Treasury I Bond Interest Calculator
A Series I savings bond earns interest from two parts: a fixed rate that stays the same for the life of the bond, and an inflation rate the Treasury resets every six months. The combined return is the composite rate, computed with the exact TreasuryDirect formula. This calculator takes the fixed rate and the announced semiannual inflation rate, applies the official equation including the small cross-product term, floors the result at zero, and shows your composite annual rate along with an estimate of six months of interest on a principal you choose. Always confirm the current fixed and inflation rates on TreasuryDirect before relying on the figures.
I bond composite rate formula
f = fixed rate (decimal), i = semiannual inflation rate (decimal)
Composite = f + (2 * i) + (f * i)
If composite below 0, composite = 0
6-month interest = principal * (composite / 2)
12-month interest = principal * composite
This is the exact TreasuryDirect equation. The composite rate applies to the bond for the six months after its semiannual reset, then a new inflation rate applies. Interest compounds semiannually in practice.
US I bond context
- I bonds are sold electronically through TreasuryDirect and earn interest for up to 30 years.
- The fixed rate is locked for the life of a given bond; the inflation rate resets every May and November.
- Interest is added to the bond value monthly and compounds semiannually.
- Bonds cashed before five years forfeit the most recent three months of interest.
- I bond interest is exempt from state and local income tax and may be tax-free for qualified education expenses.
I bond interest: frequently asked questions
How is the I bond composite rate calculated?
TreasuryDirect uses the formula: composite rate = fixed rate + (2 times semiannual inflation rate) + (fixed rate times semiannual inflation rate). The result is rounded to two decimal places. If the composite rate computes below zero it is set to zero, so an I bond can never lose value.
What is the fixed rate on an I bond?
The fixed rate is set by the Treasury for each six-month issue period (May and November) and stays the same for the entire 30-year life of that bond. This calculator uses the fixed rate you enter as a user-editable input; check the current rate on the TreasuryDirect rates page.
What is the semiannual inflation rate?
The Treasury announces a semiannual inflation rate every May and November based on the change in the Consumer Price Index for All Urban Consumers (CPI-U). It applies to every I bond for the following six months regardless of when the bond was bought.
Why is the inflation component doubled in the formula?
The published semiannual inflation rate covers a six-month period. The composite-rate formula expresses an annualized rate, so the semiannual figure is multiplied by two, plus a small cross-product term with the fixed rate.
Can an I bond rate be negative?
No. If deflation pushes the computed composite rate below zero, the Treasury floors it at zero percent for that period. Your principal is protected and never declines.
Official sources
- U.S. TreasuryDirect: Series I Savings Bonds.
- U.S. TreasuryDirect: I Bonds Interest Rates.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.