shows up in the fund's own return.