TIPS Explained: Real Returns, Inflation Adjustment, and Risks [Basic Investing I Studied #8]

A statement such as “my portfolio returned 5%” is incomplete. The return is nominal: it tells us how many more dollars we have, not how much more those dollars can buy. To measure purchasing power, the return and the inflation rate must cover the same dates and be combined multiplicatively.

This distinction is also the key to understanding Treasury Inflation-Protected Securities, or TIPS. TIPS link their principal to an official inflation index, but they are not a universal or literally risk-free investment.

Nominal and real returns over the same period

Consider one holding period from date $t$ to date $t+1$. After that period ends, define:

$i_{t,t+1}^{\mathrm{real}}$: the nominal return actually realized during that period;
$\pi_{t,t+1}$: inflation actually realized during that same period; and
$r_t^{\mathrm{ex\ post}}$: the purchasing-power return known after the period ends.

If one dollar grows to $1+i_{t,t+1}^{\mathrm{real}}$ dollars while the price level grows by a factor of $1+\pi_{t,t+1}$, the exact ex-post real return is

$$ r_t^{\mathrm{ex\ post}} =\frac{1+i_{t,t+1}^{\mathrm{real}}}{1+\pi_{t,t+1}}-1. $$

For a decision made at date $t$, neither a risky investment’s future nominal return nor future inflation is yet known. Let $i_{t,t+1}^{e}$ be a point forecast formed at $t$ for that investment’s nominal return, and let $\pi_{t,t+1}^{e}$ be a point forecast formed at $t$ for inflation over the same period. Define a corresponding planning, or ex-ante, real rate $r_t^{e}$ by the point-forecast convention

$$ 1+r_t^{e}=\frac{1+i_{t,t+1}^{e}}{1+\pi_{t,t+1}^{e}}. $$

Rearranging gives an exact multiplicative conversion under that definition:

$$ 1+i_{t,t+1}^{e}=(1+r_t^{e})(1+\pi_{t,t+1}^{e}), $$

and therefore

$$ i_{t,t+1}^{e}=r_t^{e}+\pi_{t,t+1}^{e}+r_t^{e}\pi_{t,t+1}^{e}. $$

This is where the familiar approximation comes from. When both rates are small, the product $r_t^{e}\pi_{t,t+1}^{e}$ is second order, so dropping it yields

$$ i_{t,t+1}^{e}\approx r_t^{e}+\pi_{t,t+1}^{e} \quad\Longleftrightarrow\quad r_t^{e}\approx i_{t,t+1}^{e}-\pi_{t,t+1}^{e}. $$

The approximation is about omitting the cross-product, not about “approximating equilibrium.” Equilibrium is an economic-model question. Forecast versus realized outcomes are a timing question. If the nominal return or inflation differs from its forecast, the ex-post real return differs from the ex-ante planning rate even when the exact formulas are used. Under uncertainty, this ratio of two point forecasts is only a planning convention; it is not generally the expected value of the exact realized real-return ratio.

The 5% return and 5% inflation example

Suppose an investment grows from $100 to $105 while a representative basket of goods also rises from $100 to $105. Before taxes, fees, and trading costs,

$$ r^{\mathrm{ex\ post}}=\frac{1.05}{1.05}-1=0. $$

The investor has 5% more dollars but exactly the same purchasing power relative to that basket. Subtracting the rates also happens to give 0% here, but that shortcut is not exact in general. For example, a 10% nominal return with 5% inflation produces

$$ \frac{1.10}{1.05}-1\approx 4.7619\%, $$

not exactly 5%.

How TIPS cash flows work

TIPS are marketable U.S. Treasury securities currently issued with 5-, 10-, and 30-year terms. Their stated coupon rate is fixed, while their principal is adjusted using the non-seasonally adjusted U.S. city-average Consumer Price Index for All Urban Consumers (CPI-U). Treasury publishes a daily index ratio for each security.

If $P_0$ is original principal and $IR_s$ is the index ratio on payment date $s$, adjusted principal is

$$ P_s=P_0\times IR_s. $$

For annual coupon rate $c$, each regular semiannual interest payment is

$$ C_s=P_s\times\frac{c}{2}. $$

Inflation can raise adjusted principal and the dollar coupon payment. Deflation can reduce both. The coupon rate itself does not reset.

