You are weighing two uses for $50,000: a calm dividend stock that barely moves when the market sneezes, or a high-flying growth stock that doubles the market's every swing. The growth stock should pay you more — but how much more, exactly? Guessing leaves money on the table or risk on it. The Capital Asset Pricing Model, or CAPM, answers with a number: it converts an asset's market risk into the annual return you should demand for holding it. This guide walks through the formula piece by piece, shows you how to run it in a minute, and tells you where the model breaks down so you never trust it blindly.
The One-Line Idea: Only Market Risk Gets Paid
Every investment carries two kinds of risk. The first is company-specific risk: a CEO scandal, a failed product launch, a factory fire. You can wash that risk away almost for free by holding many different stocks — when one holding stumbles, another holds steady. Because diversification eliminates it, the market pays you nothing for bearing it.
The second is market-wide risk: recessions, interest-rate shocks, inflation scares. No amount of diversification escapes it, because it hits nearly every stock at once. CAPM's core claim is that expected return compensates you only for this second kind — systematic risk — and that beta measures how much of it a given asset carries.
That single distinction already corrects a common mistake. A volatile stock is not automatically a high-expected-return stock. If its jumps come from company-specific drama that a diversified holder barely notices, the market offers no premium for the ride. Only the portion of movement that marches with the market earns extra return.
The Formula, Piece by Piece
The whole model fits in one equation:
E(Ri) = Rf + βi × (E(Rm) − Rf)
In words: the expected return on asset i equals the risk-free rate, plus beta times the market risk premium. Three inputs, one output. Each deserves its own explanation.
The risk-free rate (Rf): your starting point
The risk-free rate is the return you could earn with no market risk at all — the floor every risky investment must beat. In practice, investors use a U.S. Treasury yield: the 3-month T-bill for short horizons, the 10-year Treasury note for long-horizon stock investing.
Match the rate to your horizon. Valuing a stock you plan to hold for a decade against a 3-month bill rate mixes a long-term expectation with a short-term anchor. Most practitioners pricing equities reach for the 10-year yield, which has hovered around 4 to 5 percent in recent years.
Beta (β): how hard the asset swings with the market
Beta measures an asset's sensitivity to market movements. It comes from a regression of the asset's past returns against the market's — usually the S&P 500 for U.S. stocks — and the slope of that line is beta:
- Beta of 1.0 means the asset moves with the market. Market up 10 percent, asset up roughly 10 percent.
- Beta above 1.0 means amplified swings. A beta-1.5 stock tends to rise about 15 percent when the market rises 10 — and fall about 15 percent when it falls 10.
- Beta below 1.0 means dampened swings. A beta-0.6 utility stock rises roughly 6 percent on a 10 percent market gain, and cushions the same fraction of a decline.
- Beta near zero means no link to the market at all, like short-term Treasuries themselves.
Beta is backward-looking by construction — it describes how the asset has moved — so treat it as an estimate of future sensitivity, not a law. Betas drift as companies change: a startup that matures into a dividend payer typically watches its beta slide toward 1.0 and below.
The equity risk premium (E(Rm) − Rf): the market's price of courage
The equity risk premium is the extra annual return investors demand for holding the whole stock market instead of risk-free bonds. Multiply it by beta and you get the extra return one specific asset must offer.
History suggests a premium of roughly 4 to 6 percent a year for U.S. stocks over long periods, depending on the window and the averaging method. Forward-looking estimates — which back the premium out of today's index prices and expected cash flows — recently sit near 4 to 4.5 percent. Finance professor Aswath Damodaran's widely followed annual update put the implied premium at about 4.2 percent at the start of 2026.
For your own quick calculations, 4 to 5 percent is a defensible assumption. Just remember it is an expectation, not a promise: the premium is negative in plenty of individual years, which is precisely why holding stocks feels risky.
A Worked Example in 60 Seconds
Say the 10-year Treasury yields 4.5 percent, you assume a 4.5 percent equity risk premium, and you are eyeing a stock with a beta of 1.3.
E(R) = 4.5% + 1.3 × 4.5% = 4.5% + 5.85% = 10.35%
Your required return is about 10.4 percent a year. If your analysis says the stock will deliver less than that over your holding period, the market is not paying you enough for its risk — pass, or demand a lower price. Run the same math on a beta-0.7 consumer staple and the hurdle drops to 7.65 percent, which is why calm stocks can be fairly priced at lower yields.
That hurdle rate is CAPM's practical output. It does not predict what the stock will return; it states what it must return to justify its risk.
The Security Market Line: The Formula as a Picture
Plot beta on the horizontal axis and expected return on the vertical axis, and the CAPM formula draws a straight upward-sloping line — the Security Market Line (SML). It starts at the risk-free rate (where beta is zero) and rises one risk-premium per unit of beta.
Every fairly priced asset sits exactly on the line. The interesting cases are the ones off it:
- Above the line: potentially undervalued. The asset's expected return exceeds what its beta demands. That gap has a name — alpha — and positive alpha is what every active investor hunts.
- Below the line: potentially overvalued. You bear the beta risk but the expected return falls short. In theory, investors sell such assets until the price drops enough to push expected return back up to the line.
