What Is the US Dollar Index? (DXY)

How the Index Is Constructed

The DXY is a weighted geometric mean of six currencies measured against the US dollar:

The euro dominates the basket at nearly 58% because, when the single currency was introduced in 1999, it absorbed several constituents of the original 1973 basket — the Deutsche mark, the French franc, the Italian lira, the Dutch guilder, and the Belgian franc — into one line item. The practical consequence is that DXY is, most of the time, an inverse euro chart with three smaller cross-rates layered on top. When EUR/USD strengthens, DXY falls, and the reverse holds.

The index was set at 100.00 in March 1973 as its baseline. A reading of 85 indicates the dollar has lost 15% of its value against the basket since that date; a reading of 110, that it has gained 10%. Over the past three decades the index has ranged from a low near 71 in April 2008 to a high above 164 in February 1985, the latter a product of the Reagan-Volcker policy mix that the Plaza Accord was, later that year, organised to unwind.

Notable omission: the Chinese yuan does not appear in the basket, despite China being the second-largest US trading partner. The constituents have not been revised since 1999. This is a legitimate structural criticism — DXY overweights European currencies and underweights Asian ones, and the index is therefore better read as a measure of the dollar against the developed-market West than against the world. Trade-weighted alternatives published by the Federal Reserve (the broad and the advanced-foreign-economies indices) correct for this; DXY persists because of habit, liquidity in the futures contract, and the absence of any obvious successor. (One imagines a revision is overdue; one also imagines it will not happen, the futures contract being too liquid and the constituents too entrenched to disturb without obvious cause.)

Why the Dollar Moves

Interest Rate Differentials

The most powerful driver. When the Federal Reserve raises interest rates while other central banks hold steady, global capital reallocates toward dollar-denominated assets to capture the higher yield, and the resulting demand for dollars pushes DXY higher. The reverse holds when the Fed cuts or other central banks hike. On the historical record, the rate differential between US and Eurozone short rates explains roughly 60-70% of EUR/USD variation over the past decade, which is rather a lot for a single variable.

Economic Growth

Faster US GDP growth relative to other major economies attracts foreign investment, strengthening the dollar. If the US is expanding at 3% while Europe runs at 1%, capital flows toward US assets and bid the dollar up. The reverse holds during US recessions, with one important qualification covered below: in genuinely global crises, the safe-haven flow can dominate the growth differential entirely.

Risk Appetite

The dollar is the world’s primary reserve currency, and US Treasury bonds are the asset of last resort during episodes of generalised stress. During the 2008 crisis, the dollar strengthened despite the United States being the epicentre of the problem, because global investors sold almost everything else to buy Treasuries. This is counter-intuitive only until one notices that there is no second-best choice of comparable depth. The euro was not yet ten years old; the yen carried its own deflation problem; the renminbi was not freely tradable. The dollar won by default.

Trade Deficit

The US imports more than it exports, generating a persistent current account deficit. Dollars flow overseas to pay for imports, which over long horizons puts downward pressure on the currency. The day-to-day impact on DXY is small relative to interest rates and capital flows, but the cumulative effect across decades is non-trivial, and it is the principal reason most long-horizon forecasts for the dollar carry a structurally weakening bias.

Dollar Strength and the Stock Market

The relationship between the dollar and US equities is not mechanical, but several reliable patterns emerge.

Multinational Earnings

Approximately 40% of S&P 500 revenue is generated outside the United States. When US companies earn euros, yen, or pounds and translate the receipts back to dollars for reporting purposes, a stronger dollar reduces the headline value of those foreign earnings. A 10% rise in DXY can shave 3-5% from reported earnings at the most internationally exposed firms — Coca-Cola, Procter & Gamble, Microsoft, Pfizer are the usual examples. This is the source of the "currency headwinds" disclosure that arrives, with some predictability, on earnings calls during strong-dollar quarters.

A weaker dollar produces the opposite effect: foreign earnings translate at a more favourable rate, and multinationals tend to outperform domestic-focused names through periods of dollar weakness.

