Capital at risk. Leveraged forex is a high-risk product and most retail accounts lose money on it. This article explains a mechanism; it is not a recommendation to use it, and nothing here is advice.
Some positions pay you to keep them open. Leave the right pair on overnight and a small credit appears in the account, every night, for doing nothing at all. Leave the wrong one on and the reverse happens.
That credit or debit is the swap, and the strategy built on it is the carry trade — one of the oldest ideas in currency markets, and one of the most reliably misunderstood.
Where the money comes from
Every currency has an interest rate attached to it, set by its central bank and reflected in the money market. A currency position is really two positions: you are long one currency and short another, simultaneously.
Hold the higher-yielding currency long and the lower-yielding one short, and you receive roughly the difference. Hold the opposite, and you pay it. Each night, at rollover, your broker settles that difference on your position size, not on your deposit — which is why leverage multiplies it.
The mechanics of how brokers actually apply this, including the markup and the triple charge on one day of the week, are covered in forex swap and rollover fees explained. Two points matter here.
The credit is on the notional, not the margin. A position controlled with a small deposit still earns or pays on its full size. This is the whole reason the trade is attractive, and the whole reason it is dangerous.
You do not receive the market differential. The broker applies a markup, which typically makes what you pay larger and what you receive smaller than the underlying difference. A pair whose rate differential looks positive can still show a negative swap in both directions at some brokers.
The trade, and its honest arithmetic
The carry trade is the bet that the exchange rate will not move enough to wipe out the accumulated interest.
That is a real bet with a real edge in quiet conditions, and it is also a bet with a specific and unpleasant shape. The income is small, steady and predictable. The risk is large, rare and sudden.
| Carry income | Exchange-rate risk | |
|---|---|---|
| Size | Small per night | Can be large in one session |
| Timing | Predictable, daily | Unpredictable, clustered |
| Direction | Always the same | Either, but historically against the carry |
Put another way: the trade collects a modest amount very often and pays out a large amount very rarely. That is not an argument against it — plenty of legitimate strategies have that shape — but it does mean the equity curve looks far better than the risk actually is, right up until it does not.
Why it is not free money
The credit exists because you are taking a risk somebody else does not want. Interest rate differences are not an oversight in the market's pricing; they are compensation.
Economic theory has a name for the idea that the higher-yielding currency should weaken by roughly the interest difference, cancelling the gain. In practice that has not always held over shorter periods, which is exactly why the trade persists. But the fact that it does not hold on average is not a promise that it will not hold violently on a given Tuesday.
Three specific hazards deserve naming.
Leverage turns a small move into a large one. The carry is earned on the notional and so is the loss. A position sized to make the nightly credit feel meaningful is a position sized to make a normal daily range feel catastrophic.
Everyone holds the same trade. Carry positions concentrate in the same handful of pairs, because there are only so many rate differentials worth having. Crowded positioning is fine while it is quiet and is the mechanism of the unwind when it is not.
The unwind is self-reinforcing. When volatility rises, some participants close. Closing pushes the price against everyone still holding. That triggers margin calls, which force more closing. The move is not caused by news arriving so much as by positions leaving.
What the 2015 franc shock taught about the same class of risk
The clearest lesson available on this comes from a related event rather than a classic carry unwind. When the Swiss National Bank abandoned its floor under EUR/CHF in January 2015, the price gapped — it did not trade through the intervening levels, it jumped.
Stops did not fill where they were placed, because there was no market at those prices. Accounts went past zero. Brokers took losses. Our account of that day, the Swiss franc shock of 2015, covers what happened and why the regulation that followed exists.
The relevance to carry is direct. A carry position is a bet on nothing happening, held with leverage, in a crowded trade. Gap risk is precisely the risk that being right for months does not protect you on the one morning you are wrong.
The protections that exist, and what they do not do
If you trade as a retail client with a UK or EU-regulated firm, some of the tail risk is capped by rule rather than by your own arrangements.
The margin close-out rule requires the firm to close positions when account equity falls below 50% of the required margin — though the FCA's wording is that this happens "as soon as market conditions allow" (COBS 22.5.13R), which in a gap means after the gap. And negative balance protection limits a retail client's liability to the funds in that account (COBS 22.5.17R), so you cannot end up owing the broker.
Both are worth having and neither prevents the loss. They cap it at your account balance and stop it becoming a debt. The margin close-out rule explained covers the mechanics.
The short version
The carry trade pays you a small amount to hold a currency risk that occasionally arrives all at once. It is a legitimate mechanism, it is not free money, and the interest credit is compensation for exactly the thing that eventually happens.
If the nightly credit is the reason a position is open, the position is probably too large. If the position would be worth holding without the credit, the credit is a bonus rather than a strategy — and that is a much better place to be trading from. Position sizing and risk management is the discipline that decides which of those you are doing.
Regulatory detail in this article is from the FCA Handbook, COBS 22.5, read on 19 August 2026. Swap rates, markups and rollover times are set by each broker — check yours.
Frequently Asked Questions
What is a carry trade in forex?
Holding a position that is long a currency with a higher interest rate and short one with a lower rate, so the interest rate differential is credited to the account each night as a swap or rollover. The trade earns while the exchange rate does nothing, and loses if the exchange rate moves against the position by more than the accumulated carry.
Is a positive swap free money?
No. The credit is compensation for holding currency risk, and it is small relative to how far an exchange rate can move. A position earning carry can lose several months of it in a single session, which is precisely the risk you are being paid to take.
Why do carry trades unwind so violently?
Because the same trade is held by many participants at once, it is usually leveraged, and it is profitable only while it is quiet. When volatility rises, positions are closed and margin is called across the market simultaneously, and everyone closing the same position at the same time moves the price against all of them.
Do brokers pay the full interest rate differential?
Generally not. The swap a retail client receives or pays is derived from the market differential with the broker's own markup applied, and the markup usually makes the debit side larger and the credit side smaller. Check your broker's published swap rates for the specific pair rather than assuming the differential.
