TIPS Principal Adjustment: How Inflation Changes What You're Paid at Maturity

You bought $10,000 face of a 5-year TIPS with a 1.00% coupon. Five years later the statement says the security matured for $11,592.74, and the coupon payments along the way were never the same twice. Nothing went wrong. The coupon rate stayed fixed at 1.00% the whole time; what changed was the principal it was multiplied by, and it changed every single day via a number called the index ratio.

That is the whole mechanism. The rate is frozen. The base it applies to floats with the consumer price index, and every dollar you receive, coupon or maturity, is the frozen rate applied to the floating base.

Key points

• A TIPS coupon rate never changes. Your dollar coupon changes because the principal it multiplies is restated daily by the index ratio.
• The index ratio is reference CPI on the payment date divided by reference CPI at issue. At a ratio of 1.159274, a $10,000 face TIPS has $11,592.74 of adjusted principal.
• On a 1.00% coupon, a semiannual payment is 0.50% of adjusted principal. That is $50.00 at issue and $57.96 once the ratio reaches 1.159274 — same rate, 15.9% bigger check.
• The maturity payment is the greater of adjusted principal and original face. Cumulative deflation cannot pay you back less than $10,000, but it can and does shrink the coupons in the meantime.
• The annual principal accrual is taxable federal income in the year it accrues, even though you receive no cash for it until maturity. On the year the ratio moves 1.1240 to 1.1592, that is $352 of income and $0 of cash.

Why does the coupon check keep changing if the rate is fixed?

Because the rate is applied to a number that moves. Take the concrete case and hold it for the rest of this article.

Meet Dana. On 15 January 2021 she buys $10,000 face of a hypothetical 5-year TIPS at par, coupon 1.00%, maturing 15 January 2026. Interest pays every 15 January and 15 July. All figures below are illustrations built on assumed CPI paths, not a real security’s history.

The Treasury sets a reference CPI for every calendar day of the security’s life by interpolating the published monthly CPI-U from three months earlier. Divide the reference CPI on any given day by the reference CPI on the issue date and you get the index ratio for that day. At issue the ratio is exactly 1.000000, by construction.

Adjusted principal = face × index ratio.

Coupon payment = adjusted principal × (coupon rate ÷ 2).

That’s it. Two lines of arithmetic, and everything strange about TIPS falls out of them.

Dana’s first coupon, 15 July 2021, at an assumed ratio of 1.0284:

• Adjusted principal: $10,000 × 1.0284 = $10,284.00
• Coupon: $10,284.00 × 0.005 = $51.42

Not $50.00. She didn’t buy more bonds and the coupon rate didn’t move. The 1.00% just landed on $10,284 instead of $10,000.

What the index ratio actually looks like over five years

Here is Dana’s assumed path. One column, one unit: the adjusted principal in dollars on each 15 January.

Those correspond to index ratios of 1.000000, 1.048000, 1.115000, 1.154000, 1.124000 and 1.159274.

Look at January 2025. Principal fell from $11,540.00 to $11,240.00. That is a 2.60% drop in the index over that year, an assumed deflationary stretch. Dana’s January 2025 coupon was $11,240.00 × 0.005 = $56.20, down from $57.70 the year before. Deflation cuts your income.

How much cash did Dana actually collect?

Ten coupons, each one half a percent of whatever the principal was that day. Using the January figures above and assumed midyear ratios, here’s the full run. Each is adjusted principal × 0.005.

• Jul 2021, ratio 1.0284: $51.42
• Jan 2022, ratio 1.0480: $52.40
• Jul 2022, ratio 1.0910: $54.55
• Jan 2023, ratio 1.1150: $55.75
• Jul 2023, ratio 1.1385: $56.93
• Jan 2024, ratio 1.1540: $57.70
• Jul 2024, ratio 1.1390: $56.95
• Jan 2025, ratio 1.1240: $56.20
• Jul 2025, ratio 1.1425: $57.13
• Jan 2026, ratio 1.159274: $57.96

Sum of coupons: $556.99.

Maturity payment: $11,592.74.

Total cash back: $12,149.73 on a $10,000 outlay.

Compare that to a plain 1.00% fixed-rate note bought the same day: $50.00 twice a year for five years is $500.00 of coupons, plus $10,000 back. Total $10,500.00. The difference of $1,649.73 is entirely inflation compensation. It is not extra return. Dana’s $12,149.73 in 2026 dollars buys roughly what $10,480 bought in 2021 dollars, because the price level rose 15.93% over the period. The fixed-rate note holder ends with $10,500 nominal, which in 2021 purchasing power is $10,500 ÷ 1.159274 = $9,057.42. That holder lost 9.4% of their purchasing power. That is the entire point of the structure.

