How Inflation Shrinks Real Investment Returns

Real return is the nominal return adjusted for the change in prices over the same period, and the exact conversion is (1 + nominal) ÷ (1 + inflation) − 1. A 6% return in a year with 3% inflation is a real return of about 2.91%, not 3%, and every tax, fee and rate move changes what is left.

Key takeaways

The exact real return divides growth factors rather than subtracting rates; with positive inflation the subtraction shortcut overstates a positive real return, and the error grows with both inflation and the gap between return and inflation.
At 7% nominal and 3% inflation, $10,000 grows to about $38,697 in 20 years but is worth about $21,426 in today's dollars, a final value 44.6% lower and a gain 60.2% smaller than the headline.
Tax is charged on the nominal return, so at a 24% rate on interest a 6% return at 3% inflation falls from a 2.91% to a 1.51% real return, and below zero if inflation reaches 5%.
Over 1928 to 2025, Damodaran's data shows U.S. stocks at 6.78% a year after inflation, 10-year Treasury bonds at 1.45% and T-bills at 0.33%.
Inflation also moves bond prices through yields: a fund with a duration of 6 loses roughly 6% of its price if yields rise one percentage point, before income.

Real return: the exact formula

Nominal returns are what a statement reports. Real returns subtract the effect of rising prices, so they measure what the money can buy. Investor.gov's risk page describes inflation risk this way: inflation reduces purchasing power, which hurts investors who receive a fixed rate of interest.

The link between nominal rate, real rate and inflation is named after the economist Irving Fisher. The Federal Reserve Bank of St. Louis summarizes the Fisher equation as the nominal interest rate equaling the real rate plus expected inflation. For realized returns, the exact version divides growth factors, which is how Aswath Damodaran's historical returns workbook converts each year: (1 + nominal rate) ÷ (1 + inflation rate) − 1. Simple subtraction drops a small cross term.

Exact:        real = (1 + nominal) / (1 + inflation) - 1
Shortcut:     real is about nominal - inflation

nominal   = return on the investment over the period
inflation = change in the price index over the same period

Illustrative example. A portfolio earns 6% in a year with 3% inflation. Exact: 1.06 ÷ 1.03 − 1 = 2.91%. Shortcut: 6% − 3% = 3.00%. The 0.09-point gap is small here. At 10% nominal and 8% inflation the exact figure is 1.85% against a shortcut of 2%, so the shortcut overstates the result by about 8% of itself.

The shortcut's error equals (nominal − inflation) × inflation ÷ (1 + inflation). When the return trails inflation it runs the other way: 2% nominal against 5% inflation is −2.86% exact, not −3%.

Which inflation index to use

Two U.S. measures dominate. The Bureau of Labor Statistics' Consumer Price Index for All Urban Consumers (CPI-U) is the headline U.S. inflation figure, and its non-seasonally adjusted version is the index that TIPS and I bonds are tied to under Treasury's rules (31 CFR 356.2 for TIPS, 31 CFR 359.11 for I bonds). In its September 2026 release (published October 14, 2026), BLS reported a 12-month all-items increase of 3.7% before seasonal adjustment and 2.6% excluding food and energy. The next CPI release, for October, is scheduled for November 10, 2026.

The Bureau of Economic Analysis' PCE price index read 3.4% over 12 months for August 2026; the next release is scheduled for October 29, 2026. The two differ because of the formula, the spending weights and the scope of what is counted, as BEA's reconciliation FAQ explains. The Federal Reserve states its 2% longer-run target in terms of PCE, not CPI.

The choice matters at the margin. As an illustration, a 6% return is a 2.22% real return against the 3.7% CPI-U reading and 2.51% against 3.4%. Pick one index, use the same dates as the return, and record both. Neither index is a personal inflation rate: a household's own basket can differ from either.

What 20 years of inflation does

Illustrative example. $10,000 grows at a constant 7% nominal for 20 years while prices rise 3% a year. Both figures are assumptions, not forecasts, and the result is before fees and taxes.

