What the index is
The US Dollar Index, usually written DXY, tracks the value of the US dollar against a basket of six foreign currencies: the euro, Japanese yen, British pound, Canadian dollar, Swedish krona and Swiss franc. It was introduced in the 1970s with a starting value of 100, and each currency has a fixed weight, with the euro by far the largest. When the index rises, the dollar has generally strengthened against that basket, and when it falls, the dollar has generally weakened.
Two points are worth noting. The basket covers only six currencies and leaves out some major trading partners, so it is a partial picture of the dollar’s global strength. And because the euro dominates, the index often mirrors the euro-dollar exchange rate.
Why bitcoin is compared with it
Bitcoin is mostly quoted in US dollars, and dollar-linked tokens act as the main trading currency on many exchanges. That means a bitcoin price is, in effect, a price in dollars. A few reasons are given for expecting a link to dollar strength.
Pricing effect. If the dollar strengthens broadly, assets priced in other currencies may look cheaper to dollar holders and more expensive to everyone else, shifting demand. For an asset traded globally, a stronger dollar can mean buyers outside the US face higher local-currency prices.
Global dollar liquidity. A strong dollar is sometimes associated with tighter global financial conditions, since a lot of borrowing worldwide is denominated in dollars. Tighter conditions have often coincided with weaker risk assets. This connects to the idea of liquidity cycles.
Alternative store of value. Some argue that when people lose confidence in currencies, they may look at scarce assets. That story would predict a weaker dollar and a stronger bitcoin, though it is a narrative rather than a tested rule.
Common drivers. Interest rate expectations can lift the dollar and weigh on risk assets at the same time, producing a negative relationship without one causing the other.
What has been observed
In some periods, especially those dominated by shifts in interest rate expectations, commentators noted that bitcoin tended to fall when the dollar index rose and the reverse, giving a negative correlation. In other periods, the two moved in the same direction or showed no consistent connection. Because the comparison depends on the dates, the data frequency and the window used, any single reported figure should be viewed carefully.
A related point is that bitcoin behaves very differently from the currencies in the basket. Currency pairs among major economies typically move by small percentages, while bitcoin often moves by several percentage points in a day. A small shift in the index may therefore sit alongside a very large move in bitcoin, and the sizes are not comparable.
Why the relationship is unstable
- Short sample. There are only a limited number of dollar cycles in bitcoin’s lifetime.
- Changing driver. The dominant market theme changes. When attention is on rates, the link may appear. When attention is on crypto-specific news, it fades.
- Basket composition. The index reflects six currencies, so a move may stem from the euro or yen for reasons unrelated to US conditions.
- Third factors. Risk sentiment, inflation data and policy expectations can influence both at once.
- Measurement. Correlations computed on short windows swing widely, a feature explained further in the article on rolling correlation with stocks.
How to read the comparison
Placing a chart of DXY beside a bitcoin chart can be a useful way to understand the backdrop on a particular day, such as noticing that a sharp dollar move coincided with a broad market shift. It is much weaker as a forecasting tool. Using the dollar index as a signal for entering or leaving positions assumes a stable relationship that the evidence does not support.
Another useful reminder is the role of quote currency. When bitcoin is shown against dollars, part of any price change can reflect the dollar rather than the coin. Viewing it against other currencies or against gold can show how much of the movement is specific to bitcoin.
Related macro threads
The dollar is one of several channels discussed in the overview of macroeconomic conditions, and it interacts with the others. For example, the rate outlook and inflation releases can drive dollar moves in the first place. See the articles on interest rates and inflation data for how those influences are described.
Cryptocurrency prices are volatile, past relationships may not continue, and people can lose all they invest. This article is educational and is not financial advice.