What is the dollar index (DXY) and what does it really measure?

The DXY tracks the US dollar against a basket of six currencies — but the weights were set in 1973 and never updated. Knowing what is actually inside it explains why the index sometimes disagrees with what the dollar is doing in front of you.

What is in the basket?

The US Dollar Index measures the dollar against six currencies, with fixed weights:

• Euro — 57.6%. More than half the index is one exchange rate: EUR/USD.
• Japanese yen — 13.6%
• British pound — 11.9%
• Canadian dollar — 9.1%
• Swedish krona — 4.2%
• Swiss franc — 3.6%

That is the whole thing. The index launched in 1973 after the collapse of Bretton Woods, and the only structural change since was in 1999, when the euro replaced the German mark, French franc, Italian lira, Dutch guilder and Belgian franc in one substitution.

Why does the composition matter so much?

Because with the euro at 57.6%, the DXY is close to an inverted EUR/USD chart. If the euro is strong for reasons that have nothing to do with the United States — a hawkish ECB, say — the DXY falls, and a headline will report "dollar weakness" that is really euro strength.

The gaps are just as telling. The index contains no Chinese yuan, despite China being one of the largest US trading partners; no Mexican peso; no Korean won; no emerging-market currency at all. Sweden has a bigger weight than every emerging economy combined, because in 1973 that reflected trade patterns that no longer exist.

So the DXY is not "the dollar against the world". It is the dollar against a snapshot of the developed world as it looked half a century ago.

How should a trader read it, then?

As a directional read on the dollar, not a precise one, and always with the euro question in mind: is this the dollar moving, or the euro moving?

Three practical uses survive that caveat:

• Confirmation. If you are long USD/JPY on a dollar thesis and the DXY is falling, one of the two is telling you something. Worth checking why before you size up.
• Regime. A DXY grinding to new highs over weeks is the risk-off tide that lifts or sinks everything else, and it usually matters more than any single pair's setup.
• Sanity check on headlines. "Dollar hits three-month high" means the DXY did. It may not be true against the currency you actually trade.

How is this different from a currency strength meter?

This is the useful comparison, and it is why we built a different tool.

The DXY answers "how is the dollar doing against six fixed currencies, with the euro dominating?" A currency strength reading like our FOTSI oscillator answers a different question: "of the eight majors, which is strongest and which is weakest right now, each measured against all the others?"

Two consequences. First, the FOTSI has no basket bias — no single pair can dominate the reading the way EUR/USD dominates the DXY. Second, it reads every currency, so it can tell you the dollar is weak and the yen is weaker, which is exactly the situation where a DXY-only view puts you on the wrong side of USD/JPY.

Neither replaces the other. The DXY is the market's shared reference point, quoted everywhere and traded as a product. A strength meter is the better instrument for choosing which pair to trade.

The takeaway

The DXY is a useful, deeply imperfect number: over half of it is EUR/USD, and its weights describe the trade patterns of 1973. Read it as the market's headline for "the dollar", check whether a move is really the euro in disguise, and use a strength read when you need to know which pair actually deserves the trade.