TIPS Mechanics
TIPS Mechanics
TIPS (Treasury Inflation-Protected Securities) adjust both principal and coupon monthly using the Consumer Price Index, ensuring every payment keeps pace with inflation.
Key takeaways
- TIPS principal is multiplied monthly by the ratio of current CPI to the original issuance CPI
- Coupons are paid semi-annually on the inflation-adjusted principal, creating a dual inflation benefit
- The ratio used includes a three-month lag, so recent inflation feeds into payments with a delay
- If deflation occurs, principal can fall but is guaranteed to return to par at maturity (deflation floor)
- Interest accrual on the adjusted principal is taxed annually in the US, creating the phantom-income problem
The inflation-adjusted principal formula
When the US Treasury issues a TIPS, it assigns a reference CPI value—the Consumer Price Index from the month prior to issuance. Let's say a 10-year TIPS is issued in June 2024 with reference CPI = 320.
Each month, the Treasury calculates an inflation ratio:
Inflation Ratio = Current CPI / Reference CPI
If CPI in July 2024 is 322, the ratio is 322 / 320 = 1.00625. The bond's principal immediately becomes its par value multiplied by this ratio. A $1,000 TIPS becomes $1,006.25. The coupon, paid semi-annually, is calculated on this adjusted principal, not the original $1,000.
Three months later, in October 2024, the CPI used in the adjustment is actually from July 2024 (three-month lag). This lag is built into the Treasury's calculation to allow time for CPI data to be finalized. The lag means recently-published inflation figures don't immediately boost TIPS payments—there is a three-month delay between when inflation occurs and when TIPS investors feel its full effect.
Coupon payments on adjusted principal
Consider a TIPS issued at $100,000 par with a 1.5% coupon rate. In normal years without adjustment, this would pay $750 semi-annually ($100,000 × 1.5% ÷ 2 = $750). But TIPS coupons are paid on the adjusted principal.
If over six months the inflation ratio grew from 1.0 to 1.025 (2.5% inflation), the adjusted principal becomes $102,500. The semi-annual coupon paid is $102,500 × 1.5% ÷ 2 = $768.75, not $750. The difference ($18.75) reflects the inflation adjustment to principal multiplied by the coupon rate. Over 20 years, this compounding of inflation adjustments into coupon payments is why TIPS investors receive the full inflation hedge.
The deflation floor and principal floor protection
If deflation occurs (the CPI falls), the TIPS principal adjusts downward. The same formula applies: if CPI drops 2% over a six-month period, the inflation ratio falls to 0.98, and a $100,000 TIPS principal becomes $98,000.
However, TIPS have a deflation floor: the principal is guaranteed never to fall below par (the original $100,000 in this example). At maturity, if the inflation-adjusted principal is below par, the investor receives par. This protects against severe deflation scenarios.
The floor means TIPS investors are essentially long deflation risk. In deflationary scenarios like Japan's 1990s or the depths of the 2008 financial crisis, TIPS principal floors kicked in and protected investors. In normal inflationary periods, the floor is never reached. Economically, the floor is a small insurance cost—the real yield on TIPS is slightly lower than it would be without deflation protection, reflecting this insurance value.
Accrued interest and the lag period
TIPS accrued interest is added to principal monthly but paid out semi-annually in the coupon. This accrued interest amount (the difference between adjusted principal and the previous month's adjusted principal) is taxed as income each year for US tax purposes, even though you don't receive the cash until the semi-annual coupon date (or maturity). This is the phantom-income problem discussed in depth in a later article.
An example: A $100,000 TIPS with a 1% coupon issued in January 2024. In January, inflation accrues 0.2%, so principal adjusts to $100,200. In February, inflation accrues 0.25%, so principal adjusts to $100,450. The difference, $250, is phantom income—taxed in 2024 even though you don't receive it until the June semi-annual coupon payment.
Purchase methods and settlement
TIPS are issued by auction every few months (regular maturity dates include 5-year, 10-year, and 30-year). The Treasury publishes an auction announcement with the coupon rate and reference CPI. Investors bid competitively or non-competitively, and accepted bids settle on the issue date.
On the secondary market, TIPS trade like any Treasury bond, but the price quote reflects only the base principal (the original par), not the inflation-adjusted principal. A TIPS quoted at 102 ("102-00" in Treasury nomenclature) means 102% of the inflation-adjusted principal, not 102% of par. A TIPS worth $120,000 due to inflation adjustments trading at 102 price would cost the buyer $122,400 ($120,000 × 1.02). This can confuse retail investors who expect to multiply the quoted price directly by par.
Real yield and the auction process
At auction, the Treasury announces the coupon rate and investors bid for yield. The winning bid sets the real yield that investors accept. In May 2024, a 10-year TIPS issued with a 1.5% coupon at par means the real yield to maturity is 1.5%. If the same bond trades on the secondary market a week later and is quoted at 99, its real yield has risen (because price fell), attracting buyers who believe inflation will exceed expectations.
The spread between the coupon rate at issuance and the real yield at issuance reflects the auction conditions. If demand is strong, TIPS might be issued at a premium (coupon rate above the winning real yield), and if demand is weak, they might be issued at a discount. Over the long run, real yields on TIPS follow the market's inflation expectations and reflect supply and demand.
Index ratio and accrual dates
The Treasury maintains an index ratio for each TIPS daily. This ratio is used to calculate both the adjusted principal (for pricing and display purposes) and the coupon payments. The ratio is reset every six months on the semi-annual coupon date. If you buy a TIPS between coupon dates, you pay accrued interest reflecting the inflation adjustment since the last coupon payment.
For example, if you buy a TIPS a quarter of the way between semi-annual coupon dates, and the inflation ratio has moved 0.3% since the last coupon, the accrued interest you pay reflects that 0.3% plus the partial coupon not yet paid. This ensures the seller is compensated for the inflation adjustment earned but not yet distributed, and the buyer's cost basis reflects the true inflation-adjusted value at that moment.
TIPS in taxable and tax-deferred accounts
Because phantom income (the annual accretion of principal) is taxable each year, TIPS are most tax-efficient in retirement accounts (401(k), IRA) where the tax is deferred. In taxable accounts, the annual phantom-income tax bill can consume a large fraction of the real return, especially when real yields are low (0.5% or less). Investors in high tax brackets often hold TIPS only in tax-deferred accounts for this reason, and nominal bonds in taxable accounts to avoid the phantom-income drag.
Treasury Direct (the Fed's online platform) allows individuals to buy TIPS in their non-competitive tenders at no cost. This avoids broker commissions and is ideal for buy-and-hold investors who plan to hold TIPS to maturity in tax-deferred accounts.
Next
The mechanics of TIPS are precise and unforgive—understanding the reference CPI, the lag periods, and the phantom-income calculation is essential before committing capital. The next article dives deeper into the CPI adjustment mechanism and how the lag period shapes TIPS returns in different economic environments.