I Bond Value Calculator
Growth of a Series I savings bond at its composite rate
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Working out growth of a Series I savings bond at its composite rate is easier with I Bond Value Calculator, which answers as you type. Fill in Purchase Amount ($), Composite Rate (%) and Years Held on I Bond Value and read your answer immediately. I Bond Value prints the result and its working. I Bond Value replaces guesswork. I Bond Value runs in your browser. Finance data stays in the tab. I Bond Value is part of the Finance collection on CalcProMaster, alongside i bond calculator series i savings bond composite rate inflation, free online i bond value calculator and more. The labels in I Bond Value match the terms in its result. Try I Bond Value now and keep it handy.
What the defaults grow to
$1,000 of Series I bonds at a 4% composite rate is worth about $1,216.65 after five years.
That is $216.65 of interest. The composite rate combines a fixed rate set at purchase with an inflation rate that resets twice a year, so the 4% here is an assumption rather than a guarantee.
The formula
Value = principal × (1 + composite rate)years.
1.04⁵ = 1.21665, so $1,000 × 1.21665 = $1,216.65. This is annual compounding, a simplification of the semi-annual accrual Treasury actually applies.
Worked example
Principal $1,000
Composite rate 4%
Years 5
(1.04)⁵ = 1.21665
Value = $1,216.65
Interest = $216.65
How the composite rate is actually built
Series I bonds carry a fixed rate for the life of the bond plus an inflation rate that is reset every May and November based on CPI-U. The composite is roughly the fixed rate plus twice the half-year inflation rate plus their product — which is why the headline rate can move sharply between periods.
The rules that change the answer
- One-year lockout. You cannot redeem at all in the first 12 months.
- Three-month penalty. Redeeming between one and five years forfeits the last three months of interest.
- Five years. After that, redemption is penalty-free — which is exactly the horizon this default uses.
- Tax. Interest is exempt from state and local tax but taxable federally, and you can defer it until redemption.
- Purchase limits. Annual caps apply per person, so the model is per holding rather than unlimited.
What this estimate leaves out
The inflation component resets, so a flat 4% for five years is unlikely. Deflation can also push the composite rate to its floor of zero rather than negative, which protects the principal but stops the growth.
Frequently Asked Questions
Why is $1,000 worth $1,216.65 after five years at 4%?
Because 1.04 raised to the fifth power is 1.21665. Multiplying the principal by that gives $1,216.65, or $216.65 of interest.
Is the 4% rate fixed for five years?
No. The fixed component is locked at purchase, but the inflation component resets every May and November. A flat 4% is an assumption for illustration.
What happens if I cash out early?
You cannot redeem in the first 12 months. Between one and five years you forfeit the last three months of interest; after five years there is no penalty.
Is the interest taxable?
Federally, yes — though you can defer reporting until redemption. State and local income tax do not apply to Series I interest.
Can the value fall?
No. The composite rate floors at zero during deflation rather than going negative, so the redemption value does not drop below the accrued principal.
How is the composite rate calculated?
Roughly the fixed rate plus twice the half-year inflation rate plus the product of the two. With a 1.3% fixed rate and 1.35% half-year inflation, the composite is about 4.02% — which is what the 4% default here is standing in for.
How long until the bond doubles?
At a 4% composite rate, about 18 years by the rule of 72 (72 / 4). A 6% composite would halve that to roughly 12 years, which shows how much the inflation component matters over a long holding.
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