Measure	Calculation	Result
Nominal value	$10,000 × 1.07 ^ 20	$38,697
Price level	1.03 ^ 20	1.806 times higher
Real value (today's dollars)	$38,697 ÷ 1.806	$21,426
Real annual return	1.07 ÷ 1.03 − 1	3.88% a year

The nominal gain is $28,697, but in today's dollars the gain is $11,426, which is 60.2% smaller, and the final value is 44.6% lower. Subtracting rates gives 4% a year and about $21,911, an overstatement of roughly $485 on this path. Inflation does not subtract a fixed amount each year; it compounds against the balance like a negative return.

How taxes raise the drag

Taxes apply to the nominal return, not the real one, so inflation makes the effective tax rate on the real gain higher than the statutory rate. Illustrative example: a bond pays 6% taxed as ordinary interest each year at an assumed 24% federal rate, with 3% inflation and no state tax or fees.

After-tax nominal return: 6% × (1 − 0.24) = 4.56%.
After-tax real return: 1.0456 ÷ 1.03 − 1 = 1.51%, against 2.91% before tax.
Tax took 24% of the nominal return but 48% of the real return.
At 5% inflation the same bond earns 1.0456 ÷ 1.05 − 1 = −0.42% after tax, while the pre-tax real return would still be +0.95%.

Over 20 years the 1.51% real return turns $10,000 into about $13,507 in today's dollars, against $17,757 at the pre-tax 2.91%. Where and how a return is taxed changes this result; tax-advantaged accounts, cash holdings, fees, and gold or digital assets as hedges each raise separate questions this page does not cover.

Bond prices and the duration test

Inflation also reaches returns through yields. If the real rate holds steady, higher expected inflation means higher nominal yields under the Fisher relation, and the SEC's interest rate risk bulletin notes that bond prices fall when rates rise. A bond held to maturity returns its face value plus interest, but its purchasing power still erodes along the way.

Duration is the usual stress-test number. FINRA describes it as a rule of thumb: for each 1 percentage point change in rates, price moves in the opposite direction by roughly the duration number in percent. FINRA also frames it as an approximation.

Illustrative example. A fund with an effective duration of 6 yields 4% and market yields rise 1 point over a year.

Price effect: about −6%. Income: about +4%. Nominal return: about −2%.
With 3% inflation the real return is 0.98 ÷ 1.03 − 1 = −4.85%.

Duration is a straight-line estimate, so for ordinary fixed-rate bonds it slightly overstates losses on large yield rises (a 10-year, 4% annual-coupon bond priced at 100 has a modified duration of about 8.11, yet a 1-point rise cuts its price by 7.72%, not 8.11%). Fund fact sheets report effective duration, and the stress test only needs that figure and a few yield scenarios.

What history shows, 1928–2025

NYU Stern's Aswath Damodaran publishes annual U.S. returns for stocks (the S&P 500 with dividends), 10-year Treasury bonds and 3-month T-bills, plus CPI-based inflation, in his historical returns workbook. The figures below are geometric (compound) annual averages computed from that file's yearly return series and its CPI sheet for 1928 to 2025, before taxes and fees. The workbook's own nominal-versus-real sheet uses slightly different T-bill and inflation inputs and shows a real T-bill return of about 0.28% (6.77% for stocks, 1.45% for bonds).

Asset	Nominal a year	Real a year	Years with a negative real return
S&P 500 with dividends	10.02%	6.78%	31 of 98
10-year Treasury bond	4.53%	1.45%	42 of 98
3-month T-bill	3.37%	0.33%	40 of 98
Inflation (CPI-based)	3.04%	n/a	n/a

Averages hide long stretches. From 1966 to 1981, inflation compounded at 7.01% a year, and the real return was −0.97% a year for stocks, −3.17% for bonds and −0.19% for T-bills. In 2022, with inflation at 6.45% in this dataset, the 10-year bond's nominal return of −17.83% was a real return of −22.81%. These are past results, not a guide to what comes next.

