Nikkei 225 vs TOPIX: Why Chip Stocks Move Them So Differently

Japan has two headline stock indexes, and they weigh companies in different ways. The Nikkei 225, calculated by Nikkei Inc., is a price-weighted average of 225 stocks: a high share price means a big weight. TOPIX, calculated by JPX Market Innovation & Research, weights about 1,600 stocks by the market value of their freely traded shares. Advantest and Tokyo Electron carry large weights in the first and modest ones in the second. On September 30, 2026 the two made up about 21.29% of the Nikkei 225 but 3.97% of TOPIX. When they move, the Nikkei moves with them.

Share prices below are Tokyo Stock Exchange closing prices. Index levels are the Nikkei 225 (Nikkei Inc.).

How is the Nikkei 225 calculated?

Nikkei Inc.’s index announcements show how the pieces fit:

• Each constituent counts at its share price multiplied by a price adjustment factor (PAF), which Nikkei announces for every addition and revises when a constituent splits its shares.
• The index is the sum of those adjusted prices divided by a divisor. Nikkei changes the divisor when constituents change or a factor is revised, and announces each new value.

Membership changes at periodic reviews in spring and autumn. Nikkei’s announcements for 2026 took effect on April 1 and October 1, and explain each addition by trading liquidity or by the balance of constituents across seven broad sectors, not by market value.

Why do Advantest and Tokyo Electron weigh so much in it?

A stock’s weight is its adjusted price divided by the sum of all adjusted prices, and that sum equals the index level times the divisor. With the factors and the divisor Nikkei announced, the weights can be worked out. On September 30, 2026:

• Advantest closed at 34,260 yen, with a capped factor of 7.2.
• The Nikkei 225 closed at 66,753.72, and the divisor was 29.83073565.
• 34,260 × 7.2 ÷ (66,753.72 × 29.83073565) = 12.39%.

The same arithmetic gives Tokyo Electron (11,815 yen × 15) 8.90% and Kioxia (18,095 yen × 2.1) 1.91%.

Tokyo Electron’s factor is large because of splits. For its 1:5 stock split, with an ex-rights date of September 29, 2026, Nikkei raised its PAF from 3 to 15. Each share became five, so the factor rose fivefold to keep the adjusted price level the same.

Set side by side with TOPIX, the gap is wide. The Nikkei figures below are calculated as above; the TOPIX figures are JPX’s published weights, which come out on their own schedule:

Company (code)	Nikkei 225, Sep 30 (calculated)	TOPIX, Aug 31 (JPX)
Advantest (6857)	12.39%	1.7507%
Tokyo Electron (8035)	8.90%	2.0458%
Kioxia Holdings (285A)	1.91%	0.5308%

Kokusai Electric and JX Advanced Metals, also on the Japan map, joined the Nikkei 225 on October 1.

How does TOPIX weigh the same companies?

JPX describes TOPIX as a free-float adjusted market capitalization-weighted index, set at 100 points on January 4, 1968. A stock’s weight follows the market value of its freely traded shares, not its share price.

On September 30, 2026 TOPIX had 1,636 constituents, according to JPX’s factsheet. The largest, Mitsubishi UFJ Financial Group, was 4.02%. Tokyo Electron (2.17%) and Advantest (1.80%) ranked sixth and tenth. TOPIX also has a 10% weight cap, set once a year on the last business day of October.

TOPIX is being reshaped. Under JPX’s guidebook dated October 1, 2026, constituents come from all three Tokyo Stock Exchange markets (Prime, Standard and Growth) and are reviewed once a year. The first periodic review takes effect on October 30, 2026: 35 issues are added, and constituents that did not meet the continuation criteria are phased out in up to eight quarterly steps ending in July 2028.

How much did chip stocks move each index in October?

In the Nikkei 225, a 1-yen move in a stock shifts the index by its factor divided by the divisor. From October 1 the divisor is 29.66435840 and Advantest’s capped factor is 6.4. So a 1-yen move is worth 6.4 ÷ 29.66435840 = about 0.2157 index points for Advantest, 0.5057 for Tokyo Electron (factor 15) and 0.0708 for Kioxia (factor 2.1).

October 15 worked through:

• Advantest fell from 40,380 to 34,340 yen (−14.96%): −6,040 × 6.4 ÷ 29.66435840 = −1,303.1 points.
• Tokyo Electron fell from 11,240 to 10,365 yen (−7.78%): −875 × 15 ÷ 29.66435840 = −442.5 points.
• The Nikkei 225 fell from 67,744.29 to 64,425.95: −3,318.34 points (−4.90%).

The two stocks account for 1,745.6 points, or about 53% of the day’s fall. Kioxia (14,690 to 12,880 yen, −12.32%) added another −128.1.

TOPIX levels are not shown here. As a reference, the table uses the NEXT FUNDS TOPIX Exchange Traded Fund (1306), an ETF on the Tokyo Stock Exchange whose benchmark is the TOPIX Total Return Index. It is a fund’s price, not the index itself, so its daily move only approximates TOPIX.

Day	Nikkei 225	Advantest + Tokyo Electron, in Nikkei points	TOPIX ETF (1306)
Oct 5	+1,637.40 (+2.40%)	+696.6	+1.42%
Oct 15	−3,318.34 (−4.90%)	−1,745.6	−2.39%
Oct 16	+898.34 (+1.39%)	+175.0	+1.76%
Oct 20	−841.28 (−1.30%)	−259.1	−1.24%

On October 15 the Nikkei fell about twice as far as the ETF. On October 16, when Advantest rose only 0.55%, the ETF rose more than the Nikkei. As a rough guide on the TOPIX side, multiply each weight by the move: at the September 30 factsheet weights, Advantest’s −14.96% on October 15 is worth about −0.27 percentage point of TOPIX, and Tokyo Electron’s −7.78% about −0.17.

Doesn’t Nikkei cap a stock’s weight?

It does, but only at its reviews. Nikkei’s announcements set a weight cap threshold of 10%. A constituent above it on a review’s base date (the end of January for the spring review, the end of July for the autumn one) gets a capping ratio, and its “capped price adjustment factor” is used in the index.

Nikkei applied a capping ratio of 0.9 to Advantest from April 1, 2026, cutting its factor from 8 to 7.2. Because Advantest was still above 10% at the end of July, the ratio fell to 0.8 on October 1, and the factor to 6.4. Together with three constituent changes that day (Kokusai Electric, JX Advanced Metals and Capcom added), the divisor fell from 29.83073565 to 29.66435840.

Between reviews, the weight floats with the price. By the same formula, Advantest’s weight at the October 14 close was 40,380 × 6.4 ÷ (67,744.29 × 29.66435840) ≈ 12.86%.

How did Kioxia’s stock split change its place in the Nikkei 225?

It did not change its weight. Kioxia Holdings joined the Nikkei 225 on April 1, 2026 with a PAF of 0.7. For its split of each share into three, with an ex-rights date of September 29, 2026, Nikkei raised its factor to 2.1. The share price fell to about a third and the factor tripled, so Kioxia’s adjusted price stayed the same. How to read Kioxia’s price history across the split is covered in our Kioxia split explainer.

For how Tokyo’s close lines up with Seoul, Taipei and New York, see four closes, one day.