Beta measures how much a share tends to move when the stock market moves. Over the year to 24 September 2026, ExxonMobil's beta against the S&P 500 was −0.56. A negative beta means the share tended to move the opposite way to the market. In April 2021, ExxonMobil's beta was 1.33.
Beta is the market-risk input in the Capital Asset Pricing Model, a model that estimates the return shareholders expect. Here is what beta means, how it is measured and how much it moved from 2017 to 2026 for Nvidia, ExxonMobil and a fund of long-dated US government bonds.
What is stock beta? Beta measures how much a share tends to move when the stock market moves. A beta of 2 means the share has moved 2% on average for each 1% move in the market.
Beta is the slope of a line drawn through a share's returns against the market's. Using daily returns over the year to September 2026, Nvidia's was 1.90.
It is not stable. Measured each day over the previous year, ExxonMobil's beta fell from 1.33 in April 2021 to −0.56 in September 2026.
It only covers risk tied to the market. Over the year to September 2026, the market explained 43% of Nvidia's daily moves. For ExxonMobil it explained 8%.
What beta measures
Beta compares a share's moves with the market's:
A beta of 1 means the share has moved in line with the market, on average.
Above 1, it swings more than the market. Below 1, it swings less.
Zero means no link to the market's moves. Below zero, it tends to move the opposite way.
Beta feeds into valuation. The Capital Asset Pricing Model (CAPM) adds beta times a premium for owning shares to the safe interest rate. The result is the cost of equity, the return shareholders expect. It then feeds the Weighted Average Cost of Capital (WACC). We work through both for Microsoft in How to Estimate the Cost of Equity and What Is WACC?.
How beta is calculated
Source: YX Insights price data
Each dot is one trading day from 25 September 2025 to 24 September 2026. It plots Nvidia's return against the return of SPY (the S&P 500 exchange-traded fund, or ETF). Both include dividends. The orange line is the best straight-line fit through the 251 dots. Its slope is the beta: 1.90.
The same number comes from a formula. Beta is the covariance of the share's returns with the market's, divided by the variance of the market's returns. Covariance measures how much the two move together. Variance measures how much the market moves on its own. For Nvidia over this year, that is 0.000127 divided by 0.0000669, or 1.90.
Three choices sit behind every beta:
The window: one year, two years or five.
The return interval: daily, weekly or monthly returns.
The market: the S&P 500 here. Another index gives another answer.
Why published betas differ between sites
The table shows betas for Nvidia, ExxonMobil and TLT (the long-term US Treasury bond ETF), all ending in September 2026. TLT holds US Treasury bonds with more than 20 years left to run. Only the window and the return interval change.
Window and returns | Nvidia (NVDA) | ExxonMobil (XOM) | Long-dated Treasury fund (TLT) |
|---|---|---|---|
One year, daily | 1.90 | −0.56 | 0.19 |
Two years, daily | 1.86 | 0.18 | 0.11 |
Two years, weekly | 1.87 | −0.01 | 0.00 |
Five years, weekly | 2.07 | 0.27 | 0.11 |
Five years, monthly (Sep 2021 to Aug 2026) | 2.21 | 0.18 | 0.51 |
Source: YX Insights price data; YX Insights
Nvidia's beta runs from 1.86 to 2.21. ExxonMobil's runs from −0.56 to 0.27. TLT's runs from 0.00 to 0.51.
Yahoo Finance shows "Beta (5Y Monthly)", which was 2.22 for Nvidia on 4 October 2026, close to our 2.21. Bloomberg's default, as described in Brigham Young University's library guide, uses two years of weekly returns against the S&P 500. Bloomberg also shows an adjusted beta: 0.67 times the unadjusted beta, plus 0.33. That pulls every beta towards 1. On our two-year weekly figure of 1.87, Nvidia's adjusted beta would be 1.58.
So check the window and the interval before comparing two betas.
How beta shifted for Nvidia, ExxonMobil and bonds since 2017
A rolling beta repeats the calculation every day. Each time it uses the previous year of daily returns. Our data starts in October 2016, so the first one-year beta is for October 2017.
Source: YX Insights price data
Nvidia stayed above 1 throughout. Its one-year beta ran from 1.34 in February 2021 to 2.92 in February 2025. Nvidia's own swings were about the same at both points. The market's were not. SPY's volatility, the yearly standard deviation of its daily returns, was 33.3% in the year to February 2021, which included the fall of early 2020. In the year to February 2025 it was 12.6%. Against a calmer market, the same Nvidia moves gave a higher beta.
ExxonMobil peaked at 1.33 in April 2021. It was 0.50 at the end of February 2026. Then two things pulled it down:
Big shared days left the window. On 3 April 2025, SPY fell 4.9% in one day. On 4 April it fell 5.9%. ExxonMobil fell 5.3% and 7.2%. As those days and the rest of that week dropped out of the window, between 7 and 14 April 2026, its beta fell from 0.45 to −0.02.
Oil pushed the two apart. US crude oil averaged $64.5 a barrel in February 2026 and $91.4 in March. Over March, ExxonMobil gained 11.3% while SPY lost 4.9%. In the year to September 2026, ExxonMobil and SPY moved in opposite directions on 64% of trading days, up from 44% the year before. Its beta reached −0.56.
TLT had a negative beta until 10 November 2022. Before then, long-dated bonds tended to rise on days shares fell. In 2022 the Federal Reserve (Fed) raised its main interest rate, the Fed Funds Rate. The top of its target range went from 0.25% to 4.50%. SPY fell 18.2% that year. TLT fell 31.2%. Shares and bonds fell together. Since then, TLT's beta has stayed above zero apart from three trading days in April 2025. It was 0.19 on 24 September 2026.
A single beta hides all of this. Measured once over the full ten years, from October 2016 to September 2026, it was 1.84 for Nvidia. It was 0.75 for ExxonMobil and −0.11 for TLT.
What beta does not capture
Beta measures only the part of a share's risk that comes from the market. The rest is the company's own news.
R-squared shows how big the market's part is. It is the share of a stock's daily moves that the market explains, between 0% and 100%. Over the year to 24 September 2026, it was:
43% for Nvidia;
8% for ExxonMobil;
7% for TLT.
A low R-squared means beta explains little of the moves. ExxonMobil shows why this matters. Its beta was negative, yet it swung twice as much as the market. Its volatility was 26.1% against 13.0% for SPY. A low or negative beta does not make a share safe.
How to read a stock beta
A few checks help:
Read the label. Note the window, the return interval and the index before you use a beta.
Look at the rolling beta. One number can hide a drift, as Nvidia's 1.34 to 2.92 shows.
Check R-squared. If it is low, treat beta as a rough guide.
Test a range. In a valuation, try the beta from two or three methods and see how much the answer moves.
Beta is the slope of a share's returns against the market's. ExxonMobil's went from 1.33 in 2021 to −0.56 in 2026, so treat any single beta as an estimate with a range around it.
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