Guide · Markets
Price-Weighted vs Market-Cap-Weighted Indexes: Why the Dow Fell 1.23% and the S&P 500 Only 0.52% on October 23, 2026
The Dow Jones Industrial Average weights its 30 stocks by share price, the S&P 500 by company size, and on some days that difference decides the headline. Here is how each is calculated, what the Dow divisor does, and why Caterpillar cost the Dow more points than Nvidia on October 23, 2026.
A stock index is an average, and the way it averages decides what it measures. Most modern indexes, including the S&P 500 and the Nasdaq Composite, are weighted by market capitalisation: each company counts in proportion to its value, the share price multiplied by the shares available to trade. A $4 trillion company moves the S&P 500 roughly forty times as much as a $100 billion one for the same percentage change. The Dow Jones Industrial Average, first published in 1896 and still the index most often quoted on the evening news, does something older and simpler: it adds up the share prices of its 30 members and divides by a number called the divisor. Company size plays no part. A stock trading at $700 counts seven times as much as one trading at $100, whatever the two companies are worth.
The divisor exists to keep the index continuous. When a member splits its stock, or when one company is replaced by another, the sum of prices changes for reasons that have nothing to do with the market, so S&P Dow Jones Indices adjusts the divisor so that the index level does not jump. After decades of splits it is well below one: in October 2026 the ratio of the 30 prices to the index level implied a divisor of about 0.163. That gives the Dow its most useful rule of thumb. A $1 move in any member's share price moves the index by about 1 ÷ 0.163, or roughly 6.1 points, whether the stock is a $40 telecom share or a $900 investment bank. The percentage move of the stock does not matter; the dollar move does.
Here is a real example. On Friday, October 23, 2026, the Dow fell 652.82 points, or 1.23%, to 52,215.16, while the S&P 500 lost 0.52% and the Nasdaq Composite 0.21%. The difference was in the members with the highest share prices. Caterpillar fell $17.05, or 2.42%, to $687.00, which at 6.1 points a dollar cost the Dow about 105 points on its own. Goldman Sachs fell $15.62 to $875.68, about 96 points, and American Express fell $11.95, or 3.68%, to $313.22, about 73 points. Those three stocks explain about 42% of the Dow's decline. Nvidia, one of the two most valuable companies in the world and one of the largest weights in the S&P 500, fell $1.25 to $210.96 and cost the Dow under 8 points; Chevron's $6.53 gain, on a day Brent crude rose above $116, added about 40. In the S&P 500, where Caterpillar and Goldman are each well under 1% of the index, the same moves barely registered.
The previous day showed the opposite case. On October 22 the Philadelphia Semiconductor Index fell 6.72% and the Nasdaq 2.6%, yet the Dow finished up about 12 points, because the chip companies that led the sell-off are either absent from the Dow or, like Nvidia, carry a low share price there, while defensive and industrial members such as Procter & Gamble rose. Two days, two different stories: on Thursday the Dow understated the damage to the market, on Friday it overstated it. Neither index was wrong. They were measuring different things.
A few practical habits follow. When the Dow and the S&P 500 diverge by more than a few tenths of a percent, look at which high-priced Dow members moved before concluding that the market as a whole did something unusual. Read the S&P 500, or an equal-weighted index, when the question is how most of the market did, and the Nasdaq when it is about technology. Convert Dow point moves into percentages before comparing them across years: 652 points was 1.23% at 52,000, but would have been a 6% crash at 11,000 in 2010. And remember that ETFs tracking the Dow, such as DIA, follow the same price weighting, so a fund holder owns proportionally more of Caterpillar than of Nvidia, which is not what most people assume when they buy the Dow.