Calculate how your monthly contributions grow over time with compound interest.
$
$
%
Years
Total Principal Invested:$0.00
Total Interest Earned:$0.00
Future Portfolio Value:$0.00
Understanding Investment Growth and Compound Interest
Building wealth is not just about how much money you save, but how effectively that money works for you over time. This Investment Growth Calculator is designed to demonstrate the powerful effect of compound interest combined with consistent monthly contributions.
How This Calculator Works
Our tool uses the standard future value formula adapted for monthly compounding intervals, which reflects how most savings accounts, index funds, and brokerage accounts operate. The calculation considers three main factors:
Principal: Your starting lump sum.
Contributions: New money added to the investment every month.
Compounding: The process where interest is earned on both your principal and previously earned interest.
Why "Time in the Market" Matters
The most critical variable in this calculator is often the Growth Period. Due to the exponential nature of compounding, the earnings in the final years of an investment often exceed the contributions made in the first decade. Starting just 5 years earlier can sometimes double your potential future value, assuming a consistent annual return rate (e.g., the historical S&P 500 average of roughly 7-10% adjusted for inflation).
Investment Strategy Tips
To maximize your results:
Start Early: Give your money more time to compound.
Be Consistent: Automate your monthly contributions to remove emotional decision-making.
Reinvest Dividends: Ensure all earnings are fed back into the principal to accelerate growth.
function calculateGrowth() {
// 1. Get input values
var initialInput = document.getElementById('inv_initial').value;
var monthlyInput = document.getElementById('inv_monthly').value;
var rateInput = document.getElementById('inv_rate').value;
var yearsInput = document.getElementById('inv_years').value;
// 2. Validate and Parse
var principal = parseFloat(initialInput) || 0;
var monthly = parseFloat(monthlyInput) || 0;
var rate = parseFloat(rateInput) || 0;
var years = parseFloat(yearsInput) || 0;
// Basic validation to prevent negative numbers or zero years causing issues
if (principal < 0 || monthly < 0 || rate < 0 || years <= 0) {
alert("Please enter valid positive numbers for all fields.");
return;
}
// 3. Calculation Logic (Monthly Compounding)
var months = years * 12;
var monthlyRate = (rate / 100) / 12;
var futureValue = principal;
var totalContributed = principal;
// Iterative calculation to ensure monthly contribution timing is accurate
// (End of month contribution assumption)
for (var i = 0; i < months; i++) {
// Interest is applied to the balance at start of month
futureValue = futureValue * (1 + monthlyRate);
// Monthly contribution is added
futureValue = futureValue + monthly;
totalContributed = totalContributed + monthly;
}
var totalInterest = futureValue – totalContributed;
// 4. Update DOM
document.getElementById('res_principal').innerText = formatMoney(totalContributed);
document.getElementById('res_interest').innerText = formatMoney(totalInterest);
document.getElementById('res_total').innerText = formatMoney(futureValue);
// Show results
document.getElementById('inv_result').style.display = 'block';
}
function formatMoney(amount) {
return '$' + amount.toLocaleString('en-US', { minimumFractionDigits: 2, maximumFractionDigits: 2 });
}