How do you estimate inflation-adjusted TIPS principal?
Multiply a TIPS security’s original principal by its index ratio for the relevant date. TreasuryDirect explains that the principal can rise or fall with inflation and deflation, and its index-ratio lookup is organized by security and date. Enter the ratio that applies to your security; this page does not fetch or infer it.
For example, an original principal of $1,000 and an entered index ratio of 1.01165 produce $1,011.65 of adjusted principal. That is the basis for the next payment only when that ratio corresponds to the interest payment date. A different date can have a different ratio.
How is a semiannual TIPS coupon estimated?
Multiply the adjusted principal for the payment date by one-half of the security’s stated annual coupon rate. With $1,011.65 adjusted principal and a 0.125% annual coupon, the modeled payment is approximately $0.63: $1,011.65 × 0.00125 ÷ 2. TreasuryDirect’s example uses these same inputs and calculation steps. The tool shows an estimate to cents; actual payment rounding and records should be checked with Treasury.
What principal is paid at maturity?
TreasuryDirect says that at maturity a TIPS pays the inflation-adjusted principal if that amount exceeds original principal; otherwise, it pays original principal. The calculator applies that floor to a maturity-date index ratio that you enter. It does not assume that today’s ratio will remain unchanged through maturity. The maturity estimate is not a present-day sale value.
Where do I find an index ratio?
Use TreasuryDirect’s TIPS/CPI Indexes page and select the CUSIP and date relevant to the payment or maturity calculation. The site also links to daily index ratios and historical CPI information. Check that the selected ratio is for the right security and date before relying on a result.
Sources checked 2026-11-03: TreasuryDirect, TIPS/CPI Indexes, which describes the date-specific ratio and coupon calculation; and TreasuryDirect, Treasury Inflation-Protected Securities, which explains principal adjustments, semiannual interest, and the maturity principal floor.
Frequently asked questions
Does this calculator look up a current index ratio?
No. Enter the ratio from TreasuryDirect for the specific CUSIP and date you need.
Can adjusted principal fall below original principal before maturity?
Yes. TIPS principal can move up or down with inflation and deflation. The maturity floor applies to the amount paid at maturity.
Does the coupon rate change with inflation?
The stated interest rate is fixed. The dollar payment varies because interest is calculated on inflation-adjusted principal.
Is the maturity amount a guaranteed sale price?
No. It is a modeled maturity payment using your entered maturity ratio and the principal floor. It does not estimate a secondary-market price before maturity.