I Bond Calculator: How to Use It & What It Shows

The TreasuryDirect electronic I bond calculator works out the current value of a paper or electronic Series I savings bond, and you get back its redemption amount as of the date you choose. You enter the bond series, denomination, serial number (for paper bonds) or issue date (for electronic bonds), and the calculator returns the interest earned and the total value. It is the official tool from the U.S. Treasury for checking what your I bond is worth today, and it also projects future values if you ask for a specific date.

Formula

The I bond value is computed with the composite rate formula set by the U.S. Treasury. The composite rate combines the fixed rate (which stays the same for the life of the bond) and the semiannual inflation rate (which changes every May and November). The formula is:

Composite rate = fixed rate + (2 × semiannual inflation rate) + (fixed rate × semiannual inflation rate)

For a bond held t months, the value per $25 unit is:

Value = 25 × (1 + composite rate / 2)^(t / 6)

where:

• Fixed rate – the annual rate set at purchase, in percent (e.g., 1.30% means 0.013).
• Semiannual inflation rate – the CPI-U change over six months, in percent (e.g., 1.97% means 0.0197).
• t – months held since issue date.
• Composite rate / 2 – because the composite rate is an annual rate applied semiannually.

Note: The actual Treasury calculator uses the exact composite rate for each six-month period, and the fixed rate and inflation rate change over time. The simplified formula above assumes a constant composite rate for the entire holding period, which is fine for estimating but not for exact values.

Worked Example

Using the defaults: a $25 I bond held 24 months, fixed rate 1.30%, semiannual inflation rate 1.97%.

1. Composite rate = 1.30% + (2 × 1.97%) + (1.30% × 1.97%) = 1.30 + 3.94 + 0.02561 = 5.26561% (annual).
2. Half-year factor = 1 + 0.0526561 / 2 = 1.02632805.
3. Number of six-month periods = 24 / 6 = 4.
4. Value = 25 × (1.02632805)^4 = 25 × 1.10995 ≈ 27.75.
5. Interest earned = 27.75 – 25 = 2.75.

So the bond is worth about $27.75 after two years, earning $2.75 in interest. The effective annual yield is about 5.33% (since (1.02632805)^2 – 1 ≈ 0.0533).

How the Official I Bond Calculator Works

The TreasuryDirect Savings Bond Calculator is the official electronic tool. For paper bonds, you enter the series (I), denomination, serial number, and issue date. For electronic bonds, you enter the issue date and the bond’s face value. The calculator then applies the bond’s actual composite rates for each six-month period from issue to the valuation date. It also shows a table of values for each six-month period, so you can see how the bond grows.

Key details:

• I bonds are sold at face value (a $50 bond costs $50) and accrue interest monthly, compounded semiannually.
• Interest is added to the bond’s value, not paid out, until you redeem.
• The calculator uses the bond’s exact issue date and the current date (or a date you choose).
• If you redeem before 5 years, you lose the last 3 months of interest – the calculator shows the redemption value with that penalty applied.

When to Use the Calculator

Use the I bond calculator when you need to know:

• What a paper bond you found in a drawer is worth today.
• How much interest you’ve earned on an electronic bond you bought last year.
• Whether it’s a good time to redeem (e.g., after the 5-year mark to avoid the penalty).
• How much your bond will be worth at a future date, like a planned redemption.

It’s also useful for planning: you can compare the current value against what you’d get if you held longer, factoring in the 3-month penalty before 5 years.

Common Mistakes and Misconceptions

One common mistake is using the composite rate as a simple annual rate without dividing by 2 for semiannual compounding. Another is ignoring the 3-month interest penalty if you redeem before 5 years. The calculator on TreasuryDirect automatically applies the penalty for early redemption, but if you use a custom formula, you must subtract the last 3 months of interest.

Another misconception: the fixed rate is not the same as the current composite rate. The fixed rate is set at purchase and never changes, but the inflation rate changes every six months. So two bonds bought at different times can have very different composite rates even if the current inflation rate is the same.

Also, I bond interest is subject to federal income tax, but not state or local tax. The calculator shows the pre-tax value; you’ll need to report the interest when you file taxes.

Limits of the Calculator and the Formula

The official calculator is accurate for actual bond values because it uses the Treasury’s rate history. A simplified formula like the one above assumes a constant composite rate, which is only an estimate. Real I bonds have changing inflation rates, so the value will differ from a constant-rate projection.

Also, the calculator only works for bonds that have been issued at least 1 month old. You cannot redeem an I bond within the first 12 months, so the calculator won’t show a redemption value for a brand-new bond.

For paper bonds, you need the serial number; if you don’t have it, you can still estimate using the denomination and issue date, but the exact value may be off by a few cents due to rounding per $25 unit.

Alternatives to the Official Calculator

Besides the TreasuryDirect calculator, you can use:

• Spreadsheets – you can build your own with the composite rate formula and historical inflation rates.
• Third-party websites – many offer I bond calculators, but they may not be up-to-date with the latest rates. Always verify with TreasuryDirect.
• The Savings Bond Wizard (discontinued) – was a downloadable tool, but it’s been retired; the online calculator is now the standard.

For planning purposes, you can also use the Treasury’s “I Bond Rate� page to see the current fixed rate and inflation rate, then plug them into a simple future value formula.

Frequently Asked Questions

How do I find my I bond’s issue date?

For paper bonds, the issue date is printed on the bond (e.g., “ISSUED 05-2020�). For electronic bonds, log in to TreasuryDirect and go to your “Current Holdings� – the issue date is listed there. If you don’t know it, you can estimate from the purchase date, but the exact date matters for the calculator.

Can I use the calculator for electronic bonds?

Yes, the TreasuryDirect calculator works for both paper and electronic I bonds. For electronic bonds, you don’t need a serial number – just enter the issue date and the face value (the amount you paid). The calculator will show the current value and interest.

Why does the calculator show a lower value than I expected?

If you’ve held the bond for less than 5 years, the calculator deducts the last 3 months of interest as a penalty. Also, if you’re using a simplified formula with a constant rate, the actual value may be lower if inflation rates were lower in earlier periods. Check the issue date and the rate history.

Is the I bond calculator free?

Yes, the official TreasuryDirect calculator is completely free. There is no charge to use it, and you don’t need an account to use the paper bond calculator. For electronic bonds, you need to log in to TreasuryDirect, but that’s also free.

How often are I bond rates updated?

The Treasury announces the fixed rate and the semiannual inflation rate every May 1 and November 1. The calculator uses these rates for the six-month periods starting on those dates. If you’re checking a bond’s value, the calculator automatically uses the correct rates for each period.

Can I calculate the value of a bond that hasn’t reached 5 years?

Yes, but the calculator will apply the 3-month interest penalty. For example, a bond held 24 months will show the value as if it had been held 21 months (subtracting the last 3 months of interest). You can still see the “value� without the penalty by looking at the table of values for each period, but the redemption amount is the one with the penalty.

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in t | Months held | months | 24 | 0
in r | Fixed rate (annual) | % | 1.30 | 0
in i | Inflation rate (semiannual) | % | 1.97 | 0
out value | Bond value per $25 | USD | 25 * (1 + (r/100)/2 + (i/100))^ (t/6) | 2
out interest | Interest earned | USD | value - 25 | 2
out apy | Effective annual yield | % | ((1 + (r/100)/2 + (i/100))^2 - 1) * 100 | 2