Ask why one currency weakened over a decade and the answer usually starts with inflation. Ask why it strengthened this month and the answer usually starts with interest rates. The two forces are connected — here is how the mechanism works.
Inflation erodes a currency slowly
A currency is, at bottom, a claim on an economy's goods and services. If prices double while the amount of money stays the same, each unit of that money buys half as much — and foreign investors will pay correspondingly less for it. Over years, a country with persistently higher inflation than its trading partners tends to see its currency depreciate against theirs.
The clearest examples are the currencies that lost most of their value over a generation: the Turkish lira, the Argentine peso, the Venezuelan bolívar. In each case, years of money printing and double-digit inflation ended where purchasing-power logic said they would.
Purchasing power parity: the slow anchor
The idea behind this is called purchasing power parity (PPP): over the long run, exchange rates adjust so that the same basket of goods costs roughly the same everywhere. If a basket costs 100 units at home and the same basket costs twice as much abroad after converting, the exchange rate is out of line — and over time, either the rate or the prices move to close the gap.
PPP is useless for predicting next week. It is surprisingly useful for judging whether a currency is obviously stretched after a big move — which is why economists keep publishing Big-Mac-style comparisons.
Interest rates: the fast lever
Central banks fight inflation by raising interest rates. Higher rates make deposits and bonds in that currency more attractive, pulling foreign money in and supporting the exchange rate — at least while investors believe the central bank will stay the course.
This is why the pattern repeats worldwide: a central bank hikes hard, the currency strengthens; the bank cuts or loses credibility, the currency weakens. Turkey's lira fell through an experiment of cutting rates into rising inflation and recovered when policy turned orthodox. Brazil's real draws strength from some of the world's highest real interest rates.
Real rates are what matter
What moves money is not the nominal rate but the real rate: the policy rate minus inflation. A 40% policy rate with 50% inflation is still deeply negative — money parked there loses purchasing power every year, which is exactly why locals prefer dollars. A 5% rate with 2% inflation, by contrast, is a genuine attraction.
What this means for you
- **Travelers:** high-inflation destinations get cheaper over time in hard-currency terms — but check current rates, since the market has usually priced in part of the story
- **Savers:** keeping savings in a currency with high inflation and negative real rates is a slow loss, even if the account "pays interest"
- **Businesses:** price long contracts in hard currency, or index them to inflation, when the counterparty's currency has a credibility problem
Inflation is the slow tide that decides where currencies end up. Interest rates are the waves that decide where they go this month.