The right discount rate to use for NPV depends on who’s doing the calculation and where the money comes from. If a company is funding the project from its own balance sheet, start with its weighted average cost of capital (WACC). If you’re an individual investor using personal savings, anchor to opportunity-cost benchmarks like Treasury yields or expected stock market returns. Either way, the rate works as a hurdle: when the project’s NPV comes out positive at that rate, the investment is expected to beat your next-best use of the money.
WACC or an Opportunity-Cost Benchmark
The choice tracks how the investment is funded, not personal preference. A company financing a project with a mix of debt and shareholder equity has a measurable internal cost of capital. That cost already reflects interest payments on loans, the returns stockholders demand, and the tax break on interest. WACC rolls all of that into a single percentage. If a project can’t clear that number, it destroys value for the people who put up the money.
Private investors, sole proprietors, and anyone deploying personal savings face a different question: what would this money earn if I didn’t invest it here? That’s an opportunity cost, and the answer comes from the realistic alternative you’d actually pursue. A retiree choosing between a rental property and Treasury bonds has a different baseline than a venture investor weighing a startup against index funds. Pick the rate that reflects the alternative you’d really take, not an abstract concept.
Building WACC
WACC combines three inputs: the after-tax cost of debt, the cost of equity, and the proportion each contributes to total financing. The result is a blended percentage representing what the company pays, on average, for every dollar of capital.
After-Tax Cost of Debt
Start with the interest rate on the company’s borrowings. Because interest is deductible, the real cost is lower than the stated rate. The federal corporate income tax rate is 21 percent, so a company paying 6 percent interest effectively pays about 4.74 percent after the deduction (6% × (1 − 0.21)).
That shield has a limit. Under Section 163(j) of the Internal Revenue Code, businesses can deduct interest expense only up to 30 percent of adjusted taxable income, plus any business interest income and floor plan financing interest.1Internal Revenue Service. Questions and Answers About the Limitation on the Deduction for Business Interest Expense Heavily leveraged companies that hit the cap won’t get the full tax benefit, which pushes their effective cost of debt higher than the simple formula suggests.
Cost of Equity
Equity investors don’t send invoices, but they expect a return. The Capital Asset Pricing Model (CAPM) estimates that return by starting with the risk-free rate and adding a premium for the stock’s risk. The premium equals the company’s beta (how much its stock price swings relative to the market) multiplied by the equity risk premium (the gap between expected market returns and the risk-free rate).
As of early 2026, the 10-year Treasury yield is around 4.1 percent, which serves as the standard risk-free rate for long-term analysis.2Federal Reserve Economic Data. Market Yield on U.S. Treasury Securities at 10-Year Constant Maturity The equity risk premium for U.S. stocks is roughly 4.2 percent at the start of 2026. A company with a beta of 1.2 would show a cost of equity near 9.2 percent (4.1% + 1.2 × 4.2%). A steadier company with a beta of 0.8 lands closer to 7.5 percent.
Weighting the Pieces
Weight each cost by its share of total capital. A company financed with $600,000 in debt and $400,000 in equity weights debt at 60 percent and equity at 40 percent. If after-tax debt costs 4.74 percent and equity costs 9.2 percent, the WACC comes to about 6.5 percent (0.60 × 4.74% + 0.40 × 9.2%).
Companies that issue preferred stock add a third component. The cost of preferred stock is the annual dividend divided by the current share price. Preferred dividends aren’t tax-deductible like interest, so there’s no tax adjustment. The preferred component gets weighted alongside debt and equity in proportion to its share of financing.
Opportunity-Cost Benchmarks for Individuals
The Risk-Free Floor
The lowest reasonable discount rate for any investment is the yield on U.S. Treasuries, since the federal government is effectively guaranteed to pay you back. The 10-year Treasury note yield, around 4.1 percent in early 2026, is the most common risk-free benchmark for multi-year projects.2Federal Reserve Economic Data. Market Yield on U.S. Treasury Securities at 10-Year Constant Maturity For shorter-term investments, Treasury bills or high-yield savings accounts set the floor. Top high-yield savings accounts were paying up to about 5 percent APY as of early 2026, which represents what your cash could earn with virtually no effort or risk.
The Stock Market as Opportunity Cost
If your realistic alternative is a diversified stock portfolio, use the long-term return of the broad market. The S&P 500 has averaged roughly 10 percent annually in nominal terms over the past several decades, or about 6 to 7 percent after inflation. Which figure applies depends on whether your project’s cash flows are expressed in future dollars (nominal) or today’s dollars (real).
The 10 percent nominal number is what people usually mean by “average stock market return,” but it hides considerable year-to-year volatility. A private project with steadier cash flows than the market might justify a lower rate; a speculative venture warrants a higher one. The point isn’t to match the market exactly. It’s to be honest about what you’re giving up.
