A currency strengthens when more money wants to hold it than to leave it. Everything else is an account of why that balance shifts. Four forces do most of the work, they frequently pull against each other, and the tension between them is why a single explanation for a move is usually wrong.
Interest rates
The most direct channel. Money held in a currency earns that currency's interest rate, so higher rates attract capital and support the currency.
Two refinements turn this from a slogan into something usable.
It is the differential that matters. Not whether British rates are high, but whether they are high relative to the eurozone and the United States. A country cutting rates while everyone else cuts faster can see its currency strengthen.
It is the real rate that matters most. A 6% interest rate with 8% inflation is a guaranteed loss in purchasing power. Investors compare rates net of expected inflation, which is why a rate rise made to chase runaway inflation sometimes fails to lift a currency at all.
There is a longstanding trade built on this: borrow in a low-rate currency, hold a high-rate one, pocket the difference. The yen has been the classic funding currency for decades. It works for long stretches and then unwinds violently, which is one reason currency moves are not smooth.
Inflation
Over long horizons, a currency with persistently higher inflation loses value against one with lower inflation. It has to: if prices in one country double while prices elsewhere hold still, the same goods cost twice as much in that currency, and the exchange rate is the mechanism that reconciles them.
Over short horizons this relationship is nearly useless as a guide — it is swamped by capital flows. But it is the reason no amount of intervention holds a rate for a country whose inflation runs far above its trading partners' for years. The relationship also runs the other way, and tightly enough that it deserves its own treatment: see how rates and inflation feed each other.
Trade and the current account
A country that imports more than it exports is, in effect, selling its own currency to buy foreign goods. Sustained, that is downward pressure. A persistent surplus is the reverse.
This textbook channel has been weakened by the sheer size of financial flows. Capital moving for investment reasons dwarfs the money moving to pay for goods, so a country with a large trade deficit can have a strong currency for years if capital keeps arriving — the United States being the standing example.
Where the trade balance still bites hard is in commodity currencies. When oil, gas or metals prices move, the currencies of large exporters move with them, because the export receipts are large relative to the economy.
Risk appetite
The force that overrides all the others in a bad week.
When investors get frightened, money moves toward a small set of currencies regardless of what is happening in those countries: the dollar, the Swiss franc and the yen. This is not a judgement about Switzerland's economy. It is about deep markets, credible institutions and the ability to sell a large position without moving the price.
The consequence is genuinely counter-intuitive: bad global news can strengthen the currency of a country that is itself in trouble, if that country happens to be a haven. And a small open economy can see its currency fall on news that has nothing to do with it, purely because risk was taken off everywhere at once.
How the four combine
| Force | Timescale | Typical effect |
|---|---|---|
| Interest rate differential | Weeks to months | Strong and direct |
| Inflation gap | Years | Slow, hard to escape |
| Trade balance | Months to years | Real but often outweighed |
| Risk appetite | Hours to weeks | Overrides everything, briefly |
Read down that column and the reason forecasting fails becomes obvious. On any given day, the shortest-lived force is the loudest.
What is already in the price
The single most useful idea here: anything widely expected is already reflected in today's rate.
If markets are confident a central bank will raise rates next month, the buying has already happened. The rate moves on the announcement only to the extent that it differs from what was assumed. This is why a good number can weaken a currency — it was good, but less good than priced.
That is also why the four forces above explain moves after the fact far better than they anticipate them. They are a way of reading what happened, not a formula for what will. Where they do help concretely is in judging whether a level is unusual: comparing today against the past year's range tells you where you stand, which is a question that can actually be answered.
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Frequently asked questions
How an exchange rate is actually set, what a spread costs on a real transaction, what happens when your income and your debt are in different currencies, and what to check before paying or being paid abroad. They explain mechanisms rather than forecast rates, and each one carries the date it was last revised at the top.
No. We publish no forecasts, price targets or trading calls, and no article recommends buying or selling a currency. Exchange rates over short horizons are close to unpredictable, and a site that pretends otherwise is selling something. What the articles do instead is explain the mechanism, so that you can read the news for yourself.
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