This framework also explains why chasing past winners often disappoints. A stock that soared last year may now trade at a price whose forward expected return sits below the line — all risk, insufficient reward. Price, not past performance, decides where an asset plots.
Where CAPM Earns Its Keep: Your Cost of Capital
CAPM is not just a stock-picker's tool. Its most durable business use is setting the cost of equity — the return your money must earn to justify staying invested in your own enterprise rather than in the market.
Suppose you run a small business and are weighing a $40,000 equipment purchase that should generate extra profit for years. CAPM gives you a personal hurdle rate: with a 4.5 percent Treasury yield, a 5 percent premium, and a beta of 1.0 for an average-risk alternative, your money's next-best diversified use demands about 9.5 percent. If the equipment's projected return clears that bar with margin, invest; if not, the market pays you better for less effort.
Blending that equity cost with your borrowing cost yields the weighted average cost of capital (WACC), the standard discount rate for valuing projects and whole businesses. Even a rough CAPM-based WACC beats the most common small-business alternative, which is no hurdle rate at all.
Track the comparison in your own books
A hurdle rate only helps if you can see your actual returns beside it. If you keep your investing in Beancount alongside the rest of your finances, booking a brokerage position takes a few lines:
2026-01-05 open Assets:Brokerage:VTI
2026-01-05 open Expenses:Investing:Fees
2026-01-05 * "Buy broad-market index fund"
Assets:Brokerage:VTI 100 VTI {285.00 USD}
Assets:Checking -28500.00 USD
Expenses:Investing:Fees 0.00 USDWith positions and dividends recorded as ordinary transactions, computing your realized annual return becomes a bookkeeping query rather than a spreadsheet project — and you can hold that number directly against the CAPM hurdle you calculated. The Beancount documentation covers lot tracking and price annotations for this kind of investing ledger, and Fava charts the portfolio's performance so the comparison stays visible instead of buried in a journal file.
Where the Model Breaks Down
CAPM rests on brave assumptions: investors are rational and hold diversified portfolios, everyone agrees on expected returns, there are no taxes or trading costs, anyone can borrow at the risk-free rate, and a single market factor explains returns over a single period. Every one of these fails somewhere in the real world, and the failures matter.
Beta wobbles. Estimated from past prices, beta shifts with the measurement window — a two-year weekly beta and a five-year monthly beta for the same stock routinely disagree. For thinly traded small caps, stale prices can make beta look artificially low, understating risk exactly where risk runs highest.
One factor is not enough. In the early 1990s, Eugene Fama and Kenneth French showed that small companies and cheap "value" stocks earn premiums CAPM cannot explain. Their three-factor model adds size and value factors alongside market beta, and it fits historical returns considerably better. Later research added profitability and momentum. Each addition is an admission that beta alone leaves return patterns on the table.
The low-volatility anomaly points the wrong way. CAPM says higher beta means higher expected return, yet low-beta stocks have historically delivered surprisingly strong risk-adjusted returns — sometimes beating high-beta stocks outright. Leverage constraints offer one explanation: investors who cannot borrow to amplify calm stocks bid up volatile ones instead, overpaying for beta.
Private assets have no observable beta. Your rental property, your private business, a friend's startup — none trade daily, so no regression exists. Practitioners borrow betas from comparable public companies and adjust for leverage, but each adjustment layers judgment on judgment. Treat a CAPM hurdle for an illiquid asset as a starting bid, not a verdict.
None of this makes CAPM useless. It makes it a first approximation: the simplest model that turns risk into a required return, and the baseline every fancier model must beat.
Common Mistakes When You Use CAPM Yourself
Mixing horizons. Pairing a 3-month T-bill rate with a 10-year equity premium blends two different clocks. Pick the Treasury maturity closest to your holding period and an ERP estimated over a matching horizon.
Using a stale beta. A beta from three years ago describes a company that may no longer exist in the same form — after a merger, a pivot, or a leverage change, re-check it. Free quote pages publish current betas; use them.
Treating the historical premium as a forecast. The 6 percent historical average includes a century in which the U.S. market survived everything thrown at it. Forward-looking estimates near 4 percent arguably reflect today's high valuations better. Run your hurdle with both and see whether your decision survives the range.
Forgetting leverage. A company's observed equity beta reflects its debt load: borrowing amplifies equity swings. Comparing an unlevered project to a levered company's beta overstates the project's risk. Finance textbooks un-lever and re-lever betas across capital structures; at minimum, compare like with like.
Demanding false precision. CAPM inputs are all estimates, so quoting a hurdle of 9.63 percent implies confidence nobody has. Round to the nearest half point and think in ranges — "about 9 to 10 percent" is the honest output.
Your Next Step
Pick one holding you own — a stock, a fund, even your own business's next project. Look up the current 10-year Treasury yield, find the beta, assume a 4 to 5 percent equity premium, and compute the line: risk-free rate plus beta times premium. Compare that hurdle with what you realistically expect to earn. If the expectation clears the hurdle, you are being paid for the risk; if it falls short, you now know exactly how much cheaper the price — or how much better the outlook — needs to be before the risk is worth taking.
