Recently, while watching the markets, I heard a bunch of people discussing the rise and fall of the US dollar index. I realized that many investors actually don’t quite understand what this thing really does. Today, I want to talk about why the US dollar index is so important and how it influences your investments.

First of all, the US dollar index (USDX or DXY) is actually very simple — it’s a measure used to gauge the strength of the dollar relative to other major currencies. Think of it as a global financial market thermometer, because the dollar is the most commonly used trading currency worldwide. Commodities, energy, gold, and investment assets are mostly priced in dollars, so fluctuations in the dollar index can move the entire market.

This indicator tracks the exchange rate changes of the dollar against six major currencies. The euro accounts for over 57% of the weight, followed by the Japanese yen at 13.6%, the British pound at 11.9%, the Canadian dollar at 9.1%, the Swedish krona at 4.2%, and the Swiss franc at 3.6%. The euro’s high weight is mainly because 19 countries in the Eurozone use it, and their economies are large. So, when watching the dollar index fluctuate, the euro’s movement is often a key factor.

So, what does a rise or fall in the dollar index mean? Simply put, when the index goes up, the dollar has strengthened, and other currencies have weakened relative to it. At this time, Americans can buy imported goods more cheaply, and global hot money tends to flow into the US market to buy US bonds and US stocks, which are dollar assets. But for export-oriented economies like Taiwan, this isn’t very good — our goods become more expensive and harder to sell to the US, which can impact corporate earnings.

Conversely, when the dollar index falls, it indicates the dollar has weakened. Investors will pull money out of the dollar and shift into Asian stocks or emerging markets. This is usually good news for Taiwan stocks because more hot money flows in, capital enters the market, and stock prices may rise. However, if you hold US stocks or dollar deposits, you need to be cautious about exchange losses — a weaker dollar means converting back to TWD will get you less.

How is the dollar index calculated? It uses a “geometric weighted average method,” assigning different weights based on each country’s economic size, trading volume, and currency influence. The key point is that the dollar index is not an exchange rate or a price; it’s a relative index. A value of 100 means no change from the base period, 76 indicates a 24% decline from the base, and 176 indicates a 76% increase. So, the higher the index, the stronger the dollar; the lower, the weaker it is in the international market.

Why should you pay attention to this? Because it directly affects your various assets. Stocks, gold, bonds, and Taiwan stocks are all impacted. For example, when the dollar index rises, your US stocks and dollar-denominated bonds, when converted to TWD, become more valuable. But if the dollar weakens, the value of dollar assets shrinks.

The relationship between the dollar index and gold is especially obvious — they are usually negatively correlated. When the dollar is strong, gold prices tend to fall because gold is quoted in dollars; a rising dollar increases the cost of buying gold, reducing demand. Conversely, when the dollar weakens, gold prices tend to rise. For gold investors, keeping an eye on the dollar index is very important.

As for the relationship between the dollar index and US stocks, it’s more complex — not strictly positive or negative. Sometimes, when the dollar appreciates and capital flows into the US, stocks also rise. But if the dollar becomes too strong, it can hurt US export companies, dragging down the stock market. So, market context and economic policies need to be considered, not just a single indicator.

Several key factors influence the fluctuations of the dollar index. First is the Federal Reserve’s interest rate policy — this is almost the most direct influence. Raising interest rates makes dollar interest higher, attracting global capital into the US, strengthening the dollar. Lowering rates can lead to capital outflows, weakening the dollar. Next are US economic data, such as employment figures, CPI inflation, and GDP growth. Good data tend to strengthen the dollar; poor data can shake market confidence and weaken it.

Geopolitical and international events also play a role. Wars, political turmoil, regional conflicts can boost global risk aversion, and markets often choose the dollar as a safe-haven asset. So, “the more chaotic, the stronger the dollar” might sound counterintuitive, but it’s true. Lastly, the movements of other major currencies also impact the dollar index. Since it’s a relative measure, if other currencies depreciate, the dollar index can appear stronger.

A detail worth noting is that the Federal Reserve itself often refers to the “Trade-Weighted US Dollar Index,” rather than the general dollar index that investors usually watch. The trade-weighted index includes over 20 currencies, covering more Asian emerging markets like the Chinese yuan, Korean won, and Taiwan dollar, providing a closer reflection of US trade partners’ actual situation. The dollar index is a traditional indicator suitable for quick mood checks, but the trade-weighted index more accurately reflects the dollar’s real strength and aligns better with current global market conditions.

In summary, the dollar index is like a weather vane for global capital flows. Keeping track of its changes can help you judge asset values, risks, and even spot investment opportunities in advance. Whether you’re investing in US stocks, gold, or just want to know if the TWD will appreciate or depreciate, understanding the dollar index is a fundamental skill. Especially in forex trading, it’s a very practical indicator.