---
url: https://thresholds.pages.dev/i-bonds/calculator/
title: "I bond calculator: value, penalty and best month"
updated: 2026-09-07
site: Thresholds
sources:
  - https://www.treasurydirect.gov/savings-bonds/i-bonds/i-bonds-interest-rates/
  - https://www.treasurydirect.gov/savings-bonds/i-bonds/
---

# I bond calculator: value, penalty and best month

Enter the issue month and what you paid: the rate your bond earns now, an estimated value, the three-month penalty and the best month to cash out.

A client-side calculator for one Series I savings bond. Inputs: the issue month (September 1998 onward) and the amount paid. Outputs: the bond's fixed rate, the composite rate it is earning in its current six-month period, the rate for its next period or an explicit "not announced yet", an estimated value, the three-month early-redemption penalty, the net if cashed today, the dates it unlocks and becomes penalty-free, final maturity, a bar for every six-month period it has earned, a ranking of the next eighteen months, and a period-by-period interest ledger.

The rules it applies: an I bond cannot be redeemed for 12 months; redeeming from then until five years costs the last 3 months of interest; after five years there is no penalty; the bond stops earning after 30 years. The minimum electronic purchase is $25 and the annual limit is $10,000 per SSN or EIN. The composite rate is fixed rate + 2 × semiannual inflation + (fixed × semiannual inflation ÷ 100), floored at 0%.

Dollar amounts are estimates: Treasury rounds to the penny on a per-$25 basis with its own conventions, so the official value is the one in your TreasuryDirect account. Rates and dates are exact. Rate periods past the latest Treasury announcement are labelled, never filled in with a guess. Nothing typed into the page is uploaded; shared links carry the two numbers after the # only.

## Sources

- [TreasuryDirect — I bonds interest rates](https://www.treasurydirect.gov/savings-bonds/i-bonds/i-bonds-interest-rates/)
- [TreasuryDirect — I bonds](https://www.treasurydirect.gov/savings-bonds/i-bonds/)

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Information, not advice. Figures verified 2026-09-07. https://thresholds.pages.dev/i-bonds/calculator/