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I Bond Calculator

Enter your I Bond purchase amount, current composite rate, and hold period to see final value and effective APY.

Updated June 2026 · Editorial standards

I Bond Details

$
%
yrs
Final Value
$12,358.29
Total Interest
$2,358.29
Effective APY
4.3300%

$10,000.00 in I Bonds at 4.28% grows to $12,358.29 after 5 years — earning $2,358.29 in tax-deferred interest. Effective APY: 4.3300%.I Bond interest is exempt from state and local taxes, and federal tax is deferred until redemption (or can be excluded if used for education). The composite rate adjusts every 6 months to track inflation.

By the KalkWise Editorial Team Reviewed for accuracy Updated June 2026

What is the i bond calculator — inflation-protected savings?

In short

I Bonds are U.S. savings bonds that pay a composite rate = fixed rate + inflation adjustment, reset every 6 months. Maximum purchase is $10,000/year per person via TreasuryDirect. Interest is tax-deferred and exempt from state/local taxes.

Calculates I Bond final value, total interest earned, and effective APY over your chosen hold period.

How to use this calculator

  1. 1Enter purchase amount (max $10,000/year per person).
  2. 2Enter current composite rate from TreasuryDirect.gov.
  3. 3Enter years to hold (minimum 1 year; early redemption before 5 years forfeits 3 months' interest).

The formula

value=P×(1+r)2n

r = semi-annual rate = annual composite rate ÷ 2; n = years

Value = P × (1 + r)^n; Effective APY = (1 + r)² − 1
P
— Principal
r
— Semi-annual composite rate (annual rate ÷ 2)
n
— Periods (years × 2)

Worked example

The scenario

$10,000 I Bond, 4.28% composite rate, held 5 years.

gives

The result

Final value ≈ $12,363. Total interest ≈ $2,363. Effective APY = 4.37%.

Common use cases

  • Park emergency fund in inflation-protected savings.
  • Save for education (interest may be tax-free if used for qualified expenses).
  • Diversify fixed income with inflation protection.
  • Compare to HYSA or CD rates for short-to-medium term savings.

Limitations & assumptions

  • Maximum $10,000/year per Social Security number ($20,000 for married couples with separate accounts).
  • Cannot redeem within 12 months of purchase.
  • Forfeit 3 months' interest if redeemed before 5 years.
  • Rate changes every 6 months — future rates are unknown.

Frequently asked questions

Every 6 months (May 1 and November 1). Each bond you own earns its rate for 6 months before resetting. The fixed rate component stays the same; the inflation adjustment changes.

Disclaimer: KalkWise calculators are provided for general informational and educational purposes only and do not constitute financial, investment, tax, or legal advice. Results are estimates based on the figures you enter and the assumptions described above. Actual outcomes will vary. Consult a qualified professional before making financial decisions.