This calculator estimates the total amount you will have saved by the time you reach your desired retirement age. It takes into account:
Current Age – Your age today.
Retirement Age – The age at which you plan to stop working.
Current Savings – The amount you have already saved.
Monthly Savings Contribution – How much you plan to add to your savings each month.
Expected Annual Return Rate – The average yearly percentage gain you expect from your investments (e.g., stocks, bonds, mutual funds).
The Math Behind the Estimate
The future value of your retirement savings is calculated using the compound interest formula applied to both the existing balance and the series of monthly contributions.
Variables
n = (Retirement Age – Current Age) × 12 // total months until retirement
r = Annual Return Rate ÷ 100 ÷ 12 // monthly interest rate (decimal)
PV = Current Savings // present value
PMT = Monthly Contribution // payment each month
Future Value (FV)
If r > 0:
FV = PV × (1 + r)ⁿ + PMT × [((1 + r)ⁿ – 1) ÷ r]
If r = 0 (no return):
FV = PV + PMT × n
The first term grows the existing savings, while the second term grows the series of monthly contributions.
Real‑World Example
Imagine a 35‑year‑old who wants to retire at 65. They currently have $15,000 saved, plan to contribute $400 each month, and expect an average annual return of 5%.
According to the calculator, this individual would have roughly $453,200 by age 65.
Assumptions & Limitations
The return rate is assumed to be constant and compounded monthly.
Inflation, taxes, and changes in contribution amounts are not considered.
Results are estimates; actual market performance can vary.
Use this tool as a planning aid and adjust the inputs as your financial situation evolves.
function calculateRetirement(){
var age = parseFloat(document.getElementById("currentAge").value);
var retireAge = parseFloat(document.getElementById("retirementAge").value);
var current = parseFloat(document.getElementById("currentSavings").value);
var monthly = parseFloat(document.getElementById("monthlyContribution").value);
var rate = parseFloat(document.getElementById("annualReturn").value);
var resultDiv = document.getElementById("result");
if(isNaN(age)||isNaN(retireAge)||isNaN(current)||isNaN(monthly)||isNaN(rate)){
resultDiv.innerHTML = "Please fill in all fields with valid numbers.";
return;
}
var months = (retireAge – age) * 12;
if(months <= 0){
resultDiv.innerHTML = "Retirement age must be greater than current age.";
return;
}
var monthlyRate = rate / 100 / 12;
var futureValue;
if(monthlyRate !== 0){
var factor = Math.pow(1 + monthlyRate, months);
futureValue = current * factor + monthly * ((factor – 1) / monthlyRate);
} else {
futureValue = current + monthly * months;
}
if(isNaN(futureValue) || !isFinite(futureValue)){
resultDiv.innerHTML = "Calculation error. Check your inputs.";
return;
}
// Format with commas and two decimals
var formatted = futureValue.toLocaleString(undefined, {minimumFractionDigits:2, maximumFractionDigits:2});
resultDiv.innerHTML = "Estimated Savings at Retirement: $" + formatted;
}