Series I Bond Composite Rate Calculator

Estimate the upcoming Series I savings bond composite rate by pairing the Treasury fixed rate with the new CPI-U reading. The calculator also translates that percentage into six-month and first-year interest on a custom investment amount, with an optional prorated window if you expect to redeem before 12 months.

Fixed rate announced by TreasuryDirect, expressed as a percentage.
Semiannual inflation factor from the CPI-U release used for May/November resets.
Optional. Used to translate the composite rate into dollar interest.
Optional. Use to model cashing out before a full year; interest is prorated.

For education only. Confirm official I Bond rates and redemption rules with TreasuryDirect before investing.

Examples

  • Fixed rate 0.90%, CPI-U change 1.25%, $10,000 held 12 months ⇒ Composite rate 3.42% • Six-month credit $171.00 USD • Estimated interest over 12 months $342.25 USD • Projected value after 12 months $10,342.25 USD • Prorated yield 3.42%.
  • Fixed rate 1.30%, CPI-U change −0.15%, $5,000 held 9 months ⇒ Composite rate 2.30% • Six-month credit $57.50 USD • Estimated interest over 9 months $86.25 USD • Projected value after 9 months $5,086.25 USD • Prorated yield 1.72%.

FAQ

Which CPI-U figure should I enter?

Use the semiannual inflation component published each April and October—multiply the percentage change by 100 so a 1.25% change is entered as 1.25, not 0.0125.

Does this model the three-month interest forfeiture for redemptions before five years?

No. The calculator shows gross interest. Subtract the last three months of interest manually if you plan to redeem before year five.

Can I analyze multiple investment amounts?

Yes. Adjust the optional investment field for each scenario—the composite percentage stays the same, but dollar interest scales with the principal you input.

How often do Treasury fixed rates change?

Treasury typically resets the fixed rate each May and November. Enter the current announced rate to forecast the next composite.

Additional Information

  • Composite rate formula follows Treasury guidance: fixed rate + (2 × inflation) + (2 × fixed × inflation).
  • Interest is earned monthly and compounded semiannually; the tool assumes straight-line accrual within the first year for clarity.
  • Negative inflation readings can drive the composite rate lower but never below zero for six-month credits because Treasury floors the rate at 0.00%.

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