Export Competitiveness

A strong dollar makes US-made goods more expensive abroad. A Boeing aircraft priced in dollars costs more in euros after the dollar has rallied, which compresses foreign demand. The same logic, run in reverse, lifts US exports during periods of dollar weakness. Manufacturers, agricultural producers, and aerospace companies are the most exposed; the effect is observable in industrial-sector earnings with a lag of one to three quarters.

Commodities

Most commodities are priced in dollars on global exchanges. A stronger dollar therefore raises the local-currency price of those commodities for non-US buyers, who reduce demand at the margin and pull the dollar price lower. The empirical result is a well-documented inverse correlation between DXY and the major commodity indices. Gold, oil, copper, and the agricultural complex all tend to weaken when the dollar strengthens, and commodity-linked equities — miners, integrated energy, agribusiness — carry that sensitivity directly.

Emerging Markets

A large share of emerging-market corporate and sovereign debt is denominated in US dollars. When the dollar appreciates, the cost of servicing that debt rises in local-currency terms, and weaker borrowers are forced into balance-sheet adjustments that can spill into the broader economy. Rising-dollar episodes have triggered or amplified several emerging-market crises, the 1997 Asian crisis being the canonical instance and the 2013-2015 taper-tantrum and dollar-rally cycle a more recent echo. For US investors holding international equities or emerging-market ETFs, a rising dollar imposes a double cost: the local assets weaken in their own currency, and the translation back into dollars compounds the loss.

How to Use DXY in a Stock-Trading Process

One does not need to trade currencies directly to draw something useful from DXY. The index can be integrated into an equity process at four points.

• Check the trend ahead of earnings. If DXY has risen 5% or more over a quarter, internationally exposed companies are likely to report softer headline revenue and cite currency headwinds. This is a reason to moderate enthusiasm on multinationals heading into earnings, irrespective of fundamental strength.
• Tilt domestic when the dollar is rising. US small-cap companies that earn the bulk of their revenue at home are largely insulated from translation effects. When DXY is in a confirmed uptrend, small and mid-cap domestic names have historically outperformed large-cap multinationals.
• Read sector rotation through a currency lens. A weakening dollar tends to favour commodity-linked sectors (energy, materials, mining) and emerging-market equities. A strengthening dollar tends to favour domestically focused sectors such as healthcare, utilities, and regional financials. Sector rotation frameworks that ignore the dollar are working with one variable missing.
• Treat sharp DXY rallies as a risk signal. Rapid dollar strength frequently coincides with global risk aversion, and the safe-haven bid into Treasuries that produces it is, more often than not, bearish for equities at the same time. A sudden DXY breakout is an occasion to review position sizing and stop levels, not to congratulate oneself on the macro call.

DXY and Technical Analysis

DXY is itself a tradable instrument, via the ICE futures contract, and it responds to standard technical analysis in much the same way as any liquid index. Support and resistance levels on the DXY chart can mark inflection points for equities and commodities; a clean breakdown below a multi-year trend line is, on the historical record, supportive of commodity-linked sectors and multinationals, and a breakout above multi-year resistance tends to be a headwind for the same groups.

Moving averages on the DXY weekly chart — the 50-week and 200-week in particular — are widely tracked. The dollar trading above both moving averages is the textbook strong-dollar regime; trading below both, the textbook weak-dollar regime. Crosses between the two (golden cross, death cross) tend to mark regime changes that can persist for many months, occasionally years. As Martin Wolf has noted in a different context, currency regimes change more rarely than market commentary suggests, but when they do, they reprice rather a lot of adjacent assets along the way.

The dollar is the tide. A trader can hold an excellent equity thesis and still find, over a quarter or two, that a rising DXY has worked against the position every day in ways that do not show up in the company-level numbers. Monitor the index, factor it into the analysis, and when the dollar moves violently, treat the move as information about the global system rather than as background noise. Base case: the dollar drifts within its regime. Tail risk: the regime changes, and the rest of the portfolio finds out second.

See also: Federal Reserve · GDP · Sector Rotation · Earnings Season · Bonds