TIPS and I bonds: what they fix

Treasury Inflation-Protected Securities (TIPS) adjust their principal with the non-seasonally adjusted CPI-U, the index defined in 31 CFR 356.2, and pay interest every six months at a fixed rate on the adjusted principal. Held to maturity, they protect both principal and interest against CPI-U inflation. They do not remove market-price risk: TIPS prices still move when real yields move, so a TIPS sold early can lose money.

The tax detail is easy to miss. Under IRS Publication 550, an increase in a TIPS' inflation-adjusted principal during the year is reported as original issue discount (OID) in that year, with Form 1099-OID from the payer. The cash arrives only at sale or maturity, so a taxable account can owe tax on income not yet received. A $1,000 TIPS whose principal rises 2% to $1,020 creates $20 of reportable income for that year, on top of the interest paid. TreasuryDirect notes that TIPS income is subject to federal tax but not state or local income tax.

I bonds combine a fixed rate with an inflation rate that resets every six months. As of October 17, 2026, TreasuryDirect lists a 4.26% composite rate (0.90% fixed) for bonds issued May 1 to October 31, 2026. Under 31 CFR part 359, Treasury announces new rates each May 1 and November 1, so this rate applies to purchases through October 31, 2026. A bond keeps its fixed rate for life, while the composite rate changes every six months from the bond's issue date, so 4.26% covers only a new bond's first six months.

The market's own inflation reading comes from the gap between nominal and TIPS yields. On FRED's 10-year breakeven series (T10YIE) the value was 2.35% on October 16, 2026. The 10-year TIPS real yield (DFII10) was 2.78% on October 15, 2026, the latest reading available when this page was checked. A buyer at that yield who holds a 10-year TIPS to maturity earns roughly 2.78% a year above CPI-U inflation, before tax. Neither figure is a forecast, and both change daily.

Frequently asked questions

What happens to a TIPS if prices fall?

The principal falls with the index, but at maturity TreasuryDirect pays the greater of the adjusted principal or the original principal. Before maturity the adjusted principal can sit below the original amount, and the market price can differ from either.

Can I buy I bonds in any amount?

No. TreasuryDirect lets one Social Security number or employer ID buy up to $10,000 in electronic I bonds per calendar year, with a $25 minimum, and since January 1, 2025, I bonds are sold only electronically. You can redeem after 12 months, but redeeming before five years forfeits the last three months of interest. The interest is subject to federal tax but not state or local income tax, and you choose whether to report it each year or when you redeem.

Does the breakeven rate predict inflation?

FRED describes it as a measure of expected inflation derived from nominal and inflation-indexed Treasury yields, implying what market participants expect on average over the next 10 years. It is a market-implied figure, not a forecast, and realized inflation can come in above or below it.

Does holding a bond to maturity avoid the inflation loss?

It avoids a price loss from rising yields, since Investor.gov notes the holder receives face value plus interest at maturity. It does not avoid the loss of purchasing power. A 5% bond held through 5% inflation has a real return of 0% before tax.

When does the subtraction shortcut mislead?

The error grows with inflation and with the distance between the return and inflation. With positive inflation, it overstates a real gain and makes a real loss look slightly worse: 10% nominal against 8% inflation gives 2.00% by subtraction and 1.85% exactly, while 10% against 12% gives −2.00% against −1.79%. Over several years the difference compounds, so use the exact formula for multi-year comparisons.

A checklist for your own numbers

Choose one index (CPI-U or PCE) and the exact start and end dates, and record the index values.
Take the nominal total return for those dates, with dividends and interest reinvested.
Take off fees and an estimated tax on the nominal gain, then convert with (1 + after-tax return) ÷ (1 + inflation) − 1.
For bond holdings, read the effective duration on the fund's materials and apply 1-point and 2-point yield moves.
For TIPS in a taxable account, estimate the tax on the annual principal adjustment, which is payable before the cash arrives.
To test your own balance, the site's purchasing power calculator converts a dollar amount between years.