Recently, when reading various investment news, I often hear phrases like "the US dollar index has risen" or "the dollar has strengthened," but honestly, many people aren't quite clear on what the US dollar index actually measures. I myself only understood it later, so today I’ll break down this important financial indicator for everyone.

Simply put, the US dollar index (USDX or DXY) is like a thermometer for the global financial markets. It tracks not stocks, but the exchange rate changes of the US dollar against six major currencies—Euro, Japanese Yen, British Pound, Canadian Dollar, Swedish Krona, and Swiss Franc. You can think of it as a measure of "how strong or weak the dollar is compared to other international currencies right now."

Why is this index so important? Because the US dollar is the most commonly used trading currency in the world. Commodity trade, energy, gold, global investments—most are priced in dollars. So when the dollar index changes, it triggers a series of market impacts.

When the dollar index rises or falls, the underlying meaning can be quite different. When the dollar index goes up, it means the dollar has strengthened, and other major currencies have depreciated relative to it. At this time, international commodities priced in dollars (like crude oil, gold) seem cheaper. This is good news for the US because imports become cheaper, and capital flows into the US market to buy US bonds, stocks, and other dollar-denominated assets. But for export-oriented economies like Taiwan, it becomes harder to sell goods to the US, and corporate revenues may be affected.

Conversely, when the dollar index falls, it’s a different story. A weaker dollar means the dollar has depreciated, and market confidence is declining. Investors will withdraw money from dollars and shift to other opportunities, like Asian stock markets or emerging markets. For Taiwanese investors, this could mean more hot money flowing in to buy Taiwanese stocks, pushing prices up. But if you hold US stocks or dollar deposits, a weaker dollar means converting back to TWD will get less, so you need to watch out for exchange losses.

The dollar index is composed of six currencies, but the weightings are not evenly distributed. The Euro accounts for the largest share, over 57%, because the Eurozone countries are numerous and have large economies. The Yen is second, about 13.6%, since Japan is the third-largest economy globally. The British Pound, Canadian Dollar, Swedish Krona, and Swiss Franc together account for less than 30%. So if you see the dollar index fluctuate sharply, the first thing to check is whether there’s news affecting the Euro or Yen.

The dollar index is calculated using a "geometric weighted average," 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 represents the baseline level, 176 indicates a 76% increase from the baseline, meaning the dollar has strengthened; 76 indicates a 24% decrease, meaning the dollar has weakened.

So, how does the change in the dollar index relate to our investments? It’s actually quite impactful. When the dollar index rises, your dollar assets converted into TWD become more valuable. The value of dollar assets like US stocks and bonds increases. But if the dollar weakens, these assets’ values shrink.

The relationship between the dollar and gold is usually inverse. When the dollar is strong, gold prices tend to fall because gold is priced in dollars, and higher dollar prices increase the cost of buying gold, reducing demand. When the dollar is weak, gold prices often rise. However, gold is also influenced by inflation, wars, oil prices, and other factors, so it’s not solely determined by the dollar index.

The relationship between the dollar index and Taiwan stocks or the New Taiwan Dollar roughly is: when the dollar appreciates, capital flows back to the US, and the TWD may depreciate, putting pressure on Taiwan stocks; when the dollar depreciates, capital may flow into Asia, benefiting the TWD and Taiwan stocks. But this isn’t an absolute rule—sometimes, global economic confidence causes US stocks, Taiwan stocks, and the dollar to rise together. During black swan events, everyone panics, and stocks, forex, and bonds may all decline simultaneously.

What factors influence the movement of the dollar index? First and foremost is the US Federal Reserve’s interest rate policy, which is almost the most direct factor. Rate hikes attract capital into the US, strengthening the dollar; rate cuts may lead to outflows, weakening the dollar. Second are US economic data, like employment figures, CPI inflation, GDP growth—good data tend to strengthen the dollar. Third are geopolitical and international events; wars and political turmoil cause markets to seek safe-haven assets, with the dollar often being the first choice. Lastly, the movements of other major currencies also matter because the dollar index is a relative measure; if other currencies depreciate, the dollar index appears stronger.

A detail worth noting is that the US Federal Reserve itself often refers to the "Trade-Weighted US Dollar Index" rather than the commonly seen dollar index. The dollar index compiled by ICE uses six major currencies, leaning towards a Euro-American perspective. The trade-weighted index includes over 20 currencies, including more Asian emerging market currencies like the Chinese Yuan, Korean Won, and TWD, which better reflect the actual trade partners of the US. For most investors, watching the dollar index is sufficient. But if you’re involved in forex trading or macroeconomic research, the trade-weighted index offers more in-depth insights.

Ultimately, the dollar index is like a weather vane for global capital flows. Watching its movements can help us judge asset values, assess risks, and even spot investment opportunities early. Whether you’re investing in US stocks, gold, or simply curious about whether the TWD will appreciate or depreciate, understanding the dollar index’s changes is fundamental. Especially in forex trading, grasping what dollar depreciation or appreciation means and their impacts can help you make smarter investment decisions.