Calculate your Composite Rate and estimated 6-month earnings.
The amount you are investing (Min $25).
The fixed rate assigned at the time of purchase.
The 6-month inflation rate announced by the Treasury (May/Nov).
Annual Composite Rate:0.00%
Interest Earned (First 6 Mos):$0.00
Total Value (After 6 Mos):$0.00
*Note: I Bonds interest compounds semiannually. The composite rate applies for the first 6 months of ownership. Rates reset every 6 months based on new inflation data.
Understanding I Bonds Rates
Series I Savings Bonds (I Bonds) are a unique type of investment issued by the U.S. Treasury designed to protect your savings from inflation. Unlike standard savings accounts or fixed-rate bonds, the return on an I Bond is composed of two distinct parts: a fixed rate and a variable inflation rate.
The Composite Rate Explained
The actual annual earnings rate of an I Bond is called the Composite Rate. This rate is calculated using a specific formula that combines the Fixed Rate and the Semiannual Inflation Rate. The Composite Rate is what determines the interest accrual on your bond for a six-month period.
Composite Rate = [Fixed Rate + (2 x Semiannual Inflation Rate) + (Fixed Rate x Semiannual Inflation Rate)]
Input Definitions
Investment Principal: The cash amount you purchase the bond for. You can buy electronic I Bonds in any amount to the penny from $25 to $10,000 per calendar year.
Fixed Rate: An annual rate that remains the same for the life of the bond (up to 30 years). It is determined by the Treasury at the time you buy the bond.
Semiannual Inflation Rate: A variable rate that changes every six months (in May and November) based on the Consumer Price Index (CPI-U). This rate reflects inflation over a 6-month period.
How I Bonds Interest Compounding Works
Interest on I Bonds is earned monthly and compounded semiannually. This means that every six months, the interest earned in the previous months is added to the bond's principal value. Future interest calculations are then based on this new, higher principal amount.
Important Rule: If you cash out an I Bond before five years, you lose the last three months of interest. This calculator shows the gross value generated, not the net cash-out value if the penalty applies.
Why Do Rates Change?
While your Fixed Rate stays with the bond forever, the Semiannual Inflation Rate is updated twice a year. This ensures that your investment keeps pace with the cost of living. Consequently, your Composite Rate will change every 6 months starting from the date of purchase.
Maximizing Your I Bond Strategy
Investors often look at the current fixed rate to decide when to buy. A higher fixed rate locks in a higher real return above inflation for decades. The variable inflation component ensures the nominal dollars grow fast enough to maintain purchasing power during high-inflation periods.
function calculateIBond() {
// Get input values
var principal = parseFloat(document.getElementById('principalAmount').value);
var fixedRatePercent = parseFloat(document.getElementById('fixedRate').value);
var inflationRatePercent = parseFloat(document.getElementById('inflationRate').value);
// Validation
if (isNaN(principal) || isNaN(fixedRatePercent) || isNaN(inflationRatePercent)) {
alert("Please enter valid numbers for all fields.");
return;
}
if (principal < 0 || fixedRatePercent 9.62%)
// Usually Treasury rounds the final percentage to 2 decimal places.
var compositeRatePercent = Math.round(compositeRateDecimal * 10000) / 100;
// Calculate 6-month earnings
// Interest for 6 months is applied to the principal.
// Since Composite Rate is an annual rate, the 6-month return is roughly half,
// but the accurate calculation for the semiannual accumulation is using the Semiannual Inflation + Fixed/2 components?
// Actually, Treasury simplifies it: Value = Principal * (1 + CompositeRate/2) roughly?
// Let's stick to the official logic:
// The composite rate is an ANNUAL rate.
// Interest for 6 months = Principal * (Composite Rate / 2).
var sixMonthInterest = principal * (compositeRateDecimal / 2);
var totalValue = principal + sixMonthInterest;
// Display Results
document.getElementById('compositeResult').innerText = compositeRatePercent.toFixed(2) + "%";
// Formatting currency
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
});
document.getElementById('interestResult').innerText = formatter.format(sixMonthInterest);
document.getElementById('valueResult').innerText = formatter.format(totalValue);
// Show results section
document.getElementById('results').style.display = 'block';
}