At first glance you might assume the Dow is just the average of its 30 stock prices. It started that way in 1896 — but a special number called the Dow Divisor now does the heavy lifting. Here's how it actually works.
The original idea: a simple average
When Charles Dow created the index, he added up the share prices of the component companies and divided by the number of companies. With 30 stocks, that would mean:
Index = (sum of all 30 share prices) ÷ 30
Simple — but it breaks the moment a company splits its stock or the committee swaps one company for another. Those events change the sum of prices without reflecting any real change in market value, which would make the index jump artificially.
The fix: the Dow Divisor
To keep the index continuous through stock splits, component changes, and similar events, the "30" is replaced by the Dow Divisor — a small number that is adjusted whenever such an event occurs. The formula becomes:
DJIA = (sum of the 30 share prices) ÷ Dow Divisor
The divisor is far less than 30; over the years repeated adjustments have pushed it well below 1. Because the divisor is less than one, the index is actually larger than the raw sum of the prices, and each $1 move in any component changes the Dow by more than one point.
Why the Dow is "price-weighted"
Notice that the formula uses share prices, not company sizes. This makes the Dow price-weighted: the higher a stock's price, the more sway it has over the index.
- A stock trading at $400 has roughly four times the impact of one trading at $100.
- This is true even if the $100 stock belongs to a far larger company.
That's the key contrast with the S&P 500, which is weighted by market capitalization (price × shares outstanding), so bigger companies dominate regardless of share price.
A worked example
Imagine a simplified 3-stock index with prices $100, $200, and $300, and a divisor of 0.5:
- Sum of prices = $100 + $200 + $300 = $600
- Index = $600 ÷ 0.5 = 1,200
Now the $300 stock rises by $30 (a 10% gain):
- New sum = $630
- New index = $630 ÷ 0.5 = 1,260 — a 60-point move from a single $30 change.
The same $30 move on the $100 stock would shift the index by the identical 60 points, even though it's a 30% gain for that company. That's price-weighting in action.
How splits are handled
When a component does a 2-for-1 stock split, its price halves. To prevent the index from dropping artificially, S&P Dow Jones Indices recalculates the divisor so the index value is unchanged at the instant of the split. The same mechanism keeps the index continuous when one company replaces another.
This article is educational and is not financial advice. See our Disclaimer. The current Dow Divisor is published by S&P Dow Jones Indices.