I Bond Calculator

Find the composite rate for a Series I savings bond and estimate what it will be worth after a number of months if rates stay the same.

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Rates in percent. Use the rates for the month you bought the bond.
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Value when cashed
Composite rate
Interest earned
Early-cash penalty
Rules
Work

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How I Bond Interest Works

Composite rate = fixed + (2 × semiannual inflation) + (fixed × semiannual inflation)
Value = purchase × (1 + composite ÷ 2)^(months ÷ 6)

This is the Treasury’s formula. It gives 3.11% for Nov 2024 (1.20% fixed, 0.95% inflation), 3.98% for May 2025 (1.10%, 1.43%), 4.03% for Nov 2025 (0.90%, 1.56%) and 4.26% for May 2026 (0.90%, 1.67%). Interest is added monthly and compounds every 6 months. The fixed rate stays with the bond for 30 years; the inflation rate changes every May and November. The composite rate can’t go below zero.

Worked Example

Tips

You must hold an I bond for 12 months. If you cash it before 5 years, you lose the last 3 months of interest. The limit is $10,000 in electronic I bonds per person per calendar year at TreasuryDirect. Interest is exempt from state and local tax, and federal tax can be deferred until you cash the bond. New rates are announced each May 1 and November 1, so check TreasuryDirect for the current ones. TreasuryDirect’s own values round to the cent at each step and may differ by a few cents.