The composite rate
An I bond earns a composite rate made of a fixed rate (F) and a semiannual inflation rate (I). The rule (31 CFR 359.14) is:
composite = [F/2 + I + I × F/2] × 2 = F + 2I + F × I, rounded to 0.01%
Example from TreasuryDirect: 0.0090 + 0.0334 + 0.0001503 = 0.0425503, so 0.90% fixed and 1.67% inflation give 4.26%. The federal rule's own example: 2.00% and 0.28% give 0.025656, or 2.57%. The composite never goes below 0.00% (359.13), so deflation can pull it below the fixed rate, but a bond's value never drops from one month to the next. In May 2009 inflation was −2.78%, and bonds with a fixed rate under 2.78% earned 0.00% for 6 months.
Note: TreasuryDirect's rates page shows this example under a heading for Nov 2025 to Apr 2026 (4.03%), but the math uses the May 2026 numbers. We follow the math.
The inflation part
The inflation rate comes from the Consumer Price Index for All Urban Consumers (CPI-U), U.S. city average, all items, not seasonally adjusted. The Nov 1 rate compares September with the March before it; the May 1 rate compares March with the September before it (31 CFR 359.11):
inflation = (CPI now − CPI 6 months earlier) ÷ CPI 6 months earlier, rounded to 0.01%
| Month | CPI-U (NSA) | Used for |
|---|---|---|
| Sept 2025 | 324.800 | May 2026 rate (start) |
| Mar 2026 | 330.213 | May 2026 rate (end): 1.67%; Nov 2026 rate (start) |
| Aug 2026 | 334.980 | Latest published: +1.44% over March |
| Sept 2026 | Out Wed, Oct 14, 2026, 8:30 a.m. ET Forecast | Nov 2026 rate (end) |
Until September's index is out, the Nov 1 inflation rate is a forecast. Our default takes August's index and adds the median September change from 2014 to 2025 (+0.1877%): a September index of 335.609, which gives 1.63% and a composite of 4.17% if the fixed rate stays 0.90%. Past Septembers range from 1.29% (2015, −0.16%) to 1.98% (2017, +0.53%). A September index of 335.712 or more keeps the rate at 1.67%. Later CPI revisions are ignored (31 CFR 359 App. C). If a month's CPI isn't reported by the end of the next month, Treasury announces a substitute index based on the last 12-month change.
The fixed rate
“The Secretary, or the Secretary's designee, determines the fixed rate” (31 CFR 359.10). There is no published formula. It applies to bonds issued in the next 6 months, for each bond's whole life, and it is never below 0.00%. That's why the November fixed rate is always an Assumption on this site until Treasury announces it: we don't forecast it.
Rates are announced each May 1 and Nov 1. If Treasury is closed that day, the announcement comes the next business day, but the rates still start on the 1st (31 CFR 359.9). Nov 1, 2026 is a Sunday. Last year's November rate came out Mon, Nov 3, 2025; the May 2026 release is listed under April 2026 on TreasuryDirect's news page. The exact day and time this November are not confirmed.
When a bond's rate changes
Each bond's rate changes every 6 months, counted from its own issue month (31 CFR 359.15), and each 6-month period uses the latest rate announced when it starts. An October bond changes on Apr 1 and Oct 1; a November bond on May 1 and Nov 1. So an October 2026 bond earns 4.26% from Oct 1, 2026 to Mar 31, 2027, and gets the Nov 2026 inflation rate only from Apr 1, 2027. A November bond starts on the Nov 2026 rate right away.
The issue date is the first day of the month TreasuryDirect receives your funds (before midnight ET). Oct 31, 2026 is a Saturday, so the last business day for an October issue date is Fri, Oct 30, 2026. Funds must be received before midnight ET; end-of-month purchases may get next month's issue date.
How value grows: the $25 unit
Interest is added monthly and compounds every 6 months (31 CFR 359.16). Treasury values a $25 unit (359.39):
value = PV × (1 + rate ÷ 2)^(months ÷ 6), rounded to the cent
PV is the unit's value at the start of the current 6-month period (already rounded). A bond is then valued from the unit: amount ÷ $25 × unit value, and electronic bonds use round_half_up(amount ÷ 100 × value of a $100 bond) (359.19, 359.55). Examples from the rules: a $25 unit at 2.57% for 6 months is $25.32, so $5,000 is $5,064 (not $5,064.25); a $34.59 book-entry bond with a $100 value of 101.96 is worth $35.27. Half a cent rounds up: 25.045 → 25.05. That's why a $10,000 bond's value moves in $4.00 steps.
We compute every value exactly: rates as exact fractions, money in whole cents, and the 6th-root step checked with integer arithmetic, so no floating-point rounding decides a cent. One thing is unverified: how TreasuryDirect rounds an exact half-cent tie in the monthly step. We round it up (example: $25.50 at 4.02% for 3 months is exactly 25.755 → $25.76). A true tie needs a perfect-power growth factor and can only happen with what-if rates; official rates never hit it.
Cash-in rules
- You can't cash in before 12 months (bonds issued Feb 2003 and later). Disaster-area waivers exist.
- Cash in before 5 years and you lose the last 3 months of interest: after 18 months you get the first 15 months of interest. You never get back less than you paid.
- I bonds earn interest for 30 years. Electronic bonds are paid automatically at maturity.
- Limits: $10,000 of electronic I bonds per person per calendar year, $25 minimum, any amount to the penny. Gifts count toward the recipient's limit in the year delivered. Paper I bonds ended Jan 1, 2025.
- Interest is subject to federal income tax, not state or local income tax. Report it yearly or when you cash in.
