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Calcimator

I-Bond Calculator

Calculate Series I savings bond value from fixed rate, inflation rate, holding period, and early redemption penalty.

About this calculator

This calculator implements TreasuryDirect's actual published Series I savings bond formula: composite rate = fixed rate + (2 x semiannual inflation rate) + (fixed rate x semiannual inflation rate). Since Inflation rate (%) here is entered as an annualized figure (twice the semiannual CPI-U change), the calculator derives the semiannual rate by dividing it by 2 before applying the formula's cross term. Both Fixed rate (%) and Inflation rate (%) always raise Composite Rate -- there's no scenario in this formula where increasing either one lowers the composite rate, since every term in the real Treasury formula is additive or has a non-negative coefficient. The composite rate is floored at 0% (a real TreasuryDirect guarantee -- I Bonds cannot lose principal value from a negative rate), then compounded semiannually over Holding Period to project Redemption Value.

If Holding Period is under 5 years, the bond forfeits its most recent 3 months of interest upon redemption -- a real Treasury rule -- which this calculator applies as Early Redemption Penalty. This calculator assumes the fixed rate and inflation rate you enter stay constant for the entire holding period, but in reality the inflation-rate component resets every 6 months based on new CPI-U data, and the fixed rate only applies to bonds purchased in that specific 6-month issuance window -- so a real bond's actual return will differ from this projection once rates change. Purchase Amount has no effect on Composite Rate, Effective Annual Yield, or the tax bracket used, since none of those figures depend on how much you invest -- only the dollar totals scale with it.

Inputs

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%
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Results

Redemption value ($)

$12,080.59

≈ 8 months of rent

Composite rate (%)3.82%
Total interest earned ($)$2,080.59
Early penalty ($)$0.00
After-tax value ($)$11,622.86
Effective annual yield (%)3.85%

Figures current as of 2026. Source: U.S. Department of the Treasury, TreasuryDirect, I bonds interest rates

How to Use This Calculator
  1. Enter your I-Bond purchase amount (up to $10,000 per person per year from TreasuryDirect).
  2. Input the fixed rate and current inflation rate — check TreasuryDirect.gov for current rates.
  3. Set your expected holding period in years and your federal income tax bracket.
  4. Review Composite Rate, Redemption Value, and Total Interest to see your projected return.
  5. Check Early Redemption Penalty and Effective Annual Yield if you may need to cash out before 5 years.

How the result changes with Purchase amount ($)

Purchase amount ($)Redemption value ($)
5,000$6,040.29
7,500$9,060.44
10,000$12,080.59

What each input means

Purchase amount ($)
I-Bond purchase amount ($25 min, $10,000/year max).
Fixed rate (%)
Fixed rate component (set at purchase, lasts 30 years).
Inflation rate (%)
Annual CPI-U inflation rate (adjusts every 6 months).
Holding period (years)
Years you plan to hold (min 1 year, penalty if < 5).
Federal tax bracket (%)
Your federal income tax bracket.

What each result means

Composite rate (%)
Combined fixed + inflation rate.
Redemption value ($)
Value at redemption after any penalty.
Total interest earned ($)
Interest earned over holding period.
Early penalty ($)
3-month interest penalty if held < 5 years.
After-tax value ($)
Value after federal income tax on interest.
Effective annual yield (%)
Actual annualized return after penalties.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    5 parameters
    Purchase amount ($) = 10000, Fixed rate (%) = 1.3, Inflation rate (%) = 2.5, Holding period (years) = 5, Federal tax bracket (%) = 22 = 5 input(s) provided
  2. Calculate Redemption value
    Redemption value
    12080.59 = $12,080.59
  3. Calculate Composite rate
    Composite rate
    3.82 = 3.82%
  4. Calculate Total interest earned
    Total interest earned
    2080.59 = $2,080.59

Figures and sources

Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

Why do both Fixed Rate and Inflation Rate always raise Composite Rate?

TreasuryDirect's real formula is composite rate = fixed rate + (2 x semiannual inflation rate) + (fixed rate x semiannual inflation rate) -- every term is either directly additive or a product of two non-negative rates. Since neither Fixed Rate nor Inflation Rate can be negative enough to flip a term negative within this calculator's allowed ranges, raising either one always raises (or leaves unchanged) the resulting Composite Rate.

Can my I-Bond actually lose value if inflation goes negative?

No. TreasuryDirect guarantees the composite rate can never fall below 0%, even during a period of deflation that would otherwise produce a negative calculated rate. This calculator applies that same floor -- Composite Rate is never displayed as negative, and your redemption value can never fall below your original purchase amount from rate effects alone.

Does Purchase Amount affect my Composite Rate or Effective Annual Yield?

No. Composite Rate depends only on Fixed Rate and Inflation Rate, and Effective Annual Yield is calculated from the ratio of Redemption Value to Purchase Amount, which cancels out the purchase amount's scale entirely. Purchase Amount only changes the absolute dollar figures -- Redemption Value, Total Interest Earned, and the rest -- not any of the rate or percentage outputs.

Why does my Redemption Value drop if I cash out before 5 years?

TreasuryDirect imposes a real penalty for redeeming an I Bond before it's held 5 years: you forfeit the most recent 3 months of interest. This calculator applies that same penalty whenever Holding Period is under 5 years, which is why Redemption Value and Effective Annual Yield are both lower for an early redemption than a same-rate bond held past the 5-year mark.

Will my actual I-Bond return match what this calculator projects?

Not exactly. This calculator assumes the Fixed Rate and Inflation Rate you enter stay constant for the whole holding period, but TreasuryDirect actually resets the inflation-rate component every 6 months based on new CPI-U data, so a real bond's composite rate changes over time. Treat this as a snapshot projection using today's rates, not a guaranteed multi-year forecast.

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