A numerical principal, coupon, and maturity example

TreasuryDirect illustrates the coupon calculation with $1,000 of original principal, a 0.125% annual coupon rate, and an index ratio of 1.01165:

$$ P_s=\$1{,}000\times1.01165=\$1{,}011.65, $$$$ C_s=\$1{,}011.65\times\frac{0.00125}{2}=\$0.63228125, $$

which is paid as about $0.63 for that six-month period.

Now extend the same mechanics to maturity. If the final index ratio were 1.08, adjusted principal would be $1,080 and Treasury would redeem $1,080. If cumulative deflation instead produced a final index ratio of 0.94, adjusted principal would be $940, but Treasury would redeem the greater of adjusted principal and original principal: $1,000.

That floor applies to the principal payment at maturity. It does not lift interim adjusted principal to $1,000, and the final coupon is still calculated from the inflation-adjusted principal. In the 0.94 scenario, the final coupon would be based on $940, not the $1,000 redemption floor.

What the maturity floor does not guarantee

The original-principal floor is valuable but narrow:

It applies at maturity. A TIPS sold earlier trades at the market price, which can be above or below what the investor paid.
It protects original par, not a premium paid at auction or in the secondary market.
It does not guarantee a positive return after federal tax, transaction costs, or inflation as personally experienced by the investor.
It does not make every coupon payment immune to deflation; coupons fall when adjusted principal falls.

The security is backed by the full faith and credit of the United States, which addresses promised Treasury payments. Calling the investment simply “risk-free,” however, hides several risks that matter to an actual holder.

Risks and limitations to check before investing

Inflation-measurement and tracking risk

TIPS use non-seasonally adjusted CPI-U, a national average. A household whose spending is concentrated in housing, health care, education, or another category can experience inflation quite different from CPI-U. TIPS hedge the specified index, not every investor’s personal cost of living.

Index lag

The reference CPI for the first day of a month uses CPI-U from the third preceding month; other days are linearly interpolated. The resulting approximate three-month lag means principal does not respond to a current inflation shock in real time.

Real-yield and market-price risk

TIPS prices respond to real yields. If market real yields rise, the price of an existing lower-yield TIPS can fall. An investor who must sell before maturity can realize a loss even though the security has a maturity floor.

Liquidity and transaction risk

TIPS are marketable, but Treasury’s investment considerations note that their market may be less active or liquid than the market for fixed-principal Treasury securities and that bid-ask spreads may be larger. Trading costs and execution prices therefore matter, especially for a sale before maturity.

Tax and cash-flow risk

For a U.S. taxable account, coupon interest is generally subject to federal income tax. Increases in inflation-adjusted principal are generally reportable as original issue discount in the year they occur, even though that principal is not received in cash until disposition or maturity. This creates the possibility of tax due on non-cash principal growth. Treasury interest is exempt from state and local income taxes. Individual circumstances, account type, premiums, discounts, and transactions can change the reporting details, so this is a reason to consult current IRS guidance or a tax professional—not personal tax advice.

Reinvestment risk

Coupons arrive every six months. The rate available for reinvesting them is unknown, so a quoted real yield or coupon rate does not guarantee the compounded return an investor will achieve from reinvested cash flows.

Deflation and horizon risk

Deflation lowers adjusted principal and coupons before maturity. The floor helps only an investor who reaches maturity and only for original principal. A mismatch between the TIPS maturity and the date the money is needed can force a sale at an unfavorable price.

A practical way to read a TIPS quote

Before comparing TIPS with a nominal Treasury security, ask:

Do the securities have similar maturities and cash-flow timing?
Is the quoted figure a coupon rate, real yield, nominal yield, or market price?
Will the security be held to maturity or might it be sold early?
Is the investment in a taxable or tax-advantaged account?
Is CPI-U a reasonable match for the spending risk being hedged?
Are coupons assumed to be spent or reinvested?

TIPS are best understood as a specific contract: CPI-U-linked principal, a fixed coupon rate applied to adjusted principal, and an original-principal floor at maturity. That contract can reduce exposure to measured U.S. inflation over a matched horizon. It cannot eliminate price, liquidity, tax, timing, tracking, or reinvestment risk.

Primary sources

Mechanics and tax details were checked on 2026-09-23 against these official U.S. government sources: