What is the Nikkei 225?

The Nikkei 225 is Japan's benchmark stock index — the Japanese equivalent of the Dow Jones or the S&P 500. It tracks the share prices of 225 of the largest, most actively traded companies on the Tokyo Stock Exchange, from Toyota and Sony to SoftBank and Uniqlo-owner Fast Retailing. When traders say "the Nikkei," this is the number they mean, and it is the single most-watched gauge of how Japanese equities — and Asian risk sentiment — are doing.

Why should a forex trader care about a stock index?

Because the Nikkei and the Japanese yen move together in a way few other index/currency pairs do. Most of Japan's blue chips are exporters that earn a large share of their revenue abroad. When the yen weakens, those foreign earnings translate into more yen, corporate profits rise, and the Nikkei tends to climb. When the yen strengthens, the opposite happens and the index sags. For years this has produced a striking positive correlation between USD/JPY and the Nikkei: dollar up, yen down, often means Nikkei up.

That link makes the index a live read on the same forces you already trade on the yen — the rate gap that drives yen weakness and the carry trade that funds itself in cheap yen. A sharp move in the Nikkei during the Tokyo session is often the first sign that something is shifting in yen positioning.

What is actually inside it?

The index holds 225 companies drawn from across the Tokyo Stock Exchange's Prime Market, spanning technology, autos, banking, retail and pharmaceuticals. The list is reviewed once a year, with names swapped in and out to keep it representative of the Japanese economy.

How is it calculated?

Here is the quirk that surprises most people: the Nikkei is price-weighted, not market-cap weighted. Like the Dow Jones, it simply adds up the share prices of its members and divides by a set divisor. That means a company with a high share price moves the index far more than one with a low share price — regardless of which is actually the bigger business. A handful of high-priced stocks can therefore swing the entire index.

It is calculated and published by Nikkei Inc., the company behind Japan's largest business newspaper — which is where the index gets its name.

When does it trade?

The Nikkei trades on the Tokyo Stock Exchange during Japanese hours, which overlap the forex Tokyo session — the first major session of the trading day. Nikkei futures trade nearly around the clock, so the index keeps ticking even when the cash market is closed, making it a handy risk-sentiment barometer between sessions.

The Barings connection

The Nikkei has its own place in trading folklore. In 1995 a single trader, Nick Leeson, bet enormous, unauthorised sums on the Nikkei staying calm. When the Kobe earthquake sent the index tumbling, the losses destroyed Barings — a 233-year-old British bank — in a matter of weeks. It remains the classic cautionary tale about unhedged index exposure and the absence of risk controls.

The takeaway

You do not need to trade the Nikkei to use it. Treat it as a free, real-time gauge of Japanese risk appetite and a companion to the yen: when the index and USD/JPY start pulling in the same direction with conviction, something is moving in Japan — and the yen crosses usually follow.