The 1.00% is a real yield, not a nominal one

A sceptical reader will say: a 1.00% coupon is absurd, nobody accepts that. Correct, if it were nominal. It isn’t. The coupon on a TIPS is quoted on top of inflation, so the security’s stated rate is a real rate. A 1.00% TIPS and a 4.00% nominal Treasury of the same maturity are pricing the same expectation if the market thinks inflation will run about 3% a year. The gap between them has a name, the breakeven inflation rate, and it’s simply nominal yield minus real yield.

If Dana’s TIPS yields 1.00% real and the 5-year nominal Treasury yields 4.00%, the breakeven is 3.00%. Inflation above 3% annualised and Dana beats the nominal. Below 3% and she trails it. Her realised inflation was 15.93% over five years, which annualises to 1.1592741/5 − 1 = 2.998%. Almost exactly the breakeven. Dana and the nominal holder effectively tied on that assumed path, which is what an efficiently priced breakeven is supposed to produce.

What happens if prices fall the whole time?

You get your original face back. The maturity payment is defined as the greater of adjusted principal and par, so a TIPS carries a deflation floor at 100% of face.

Suppose Dana’s assumed path had gone the other way and the final index ratio on 15 January 2026 was 0.9400. Adjusted principal would be $9,400.00. The floor overrides it and she receives $10,000.00.

The floor applies only to the principal payment. It does not apply to the coupons. On that deflationary path her final coupon would be $9,400.00 × 0.005 = $47.00, and every coupon along the way would have shrunk in the same way. Total coupons on a steadily deflating path would come in under $500. So the floor protects your capital and not your income.

There’s a subtlety that trips people up in the secondary market. The floor is measured against the security’s original issue principal, not against what you paid. If Dana had instead bought a seasoned TIPS in 2024 whose index ratio was already 1.1540, she’d have paid roughly $11,540 for $10,000 face plus or minus the price move, and the floor would still sit at $10,000. She’d be exposed to the full $1,540 of accrued inflation compensation unwinding. Buying a TIPS with a large accrued index ratio means buying something whose deflation protection is already spent.

Why is the tax bill bigger than the cash?

Because the principal accrual is taxable in the year it accrues, not in the year you receive it.

Take Dana’s 2023 calendar year on the assumed path. Principal went from $11,150.00 to $11,540.00, an accrual of $390.00. She also collected two coupons that year, $56.93 in July and $57.70 in January 2024 — take the two paid within 2023, $55.75 and $56.93, totalling $112.68.

Federal taxable interest income for 2023: $390.00 + $112.68 = $502.68.

Cash actually received in 2023: $112.68.

At a 24% marginal federal rate she owes $120.64 in tax, which is more than the $112.68 of cash the bond gave her. She has to fund $7.96 of the tax from somewhere else. That’s phantom income, and it’s why TIPS held directly are commonly placed in tax-deferred accounts. In a year of high inflation the gap widens sharply; on the 2022 stretch where principal ran from $10,480.00 to $11,150.00, the accrual alone was $670.00 against $106.95 of coupons.

The deflation year runs the other way. In 2024, principal fell from $11,540.00 to $11,240.00, a negative accrual of $300.00, against coupons of $57.70 and $56.95 totalling $114.65. Net taxable interest for the year is negative $185.35 under the standard treatment of deflation adjustments, which offsets other interest income from the same security. TIPS interest and accrued principal are both exempt from state and local income tax, like all Treasury interest.

Why does the price still move if inflation is covered?

Because the real yield moves. This is the part people miss, and it’s where the risk actually lives.

Dana’s TIPS has two prices. There’s the quoted price, expressed as a percentage of par, which responds to changes in real yields the same way any bond price responds to changes in nominal yields. And there’s the invoice price, which is quoted price × index ratio × face, plus accrued interest.

If real yields rise from 1.00% to 2.00% while Dana holds a security with about 3 years of duration remaining, the quoted price falls roughly 3%. On an adjusted principal of $11,540.00 that is a mark-to-market loss near $346, and it lands even if inflation over the same period was running hot. Inflation protection is not price protection. It protects the purchasing power of the cash flows if you hold to maturity; it does nothing about what the security is worth on a Tuesday in March.

That’s the same dynamic FINRA describes for any market-traded security in its investor education on stocks and market pricing: what something is worth today reflects what buyers will pay today, not what it will pay you later.

The settlement and leverage details that bite

Two operational points, because they change what the cash flow actually looks like in your account.

Treasury securities settle T+1, and US equities moved to T+1 in May 2024 under the SEC’s shortened settlement cycle rule, described in the SEC’s press release on the T+1 final rules. Practical effect for Dana: the accrued interest and index ratio applied to her purchase are computed as of settlement date, not trade date. A one-day difference in an inflationary month is small, a few cents on $10,000, but it’s why the invoice never exactly matches the number you calculated at 3pm on trade day.

