Anatomy of a Rate Differential

Anatomy of a rate differential: the interest rate gap between two economies pulls on their currency pair, but the expected gap after inflation does most of the pulling, not the rate posted today. This guide takes the gap apart: expected against current, real against nominal, and the front of the yield curve against the long end.

How the gap pulls on a pair

Money tends to flow toward the best return for a given risk. If one country pays 5% to hold its currency and another pays 1%, capital drifts toward the 5% currency and bids it up. That 4-point gap is the interest rate differential. How to work it out is in What Is an Interest Rate Differential?, and today’s gap for every major pair is on Interest rate differential by currency pair.

What counts is the gap, never one rate alone: it is the extra yield for holding one currency instead of the other. That is why a pair is always a relative price.

The gap you trade is the expected one

Today’s differential is largely in the exchange rate already. What moves a pair is a change in where the gap is heading.

A central bank can leave its rate unchanged and still move its currency, by signalling that future rates will be higher or lower than markets thought. The Fed’s dot plot is one such signal.

The two-year yield reflects what markets expect the policy rate to average over the next two years, so the gap between two-year yields is often a closer guide to a pair than the gap between policy rates. The differentials page shows both side by side, and Major forex pairs: prices and fundamentals puts the gap beside each pair’s last price.

So when an inflation or jobs report comes out, the useful question is not where rates are now. It is whether the release widens or narrows the expected gap between the two central banks.

Nominal against real: subtract the inflation

A headline rate is the nominal rate. A 6% rate with 5% inflation leaves about 1% after inflation: that is the real rate. Longer-horizon money, the kind that can move a currency for months, tends to weigh it more.

Real rate ≈ nominal rate − expected inflation. Two invented central banks, both at 4%:

• Currency A: expected inflation 2%, so a real rate of +2%.
• Currency B: expected inflation 5%, so a real rate of −1%.

On the nominal screen they look the same. In real terms A pays you to hold it and B costs you. A currency with a lower headline rate but a higher real rate can draw more capital.

Real interest rates by currency subtracts the latest headline inflation, year on year, from each currency’s policy rate and two-year yield: inflation as reported, in place of the expected figure.

Which part of the curve is moving

A differential exists at every maturity. The policy rate sets the very front; two-year and ten-year yields carry the expected path further out. Short-term money and carry positions watch the front, while reserve managers and pension funds weigh the long end more.

When the two disagree, read with care. A widening two-year gap with a flat ten-year gap says markets expect the two banks to diverge soon and converge later: more likely a repricing than a long trend.

Where the differential gives way

The differential is a pull, not a certainty. In calm markets it tends to dominate. In a risk-off shock, money can move into currencies seen as safe havens, historically the yen and the Swiss franc and often the US dollar, for the return of capital rather than the return on it.

The sharpest version is a carry unwind, when crowded leveraged positions are closed at once. August 2024’s is in Carry Trade Guide: How It Works and Why It Unwinds.

A short checklist

• Use the expected gap, not today’s: it is what new data reprices.
• For anything beyond a few days, prefer the real gap.
• Check which part of the curve is moving: the front and the long end mean different things.
• Respect the mood: the gap leads in calm markets, and safe-haven flows can override it.