I Bond Rate Calculator
I bond growth from composite rate and years, with early penalty.
What it does
Series I bonds pair a fixed rate that never moves with an inflation rate reset every 6 months, so savings hold purchasing power while prices shift. Interest accrues monthly and compounds semiannually for up to 30 years.
The tool above takes buy amount with composite rate and years held. It returns bond value, interest earned, and the early penalty line. The default $1,000 at 4.26% for 5 years gives $1,234.62 with $234.62 interest and no penalty.
Rules and numbers follow TreasuryDirect pages: composite as fixed plus twice semiannual inflation plus their product, monthly accrual with semiannual compounding, redemption locked 12 months, and 3 months interest lost under 5 years. The worked example on those pages builds 4.26% from 0.90% fixed with 1.67% inflation exactly as this page does.
Taxes split by level: federal income tax yes with education exclusions possible, state and local no. Electronic buys cap at $10,000 per person per year from a $25 minimum.
How to use
- Type the buy amount, $25 and up.
- Type the composite rate for the issue window.
- Type whole years held, 1 to 30.
- Read value first, interest beside it.
- Check the penalty line before early redemption.
- Confirm current rates each May and November.
Formula
- composite
- Annualized combined fixed plus inflation rate.
- periods
- Semiannual compounding steps minus penalty half step.
- penalty
- Last 3 months interest lost when cashing under 5 years.
Composite itself is fixed plus twice semiannual inflation plus fixed times inflation, per Treasury. Rate resets apply per bond on its own 6 month clock from issue month.
Worked example
Example: $1,000 at 4.26% for 5 years
Defaults: $1,000, 4.26%, 5 years.
- Half rate: 4.26 / 2 = 2.13% per period.
- Periods: 5 x 2 = 10, no penalty cut at 5 years.
- Growth: 1.0213^10 = 1.23462.
- Value: 1,000 x 1.23462 = $1,234.62.
- Interest: 1,234.62 minus 1,000 = $234.62.
- Tool shows $1,234.62, $234.62, no penalty.
Reference table
| Item | Value | Source |
|---|---|---|
| Composite May-Oct 2026 | 4.26% | TreasuryDirect |
| Fixed May 2026 | 0.90% | TreasuryDirect |
| Inflation May 2026 | 1.67% | TreasuryDirect |
| Inflation Nov 2025 | 1.56% | TreasuryDirect |
| Fixed May 2025 | 1.10% | TreasuryDirect |
| Lock and penalty | 12 mo lock, 3 mo lost under 5 yr | TreasuryDirect |
| Buy limits | $25 min, $10,000/yr electronic | TreasuryDirect |
Limits
- Fixed composite assumed; real resets every 6 months.
- Taxes excluded from value.
- Education exclusion rules ignored.
- Deflation floor at zero not modeled.
Common mistakes
- Compounding annually instead of semiannually.
- Forgetting the 3 month haircut under 5 years.
- Pricing redemption inside 12 months.
- Mixing fixed with composite in math.
- Ignoring the $10,000 yearly cap.
Try it yourself
Quick checks. Answers sit tucked under each question.
$1,000 at 4.26% for 1 year?
$1,032.12 with $10.93 lost to penalty.
$5,000 at 4.26% for 5 years?
$6,173.10 with $1,173.10 interest.
$10 buy?
Rejected: enter $25 to $1000000.
Questions
Default value?
$1,234.62 with $234.62 interest, no penalty.
Composite math?
Fixed plus twice semiannual inflation plus their product: 0.90, 1.67, and 4.26 now.
Early penalty?
Last 3 months interest lost when cashing under 5 years.
Lock period?
No redemption in the first 12 months.
Buy limits?
$25 minimum, $10,000 electronic per person per year.
Taxes?
Federal yes, state and local no; education use may exclude.