The October vs November comparison
We value the same amount bought in October 2026 (fixed 0.90%, locked) and in November 2026 (fixed = your assumption) on the same calendar date: Nov 1 of 2026 plus your hold (Oct 1, 2056 for 30 years, when the October bond matures). Both values include the 3-month penalty when the date is under 5 years. Every announcement after Nov 2026 uses the “inflation after April 2027” assumption (by default the same as Nov 2026), for both bonds.
The break-even is the lowest November fixed rate, in steps of 0.01% from 0.00% to 5.00%, at which waiting is worth at least as much as buying in October. The November value never falls as the fixed rate rises, so a binary search finds the same answer as checking every step. We show it as a number up to 3.00%.
Forecast, calculated, official
- Forecast until BLS publishes September's CPI (Wed, Oct 14, 2026, 8:30 a.m. ET).
- Calculated from BLS data after that: the Nov 1 inflation rate from the formula above. Treasury confirms around Nov 1.
- Official once Treasury announces the November rates.
This page switches when we update the data and publish, never by your device's clock. Right now it shows: Forecast.
Rate history
Fixed and semiannual inflation rates of every announcement since Sept 1998, and the composite for new bonds.
Show all 57 announcements
| Announced | Fixed | Inflation (6 months) | Composite, new bonds |
|---|---|---|---|
| May 2026 | 0.90% | 1.67% | 4.26% |
| Nov 2025 | 0.90% | 1.56% | 4.03% |
| May 2025 | 1.10% | 1.43% | 3.98% |
| Nov 2024 | 1.20% | 0.95% | 3.11% |
| May 2024 | 1.30% | 1.48% | 4.28% |
| Nov 2023 | 1.30% | 1.97% | 5.27% |
| May 2023 | 0.90% | 1.69% | 4.30% |
| Nov 2022 | 0.40% | 3.24% | 6.89% |
| May 2022 | 0.00% | 4.81% | 9.62% |
| Nov 2021 | 0.00% | 3.56% | 7.12% |
| May 2021 | 0.00% | 1.77% | 3.54% |
| Nov 2020 | 0.00% | 0.84% | 1.68% |
| May 2020 | 0.00% | 0.53% | 1.06% |
| Nov 2019 | 0.20% | 1.01% | 2.22% |
| May 2019 | 0.50% | 0.70% | 1.90% |
| Nov 2018 | 0.50% | 1.16% | 2.83% |
| May 2018 | 0.30% | 1.11% | 2.52% |
| Nov 2017 | 0.10% | 1.24% | 2.58% |
| May 2017 | 0.00% | 0.98% | 1.96% |
| Nov 2016 | 0.00% | 1.38% | 2.76% |
| May 2016 | 0.10% | 0.08% | 0.26% |
| Nov 2015 | 0.10% | 0.77% | 1.64% |
| May 2015 | 0.00% | −0.80% | 0.00% |
| Nov 2014 | 0.00% | 0.74% | 1.48% |
| May 2014 | 0.10% | 0.92% | 1.94% |
| Nov 2013 | 0.20% | 0.59% | 1.38% |
| May 2013 | 0.00% | 0.59% | 1.18% |
| Nov 2012 | 0.00% | 0.88% | 1.76% |
| May 2012 | 0.00% | 1.10% | 2.20% |
| Nov 2011 | 0.00% | 1.53% | 3.06% |
| May 2011 | 0.00% | 2.30% | 4.60% |
| Nov 2010 | 0.00% | 0.37% | 0.74% |
| May 2010 | 0.20% | 0.77% | 1.74% |
| Nov 2009 | 0.30% | 1.53% | 3.36% |
| May 2009 | 0.10% | −2.78% | 0.00% |
| Nov 2008 | 0.70% | 2.46% | 5.64% |
| May 2008 | 0.00% | 2.42% | 4.84% |
| Nov 2007 | 1.20% | 1.53% | 4.28% |
| May 2007 | 1.30% | 1.21% | 3.74% |
| Nov 2006 | 1.40% | 1.55% | 4.52% |
| May 2006 | 1.40% | 0.50% | 2.41% |
| Nov 2005 | 1.00% | 2.85% | 6.73% |
| May 2005 | 1.20% | 1.79% | 4.80% |
| Nov 2004 | 1.00% | 1.33% | 3.67% |
| May 2004 | 1.00% | 1.19% | 3.39% |
| Nov 2003 | 1.10% | 0.54% | 2.19% |
| May 2003 | 1.10% | 1.77% | 4.66% |
| Nov 2002 | 1.60% | 1.23% | 4.08% |
| May 2002 | 2.00% | 0.28% | 2.57% |
| Nov 2001 | 2.00% | 1.19% | 4.40% |
| May 2001 | 3.00% | 1.44% | 5.92% |
| Nov 2000 | 3.40% | 1.52% | 6.49% |
| May 2000 | 3.60% | 1.91% | 7.49% |
| Nov 1999 | 3.40% | 1.76% | 6.98% |
| May 1999 | 3.30% | 0.86% | 5.05% |
| Nov 1998 | 3.30% | 0.86% | 5.05% |
| Sept 1998 | 3.40% | 0.62% | 4.66% |
How we checked it
Our calculator reproduces, to the cent and the basis point:
- all 1,653 composite rates in TreasuryDirect's I bond rate chart (every issue window × every announcement);
- 34 bond values from TreasuryDirect's Savings Bond Calculator ($50 to $5,000, issued 1998 to 2025);
- all 57 inflation rates from the CPI values Treasury used;
- the examples in 31 CFR 359 and the last three Fiscal Service press releases.
The comparison itself uses assumptions: the November fixed rate and future inflation. Values for past dates match TreasuryDirect; values for future dates are projections. Try it on your own bond or the October vs November comparison. Treasury's own page: I bonds interest rates (opens in a new tab).