Nominal vs Real Cash Flows and Discount Rates
A DCF breaks quietly if inflation-adjusted cash flows get discounted with an inflation-inclusive rate, or vice versa — the fix is a simple rule: match the flavor of cash flow to the flavor of discount rate.
A cash flow can be measured two honest ways: in the actual dollars that will change hands in the future ("nominal"), or in today's purchasing power with inflation stripped out ("real"). Both are legitimate. The mistake that quietly wrecks valuations is mixing them — discounting a real cash flow with a nominal discount rate, or a nominal cash flow with a real discount rate. Either combination either double-counts inflation or ignores it entirely, and the error can move a valuation by a large multiple without any obviously wrong-looking step in the model.
Nominal cash flows must be discounted with a nominal rate; real cash flows must be discounted with a real rate. The rule survives being restated a hundred ways, but the underlying reason is simple: the discount rate already carries an inflation assumption baked into it (via the risk-free rate), and that assumption must match whatever inflation assumption is or isn't baked into the cash flow.
Why the mismatch is dangerous
A typical WACC or CAPM cost of equity is built from an observed risk-free rate — a government bond yield — which itself already prices in the market's expected inflation. That makes a standard discount rate nominal by construction. If an analyst forecasts cash flows in "today's dollars" (real, no inflation growth) but discounts them with that same nominal rate, they are discounting away inflation twice: once because real cash flows already exclude it, and again because the nominal rate assumes it's there. The result systematically understates value, sometimes severely over a long horizon.
The reverse error — inflating cash flows for expected price increases but discounting with a real, inflation-stripped rate — overstates value just as badly by never taking inflation back out.
Worked example
A project generates a real (inflation-adjusted) cash flow of $10 million a year, forever, starting one year from now. Expected inflation is 3%, and the nominal discount rate (built from a nominal risk-free rate plus a risk premium) is 8%.
Wrong approach: discount the $10 million real cash flow at the 8% nominal rate as a perpetuity: $10m / 0.08 = $125 million. This silently subtracts inflation twice.
Correct approach: first convert the discount rate to a real rate using the Fisher relationship, , giving roughly 8% − 3% = 5%. Now discount the real $10 million cash flow at the real 5% rate: $10m / 0.05 = $200 million.
The mismatched version understates the true value by $75 million, or 37.5%, purely from an inflation bookkeeping error, with no other assumption in the model changed at all.
What this means in practice
The safer default in practice is to forecast everything in nominal terms — build inflation explicitly into revenue growth, cost growth, and use a nominal discount rate — because it's easier to audit than tracking whether every cash-flow line and the discount rate all agree on being "real." Whichever convention is chosen, the check before trusting any DCF output is simple: does the discount rate's implied inflation assumption match the cash flow's?
Analysts sometimes forecast nominal revenue growth but then discount using a rate an analyst mentally treats as "real" because it was benchmarked against a real cost of capital estimate from an older report. Always trace where the discount rate came from and confirm it matches the cash-flow convention actually used in the model, rather than assuming consistency.
Further reading
- Brealey, Myers & Allen, Principles of Corporate Finance (ch. on inflation and capital budgeting)