function calculateIBond() {
// Get Inputs
var fixedRateInput = document.getElementById('fixedRate').value;
var semiInflationInput = document.getElementById('semiInflationRate').value;
var principalInput = document.getElementById('principalAmount').value;
// Validation
if (fixedRateInput === "" || semiInflationInput === "" || principalInput === "") {
alert("Please fill in all fields to calculate the rate.");
return;
}
var fixedRate = parseFloat(fixedRateInput) / 100;
var semiInflation = parseFloat(semiInflationInput) / 100;
var principal = parseFloat(principalInput);
if (isNaN(fixedRate) || isNaN(semiInflation) || isNaN(principal)) {
alert("Please enter valid numbers.");
return;
}
// Treasury Formula: Composite rate = [fixed rate + (2 x semiannual inflation rate) + (fixed rate x semiannual inflation rate)]
// The formula results in a decimal that represents the ANNUAL rate.
var term1 = fixedRate;
var term2 = 2 * semiInflation;
var term3 = fixedRate * semiInflation;
var compositeRaw = term1 + term2 + term3;
// Treasury rounds the composite rate to the nearest 0.0001 (basis point)
var compositeRounded = Math.round(compositeRaw * 10000) / 10000;
// Convert back to percentage for display
var compositePercent = (compositeRounded * 100).toFixed(2);
// Earnings Calculation
// Interest on I bonds is earned monthly and compounded semiannually.
// For a 6-month period, the interest is roughly half of the annual composite rate applied to the principal.
// Note: Actual treasury calculations for monthly accruals are more complex, but for a 6-month projection:
var sixMonthRate = compositeRounded / 2;
var interestEarned = principal * sixMonthRate;
// Interest is usually rounded to the nearest cent per bond, but we will round total for estimation
interestEarned = Math.round(interestEarned * 100) / 100;
var totalValue = principal + interestEarned;
// Display Results
document.getElementById('compositeRateResult').innerText = compositePercent + "%";
document.getElementById('sixMonthRateResult').innerText = (sixMonthRate * 100).toFixed(2) + "%";
// Format Currency
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
});
document.getElementById('interestEarnedResult').innerText = formatter.format(interestEarned);
document.getElementById('finalValueResult').innerText = formatter.format(totalValue);
// Show Results Div
document.getElementById('results').style.display = "block";
}
Understanding the I Bond Composite Rate Calculation
Series I Savings Bonds ("I Bonds") are a unique investment vehicle issued by the US Treasury that protects your savings from inflation. Unlike standard savings accounts or fixed-rate bonds, the return on an I Bond is determined by a combination of two distinct rates: a fixed rate and an inflation rate.
The I Bond Formula
The Treasury does not simply add the two rates together. Instead, they use a specific composite rate formula to determine your actual annual yield. This formula accounts for the compounding effect of the inflation rate on the fixed rate.
Composite Rate = [Fixed Rate + (2 x Semiannual Inflation Rate) + (Fixed Rate x Semiannual Inflation Rate)]
Component 1: The Fixed Rate
The Fixed Rate is established at the time you purchase the bond and remains the same for the entire life of the bond (up to 30 years). For example, if you buy an I Bond with a 1.30% fixed rate, that portion of your return will never change, regardless of economic conditions.
Component 2: The Semiannual Inflation Rate
The Inflation Rate changes every six months (on May 1 and November 1). It is based on the Consumer Price Index for All Urban Consumers (CPI-U). The Treasury announces a semiannual inflation rate, which reflects the change in CPI-U over a 6-month period.
Note for the Calculator: When using the tool above, ensure you input the Semiannual inflation rate, not the annualized version, as the official formula relies specifically on the 6-month change data.
Why Is There a Third Term in the Formula?
You might notice the third part of the equation: (Fixed Rate x Semiannual Inflation Rate). This represents the interest earned on the inflation adjustment itself. While this number is often small, it ensures that the purchasing power of the interest you earn is also maintained against inflation, providing a mathematically "real" return equivalent to the fixed rate.
How Interest Is Paid
I Bonds earn interest monthly, but the interest is compounded semiannually. This means that every six months, the interest you earned in the previous period is added to the principal value of the bond. Future interest calculations are then based on this new, higher principal amount.
The composite rate calculated above applies for a full six-month period. If you hold the bond for five years or less, you will forfeit the last three months of interest when you cash it out.