Estimate the composite rate and future value of your Series I Savings Bond.
Minimum $25, Maximum $10,000 per calendar year per SSN.
The rate that stays with the bond for its life (check TreasuryDirect for current rate).
The 6-month variable inflation rate component.
Must hold for at least 1 year. Penalty applies if less than 5 years.
Composite Annual Rate:0.00%
Total Interest Earned:$-
Early Withdrawal Penalty:$-
Final Redemption Value:$-
Understanding I Bond Returns
Series I Savings Bonds (I bonds) are a unique investment vehicle issued by the US Treasury designed to protect your savings from inflation. Unlike standard savings accounts or fixed-rate loans, the return on an I bond is composed of two distinct parts combined into a "Composite Rate."
How the Composite Rate is Calculated
The earnings of an I bond are determined by the composite rate formula. This formula combines a fixed rate and a variable inflation rate. The calculation ensures that your money grows in real terms relative to purchasing power.
Composite Rate = [Fixed Rate + (2 x Semi-Annual Inflation Rate) + (Fixed Rate x Semi-Annual Inflation Rate)]
Here is what the variables mean:
Fixed Rate: This rate is set when you buy the bond and never changes for the 30-year life of the bond.
Semi-Annual Inflation Rate: This rate changes every six months (in May and November) based on the Consumer Price Index for All Urban Consumers (CPI-U).
Important Rules for Redemption
When calculating your I Bond rate of return, it is critical to account for liquidity rules and penalties imposed by the Treasury:
1-Year Lock: You cannot cash out (redeem) your I bond for one year after purchase.
5-Year Penalty: If you redeem the bond before it is 5 years old, you lose the last 3 months of interest.
30-Year Maturity: I bonds earn interest for up to 30 years. After that, they cease to earn interest.
Example Calculation
Assume you purchase a $10,000 I bond with a Fixed Rate of 0.90% and the current Semi-Annual Inflation Rate is 1.25%.
First, calculate the Composite Rate:
0.0090 + (2 x 0.0125) + (0.0090 x 0.0125) = 0.0341125
This results in an annualized composite rate of approximately 3.41%. If you hold this bond for 4 years (assuming rates remained constant for this example), you would be subject to the 3-month interest penalty upon withdrawal.
Why is the "Combined" term usually higher?
You might notice the formula includes a third term: (Fixed Rate x Inflation Rate). This reflects the compounding effect of the inflation adjustment on the fixed return portion, ensuring a mathematically precise adjustment for inflation.
function calculateIBondReturn() {
// Get inputs
var principalStr = document.getElementById("bondPurchaseAmount").value;
var fixedRateStr = document.getElementById("bondFixedRate").value;
var inflationRateStr = document.getElementById("bondInflationRate").value;
var yearsStr = document.getElementById("bondHoldingPeriod").value;
// Validate inputs
if (!principalStr || !fixedRateStr || !inflationRateStr || !yearsStr) {
alert("Please fill in all fields correctly.");
return;
}
var principal = parseFloat(principalStr);
var fixedRate = parseFloat(fixedRateStr) / 100; // Convert % to decimal
var inflationRate = parseFloat(inflationRateStr) / 100; // Convert % to decimal
var years = parseFloat(yearsStr);
if (principal < 0 || fixedRate < 0 || inflationRate < 0 || years < 0) {
alert("Values cannot be negative.");
return;
}
// 1. Calculate Composite Rate
// Formula: Fixed + (2 * SemiInflation) + (Fixed * SemiInflation)
var compositeRateDecimal = fixedRate + (2 * inflationRate) + (fixedRate * inflationRate);
// 2. Calculate Growth
// I Bonds earn interest monthly and compound semiannually.
// For simplicity in projection, we assume the rates stay constant (which they don't in reality, but this is a projection tool).
// We compound semiannually using the composite rate.
var totalMonths = years * 12;
var semiAnnualPeriods = Math.floor(totalMonths / 6);
var remainingMonths = totalMonths % 6;
// The semiannual interest rate is half the composite rate
var semiAnnualRate = compositeRateDecimal / 2;
// Calculate value after full semiannual periods
var currentValue = principal * Math.pow((1 + semiAnnualRate), semiAnnualPeriods);
// Simple interest for remaining months (Treasury actually accrues monthly, but compounds semiannually)
// Monthly rate approximation for the partial period
// Effectively, the bond value increases monthly.
// Simplified logic: The value increases every 6 months.
// To be more precise for a calculator: Compounded semiannually means interest is added to principal every 6 months.
// Between compounding dates, interest accrues but isn't part of the compounding base yet.
// We will calculate exact monthly accrual.
// Reset to perform monthly calculation simulation
var simulatedValue = principal;
var monthlyRate = semiAnnualRate / 6; // Approximation for monthly accrual
// Treasury actually applies the earnings every month based on the semiannual rate logic.
// We will use standard compounding for the projection.
// Value = Principal * (1 + semiAnnualRate)^(years * 2)
// Let's stick to the 6-month compounding logic
var projectedValue = principal * Math.pow((1 + semiAnnualRate), (years * 2));
// 3. Handle Penalty Logic
var penalty = 0;
var finalValue = projectedValue;
var penaltyMsg = "";
if (years < 1) {
finalValue = principal;
penalty = projectedValue – principal; // You lose all interest
penaltyMsg = "I Bonds cannot be redeemed within the first 12 months. Value shown is principal only.";
} else if (years < 5) {
// Penalty is the last 3 months of interest.
// We need to approximate the last 3 months of interest.
// We take the projected value and reverse calculate 3 months of simple interest roughly.
// Or better: Calculate value at (Months – 3).
var periodsMinus3Months = (years * 2) – 0.5; // 0.5 periods = 3 months
var valueAtPenaltyTime = principal * Math.pow((1 + semiAnnualRate), periodsMinus3Months);
// The penalty amount is the difference between the full projected value and the value 3 months prior.
penalty = projectedValue – valueAtPenaltyTime;
finalValue = projectedValue – penalty;
penaltyMsg = "Includes penalty of last 3 months interest for holding less than 5 years.";
}
var totalInterest = finalValue – principal;
// Display Results
document.getElementById("resCompositeRate").innerText = (compositeRateDecimal * 100).toFixed(2) + "%";
document.getElementById("resInterest").innerText = formatMoney(totalInterest);
document.getElementById("resPenalty").innerText = "-" + formatMoney(penalty);
document.getElementById("resFinalValue").innerText = formatMoney(finalValue);
document.getElementById("penaltyMsg").innerText = penaltyMsg;
document.getElementById("result-area").style.display = "block";
}
function formatMoney(amount) {
return "$" + amount.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
}