Hidden Conversion Spreads
Short answer: A "0% FX fee" card can still lose you 0.5-2% per foreign transaction inside the exchange rate itself; compare the card's effective rate against a reference rate, not against zero.
The most expensive number on a card is often the one that isn't printed. "0% FX fee" and "no conversion fee" are both true and both misleading when the rate itself is marked up.
What a spread is
When your USD card pays a EUR merchant, a conversion happens. Two things can cost you money:
- The explicit FX fee - "0.5% foreign transaction fee". Printed, known.
- The spread inside the rate - the provider (or its bank) applies a rate slightly worse than the mid-market rate. Not printed. Usually 0.5-1.5%.
A card can honestly advertise 0% FX fee and still cost you 1% per transaction. The fee table shows the fee, not the rate.
Where spreads hide
- Inside the "network rate". The card network publishes reference rates. Providers using their own rate instead of the network's can capture 0.5-1.5% invisibly.
- Weekend markups. Crypto markets trade 24/7, but fiat FX markets don't. Some providers add a markup on Saturday-Sunday conversions.
- Dynamic Currency Conversion (DCC). At the terminal, the merchant offers to charge you in your home currency "for convenience". Always decline - DCC rates are typically 3-8% worse. Choose the local currency every time.
- Stablecoin conversion. Cards that spend from stablecoin balances convert stablecoin-to-fiat at their own rate. COCA documents theirs; several providers do not.
How to measure your card's real rate
- Spend a known EUR amount on your USD card.
- Check your statement for the USD amount.
- Divide statement amount by the mid-market rate at the transaction time (any FX site gives you the historical mid).
- The difference from 1.000 is your effective spread - explicit fee plus hidden markup.
Example: you paid a €100 restaurant bill. The mid rate is 1.0800, so a fair charge is $108.00. Your statement shows $109.50. Effective cost: 1.4% - even though the card advertises "no FX fees".
Which cards actually publish the spread
- Bitpanda documents 0% own FX markup - their EUR card converts at the network rate.
- Xapo documents no FX fees on worldwide spending.
- Most exchange cards (Coinbase, Bybit, OKX) document an explicit FX percentage but not the internal spread; assume 0.5-1% there.
Where we don't know, our comparison says "not documented" - because it isn't.
The defense
- Get a card in your home currency when possible - zero conversions beat low spreads.
- For travel, compare your card's measured effective rate against a multi-currency card or a no-foreign-fee bank card.
- Always decline DCC at the terminal.
- If a provider won't document its conversion basis, assume a markup - or test it with a small amount and measure.