Open any finance app or do a quick search for “USD to EUR,” and you’ll get a clean number instantly. What most people don’t realize is that number is really just a snapshot of something that never actually stops moving.

Exchange rates are set by supply and demand — constantly.

Currencies trade against each other on a global market that never fully closes, shifting based on everything from interest rate decisions to political news to how much of a currency is being bought or sold in a given hour. The number you see online is the mid-market rate — essentially the midpoint between what buyers and sellers are currently agreeing on. It’s accurate, but it’s also already slightly out of date by the time you read it.

Here’s the part that actually costs you money: no one gives you the mid-market rate.

Banks, currency exchange counters, and money transfer apps all build in their own margin on top of that real rate. This is why your bank’s number never quite matches what a quick search shows you — they’re not being dishonest, they’re just charging for the service of doing the exchange, and that charge is baked into the rate itself rather than shown as a separate fee. [Currency Converter]

Where this margin hurts the most is airport kiosks and last-minute exchanges.

Because these services know travelers are exchanging out of necessity rather than choice, the margins tend to be the widest exactly where you’re most likely to use them. Comparing rates a few days before a trip, rather than exchanging cash the moment you land, is one of the simplest ways to avoid losing money to a bad rate.

A quick example makes this concrete. Say the mid-market rate has $500 converting to €460. A bank might offer you €445 for that same $500 — the difference isn’t a mistake, it’s their margin. Multiply that gap across a bigger transaction and it becomes real money.

If you’re doing anything more official than casual travel money — invoicing an overseas client, for example — the timing of the rate you use can genuinely matter for your records, so it’s worth using whatever source your accountant specifies rather than whatever number happened to show up in a search that morning.

Currency conversion isn’t complicated once you understand this one idea: the number you see and the number you get are rarely the same, and the gap between them is where the actual cost lives.