Recently, when looking at investment-related discussions, I found that many people are still a bit unclear about the concept of the US dollar index. I often hear news saying "the dollar is strengthening" or "the dollar index has risen," but there aren't many who truly understand what this actually represents. In fact, this indicator has a huge impact on global markets, so today I want to talk about it.

Simply put, the US dollar index is a measure used to gauge the strength of the dollar relative to other major currencies. It tracks the exchange rate changes of the dollar against six major currencies—Euro, Japanese Yen, British Pound, Canadian Dollar, Swedish Krona, and Swiss Franc. You can think of it as a global financial market thermometer, because the dollar is the most commonly used trading currency worldwide, so its fluctuations directly influence the entire market.

The weightings of these six currencies are not evenly distributed. The Euro accounts for the largest portion, nearly 58%, because the Eurozone has a large economy and many countries. The Yen is second at about 14%, the Pound at 11.9%, the Canadian Dollar at 9.1%, and the other two combined account for less than 8%. Therefore, the rise or fall of the Euro has the greatest impact on the overall trend indicated by the dollar index.

So, what does a rise in the dollar index mean? Simply put, it means the dollar has strengthened, and other currencies have depreciated relative to it. What happens then? Commodities priced in dollars, like crude oil and gold, become more expensive for foreign buyers. At the same time, global hot money flows into the US because dollar assets become more attractive. For export-oriented economies like Taiwan, this isn't very optimistic—our goods become more expensive to US buyers, reducing competitiveness.

Conversely, a decline in the dollar index indicates the dollar has weakened in the international market. Investors then withdraw money from the dollar and shift to other opportunities, such as Asian stock markets or emerging markets. Taiwan stocks might benefit from hot money inflows, and the New Taiwan Dollar could appreciate. But if you hold US stocks or dollar deposits, you need to watch out for exchange losses—dollar depreciation means converting back to TWD will yield less.

Why should you pay attention to this indicator? Because it directly affects the performance of gold, US stocks, and even Taiwan stocks. The dollar and gold usually have an inverse relationship—when the dollar is strong, gold tends to fall; when the dollar weakens, gold tends to rise. The relationship between US stocks and the dollar is more complex—sometimes a rising dollar attracts capital into US equities, but if the dollar becomes too strong, it can hurt US exporters and drag down the stock market.

The movements of the dollar index are driven by several key factors. First, the US Federal Reserve’s interest rate policies—raising rates attracts global capital inflows, strengthening the dollar; lowering rates can lead to outflows. Second, US economic data—employment figures, inflation, GDP—also play a role. Third, geopolitical events—wars or political turmoil—often cause the dollar to appreciate due to safe-haven demand. Lastly, don’t forget, the dollar index is a relative measure; issues with other currencies also influence it—if the Euro or Yen depreciates due to domestic economic weakness, it can make the dollar index look stronger.

An interesting detail: many people only look at the dollar index, but the Federal Reserve itself more often references the “Trade-Weighted US Dollar Index.” This index includes over 20 currencies, such as the Chinese Yuan, Korean Won, and Taiwanese Dollar, providing a more accurate reflection of the actual exchange rate changes with US trading partners. Traditional dollar index calculations tend to be Euro-American focused, but the trade-weighted index is what the Fed truly considers.

In summary: a rise in the dollar index indicates dollar appreciation, capital flowing back to the US, and pressure on global risk assets. If you invest in US stocks, gold, or are concerned about the TWD trend, this indicator is definitely a must-watch. Especially in forex trading, understanding the dollar index’s movements can help you anticipate market directions in advance. Whether for long-term investing or short-term trading, grasping the logic behind the dollar index is fundamental.