The second point is leverage. TIPS are sometimes bought on margin because the real yield looks modest and the borrow cost seems manageable. Understand what that does to the deflation floor: the floor guarantees $10,000 of face back, it does not guarantee anything about a margin call triggered by a real-yield move in month eighteen. The SEC’s explainer on buying stock on margin sets out the mechanic that matters here, which is that the broker can require more collateral, or liquidate, on their timetable rather than yours. A hold-to-maturity guarantee is worth nothing if you’re forced out at month eighteen.

What this does not tell you

The CPI path used here is assumed. I made it up to produce clean arithmetic. Real inflation over any five-year window will not look like this, and the entire difference between Dana’s outcome and the nominal note holder’s outcome depends on which side of the breakeven realised inflation lands.

The security is hypothetical. Actual auctioned TIPS have specific coupons set at auction, and many trade well above or below par with index ratios already far from 1.0000. The 1.00% coupon here is a round number for legibility, not a market quote.

The three-month CPI lag is not modelled. Reference CPI on any date derives from a CPI-U print released roughly three months earlier, interpolated across the month. So the adjustment you receive in March reflects prices from around December. In a sharp inflation spike, you are compensated late. That lag is real money in a fast-moving month and I have skipped it entirely for arithmetic clarity.

Tax figures assume a 24% federal marginal rate on ordinary interest and ignore the alternative minimum tax, net investment income tax, state variation, and the specific elections available for handling negative inflation adjustments. Rules on offsetting a negative adjustment against interest from the same security have detail that a tax professional should check against your return.

Nothing here covers TIPS mutual funds or ETFs, which behave differently. A fund has no maturity date and therefore no deflation floor, distributes inflation accruals as income, and has a duration that resets rather than shortening as you hold it.

And I have not priced the liquidity difference. TIPS trade in a thinner market than nominal Treasuries, and bid-ask spreads widen in stress. That’s a real cost if you sell early and I have not quantified it, because it varies too much by size and by day to state honestly.

FAQ

Does the deflation floor mean a TIPS can’t lose money?

No. It means the maturity payment won’t fall below original face. Three ways to lose anyway: you paid above par and receive par back; you sold before maturity after real yields rose; or realised inflation came in under the breakeven, so a nominal Treasury bought the same day would have paid you more. Dana’s floor of $10,000 says nothing about the $11,540 mark on her statement in 2024.

Why did my coupon go down when inflation was positive?

Check the three-month lag. The coupon you receive in July reflects reference CPI derived from the CPI-U print for roughly April, interpolated. If prices dipped in that earlier window, your July payment falls even though the headline number reported the week of payment was positive. On Dana’s path the July 2024 coupon of $56.95 was smaller than the January 2024 coupon of $57.70 for exactly this reason.

What is the index ratio on the day I buy, and does it cost me anything?

It’s reference CPI on your settlement date divided by reference CPI on the security’s original issue date, carried to six decimals. It’s not a fee. You pay for accrued inflation compensation in the invoice price, because you’re buying a claim on a principal balance that has already grown. Buying at a ratio of 1.1540 means paying about $11,540 per $10,000 face before the price move and accrued interest.

How do I work out my breakeven inflation rate?

Subtract the TIPS real yield from the nominal Treasury yield of the same maturity. At 4.00% nominal and 1.00% real, the breakeven is 3.00% annualised. Above that, the TIPS pays more over the life; below it, the nominal does. Dana’s realised path annualised to 2.998%, which is why the two ended effectively level.

Do I pay tax on principal growth I haven’t received?

Yes, federally, in the year it accrues. In Dana’s 2023 the accrual was $390.00 against $112.68 of cash coupons, so she reported $502.68 of interest income and received $112.68. State and local income tax does not apply to Treasury interest, including the accrual.

What happens if the CPI is revised or the index gets discontinued?

Treasury uses the CPI-U as first published and does not restate for later revisions, so a revision to a past month doesn’t change payments already made. If the index were discontinued or substantially altered, the terms provide for a substitute index determined by the Treasury. That’s a documented contingency, not a hypothetical hole, but the substitution terms are worth reading in the offering circular for any specific security.

What to look at next

Three things worth understanding before the arithmetic here is useful to you.

Pull the actual reference CPI table for a real TIPS and recompute one of its past coupons yourself. If your number matches the payment, you understand the mechanism. If it doesn’t, the gap is almost always the three-month lag or the daily interpolation.

Look at where the current breakeven sits across maturities, because that is the number you are actually taking a view on. Buying a TIPS instead of a nominal is a statement that realised inflation will exceed the breakeven, and it’s worth being explicit with yourself about whether you hold that view.

Read the tax treatment of accruals against your own account structure. The phantom income problem is entirely a function of which account holds the security, and it changes the after-tax arithmetic more than a 25 basis point difference in real yield does.

This article is general information, not financial advice. See our disclaimer.

Read next

• Series I Bonds: How the Fixed and Inflation Rate Combine
• Bond Duration: How a 1% Rate Rise Moves Your Bond Price
• What Inflation Does to Cash Savings Over a Decade
• Social Security at 62 vs 70: Where Breakeven Lands

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