Adjusting for Project-Specific Risk
This is where most discount rate decisions go wrong. Neither WACC nor a market benchmark automatically reflects the risk profile of the specific project you’re evaluating. A pharmaceutical company’s WACC might be 8 percent, but using that rate to discount cash flows from an experimental drug in early-stage trials would be wildly optimistic. The project is far riskier than the company’s existing product line.
The standard approach is to add a risk premium on top of the base rate. How large that premium should be is partly judgment:
- Expansion of an existing business line carries similar risk to current operations. Little or no adjustment beyond WACC is needed.
- Entry into an adjacent market brings more uncertainty about demand, competition, and execution. A premium of 2 to 5 percentage points above the base rate is common.
- Early-stage ventures or unproven technology rely on cash flow projections that are essentially educated guesses. Discount rates of 20 to 40 percent are not unusual in venture capital for this reason.
For private and closely held businesses, an illiquidity premium accounts for the fact that you can’t sell your ownership stake on a public exchange tomorrow. Rules of thumb put that premium at roughly 2 to 5 percentage points.
Risk adjustment is the single biggest judgment call in the whole exercise. Get everything else right but use a base rate on a high-risk project, and the analysis will tell you a comforting lie.
Keep Nominal and Real Consistent
A discount rate and the cash flows it’s applied to must be expressed in the same terms. If projected cash flows include inflation (nominal dollars), use a nominal discount rate. If you’ve stripped inflation out (real dollars), use a real discount rate. Mixing them produces garbage.
The mistake happens constantly. Someone projects revenue growing 5 percent per year (which implicitly includes inflation), then discounts those cash flows at a real rate of 4 percent, and concludes the project is a winner. They’ve understated the discount rate relative to their cash flows and made the project look better than it is.
To convert a nominal rate to a real rate, divide (1 + nominal rate) by (1 + inflation rate), then subtract 1. With an 8 percent nominal rate and 3 percent inflation, the real rate works out to about 4.85 percent: (1.08 ÷ 1.03) − 1 = 0.0485.
For inflation inputs, the Bureau of Labor Statistics publishes the Consumer Price Index monthly. The CPI showed a 2.4 percent year-over-year increase as of January 2026.3U.S. Bureau of Labor Statistics. Consumer Price Index Home For longer-term projections, the Federal Reserve targets 2.0 percent annual PCE inflation over the longer run, and the March 2025 FOMC projections estimated 2.2 percent PCE inflation for 2026.4Board of Governors of the Federal Reserve System. March 19, 2025 FOMC Projections
Test How Sensitive Your NPV Is to the Rate
Small changes in the discount rate can flip a project’s NPV from positive to negative, especially when cash flows arrive years into the future. A 10-year project that looks profitable at 7 percent might break even at 9 percent and lose money at 11 percent. The further out the cash flows land, the harder the discount rate hits them.
Because of this, locking in a single rate and treating the output as gospel is a mistake. Run the NPV at several rates: your base case, a rate one or two points higher, and a rate one or two points lower. If the project only works in the optimistic scenario, you’re leaning on your most generous assumptions to justify the investment. A project that stays NPV-positive across a reasonable range of rates is a fundamentally different proposition than one that needs everything to go right.
Sensitivity testing also shows which variable matters most. In some projects the discount rate barely moves the result because the cash flows themselves are the dominant uncertainty. In others the rate is the fulcrum. Knowing which situation you’re in tells you where to spend your analytical effort.
Picking the Final Number
Once you’ve gathered a base rate, adjusted for risk, and settled on nominal or real, commit to a figure. A short framework:
- Corporate project funded by the balance sheet: start with WACC and adjust upward if the project is riskier than existing operations. If it’s funded entirely from retained earnings with no new debt or equity, the cost of equity alone may be the better rate.
- Individual investor choosing between a project and the market: use the expected return of your realistic next-best alternative, which for most people is either a Treasury yield (conservative) or a broad equity index return (moderate risk).
- Private business with no public comparables: build up from the risk-free rate, add the equity risk premium, add a size or illiquidity premium, and add a company-specific risk premium. The build-up method is standard in private business valuation and commonly produces rates between 15 and 30 percent for small businesses.
A common rule of thumb is to use the highest applicable rate as your base case, forcing the project to clear every alternative use of capital and every layer of risk. There’s real wisdom in that conservatism, because most cash flow projections are optimistic and a stiff rate provides a margin of safety. But pushing the rate too high can also kill perfectly good investments. The goal isn’t to make the hurdle impossible. It’s to make sure you’ve honestly accounted for risk, opportunity cost, and the time value of money before you commit the capital.