How Exchange Rates Work
A rate is a price, and a price has a direction, a date and a spread.
Every rate has a base
A quoted exchange rate is always "units of one currency per one unit of another". The second currency is the base; the first is the price. "EUR/USD = 1.12" means one euro costs 1.12 US dollars. Reverse the pair and the number becomes its reciprocal, 0.89 euro per dollar. Both describe the same market from opposite sides.
Most published tables use one base, usually the US dollar, because storing a rate for every ordered pair would be 140 times more data than storing 140 rates against a single base. A cross rate is what you get when you divide two entries in that table.
A/B = (units of B per USD) ÷ (units of A per USD)Cross rates in practice
Suppose you want to know how many Japanese yen one euro buys, and the table gives EUR 0.89048 and JPY 157.835475 per USD. Divide the yen rate by the euro rate: 157.835475 ÷ 0.89048 = 177.25. The dollar disappears from the answer, but it was the common unit that made the calculation possible.
This is not a trick of the tool; it is how interbank desks quote most pairs. Only the pairs involving the dollar are quoted directly in size. Everything else is a cross, calculated from two dollar rates, and that is why a small error in either dollar rate moves the cross.
Bid, ask and the spread
A market maker quotes two prices: the bid, at which it buys the base currency, and the ask, at which it sells it. The difference is the spread, and it is the market maker's compensation for taking the other side of the trade. The mid-market rate is the midpoint between them, and it is the number most reference tables publish.
You do not get the mid-market rate. A bank, a card network or a transfer service buys at one price and sells at another, then may add a fee. A 0.5% spread on a 10,000 transfer is 50, which is why comparing the "fee-free" claim without comparing the rate is a mistake.
| Channel | Typical cost above mid-market | Where it hides |
|---|---|---|
| Interbank / mid-market | 0% | Not available to retail customers |
| Large bank transfer | 0.3–1.5% | Rate margin, plus a fixed wire fee |
| Card payment abroad | 0–3% | Network rate plus an issuer foreign-transaction fee |
| Airport cash desk | 5–12% | Buy and sell rates far apart |
| Peer-to-peer transfer | 0.5–1.5% | Rate margin, sometimes a small fixed fee |
The ranges are indicative. The point is the order of magnitude: the same 1,000 can cost 5 or 100 depending on where it is exchanged, and the headline rate is only part of the answer.
Why some currencies barely move
A pegged currency is fixed to another by policy. The UAE dirham and the Saudi riyal have long been pegged to the US dollar; the West and Central African CFA francs are pegged to the euro. Their quoted rate can sit at the same value for years, and a table that shows 3.6725 for the dirham is not stale, it is a peg.
A managed float is different: the central bank allows movement but intervenes to limit it. A free float is set by the market, sometimes with sharp moves around elections, sanctions or commodity prices. The table does not label which is which, because the label can change faster than the snapshot.
Time zones and settlement
The foreign-exchange market runs continuously from Sunday evening to Friday evening in New York time, with the deepest liquidity in the London and New York overlap. A rate quoted on a Saturday is usually Friday's close. A rate quoted during a holiday in one centre may be the last available print rather than a tradeable price.
Two rates with the same date can still differ if one was captured at 09:00 and the other at 16:00. When a difference matters, the timestamp matters as much as the number.
Inflation is not an exchange rate
A currency losing half its value against the dollar does not mean prices in that country doubled. Local prices and the exchange rate are different measurements. Over long periods, countries with much higher inflation tend to see their currency weaken, but the link is loose and slow. A converter answers "what is this worth in another currency now", not "what can I buy with it".
How to compare two ways of sending money
- Ask for the final amount received. Not the rate, not the fee: the number that lands in the recipient's account.
- Convert that amount back at the mid-market rate. The gap is the total cost, including the margin and the fee.
- Check the date and time. A quote from this morning is not a quote for a transfer this afternoon.
- Read the small print on the receiving side. Some banks deduct a fee from the incoming amount, which the sender never sees.
A reference table is a starting point. It tells you whether a quoted rate is plausible and how large the margin is. It cannot tell you what a specific provider will charge, because that depends on the provider, the amount, the corridor and the day.