Real Rate of Return

A nominal return counts dollars. A real return counts what those dollars buy. If your account grows 6% while prices also rise 6%, you have more dollars but no more buying power, so your real return is zero.

The usual yardstick for prices is the Consumer Price Index for all urban consumers (CPI-U), published monthly by the Bureau of Labor Statistics. In its September 11, 2026 release, the BLS reported that the CPI-U rose 3.4% over the 12 months ending August 2026. The Federal Reserve’s goal is 2% Inflation over time, but an investment has to beat the inflation that actually happens, not the target.

Definitions differ on two points. The SEC’s investor.gov glossary describes real return as what you earn after both taxes and inflation, while economists and many planners use “real” for the inflation adjustment alone and say “after-tax real return” when taxes also come out. And for a bond or deposit, the same idea is usually called the real interest rate or real yield, quoted in advance against expected inflation; a real return is measured afterward, against the inflation that actually happened. A comparison only works when both numbers use the same definition.

The exact method divides rather than subtracts. Add one to the nominal rate and one to the inflation rate, divide the first by the second, and subtract one. For a 6% return and 3.4% inflation: 1.06 ÷ 1.034 − 1 = 2.51%. The shortcut, 6% − 3.4% = 2.6%, is close when both rates are small and drifts further off as they rise, as the table below shows.

Over several years, compound first and then adjust. Grow the money at its nominal return, then divide by cumulative inflation over the same years. $100,000 earning 7% a year for 30 years becomes $761,226. With 3% inflation, prices rise by a factor of about 2.43 over that time, so the balance buys what $313,615 buys today. That is a real return of about 3.88% a year, which is why a plan that assumes a 7% nominal return and shows results at today’s prices is really assuming less than 4% of real growth.

Use the same period for both inputs. A one-year return should be deflated by one year of CPI, and a 10-year compound annual growth rate by 10 years of average inflation.

Federal income tax is charged on nominal income. The IRS treats most interest as taxable in the year it becomes available to you, and it figures a capital gain as the difference between your sale price and your adjusted basis, with no allowance for inflation. Part of a long-held gain can simply reflect higher prices, yet it is taxed as profit.

That makes the tax heavier in real terms than the bracket suggests. The after-tax real return is (1 + nominal return × (1 − tax rate)) ÷ (1 + inflation) − 1. In the worked example below, a 4% savings account earns a real 0.58% before tax, but a saver in the 22% bracket owes 0.88 percentage points of tax, more than the whole real gain. The balance grows in dollars and shrinks in buying power, and the higher inflation runs, the larger the share of real income the tax takes.

Tax-advantaged accounts soften the effect. Growth inside a Roth IRA that comes out as a qualified distribution is never taxed, so the full real return stays with you. Some Treasury securities quote a real rate directly: the fixed rate on I bonds and the yield on TIPS are already real, although federal tax on the inflation part of their return still trims what you keep.

Retirement targets are usually set at today’s prices. Your FI number and a spending goal of, say, $60,000 a year describe what you want to buy, so they pair naturally with real returns. The research behind the 4% rule worked the same way: Bengen’s 1994 study raised each year’s withdrawal with inflation, and the best-known results of the 1998 Trinity study did too.

The rule is consistency. Either use nominal returns with costs that grow with inflation, or real returns with costs held flat at today’s prices. Mixing the two, such as a 7% nominal return against spending that never rises, quietly adds several percentage points of growth a year and can make a plan look far more comfortable than it is.

Small real differences compound into large gaps. Over 30 years, $100,000 grows to about $181,000 of today’s buying power at a 2% real return and about $324,000 at 4%. At the lower rate, buying power doubles in about 35 years; at the higher one, in about 18.

Most errors come from mixing bases, and nearly all of them flatter a plan. Before you trust a real-return figure, or compare two, check three things: which price index it used, whether taxes and fees have already come out, and whether the inflation covers exactly the same months as the return. A fund’s main published return is nominal and net of fund expenses but before your taxes; a TIPS yield is already real; a return in a planning tool may be either.