How to Calculate Return on Indices in a Stock Market

By Motley Fool Staff – Updated Apr 29, 2025 at 11:49 PM EST

Key Points

Stock indexes like S&P 500 and Dow Jones track market trends, aiding in performance comparison.
Sector-specific indexes gauge various industries, useful for targeted investment insights.
Return on indexes can be calculated using initial and final prices, reflecting investment value change.

A stock index can give you a good idea of how the overall stock market, or a certain portion of the stock market, is performing. The Dow Jones Industrial Average is perhaps the best-known index, but it isn't widely considered to be a great snapshot of the entire market, as it consists of only 30 companies.

Other popular market indexes include the S&P 500, which is generally considered to be a great indicator of how the overall market is performing, the tech-heavy NASDAQ Composite index, and the Russell 2000 index, a broad index of small-cap companies. There are also indexes for individual sectors, such as technology, healthcare, and finance, which can help track the performance of certain types of companies.

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A digital board displaying stock market indexes and what they're trading for. — Image source: Getty Images.

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Indexes can also be useful for measuring the performance of your own portfolio against a benchmark. For example, if your stock portfolio drops by 7% in a certain month, but the S&P 500 drops by 10%, you are still beating the market even though the value of your holdings fell.

Calculating the return of stock indexes

To calculate the return of a stock index between any two points in time, follow these steps:

First, find the price level of the chosen index on the first and last trading days of the period you're evaluating. Be sure to use the opening price on the first day and the closing price on the last in order to make sure your calculation is as accurate as possible.

Next, subtract the starting price from the ending price to determine the index's change during the time period.

Finally, divide the index's change by the starting price and multiply by 100 to express the index's return as a percentage.

Putting the formula together, we have:

Return (%) = [(Ending Price – Starting Price) ÷ Starting Price] × 100

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Screenshot 2024-06-06 at 12.52.56 PM — Note: This formula is useful for determining the return of individual investments as well.

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An example

Let's say that you want to calculate the return of the S&P 500 index during the month of October 2015.

First, using an accurate price chart, determine the starting and ending price. In this case, on Oct. 1, the S&P 500 opened at 1,919.65, and on Oct. 30, the index closed at 2,079.36.

Using the formula mentioned above, we can see that:

October S&P 500 Return = [(2079.36 – 1919.65) ÷ 1919.65] × 100 = 8.3%

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