.ci-calculator-container {
font-family: inherit;
border: 1px solid #e0e0e0;
background-color: #f9f9f9;
padding: 25px;
border-radius: 8px;
max-width: 800px;
margin: 20px auto;
box-shadow: 0 2px 5px rgba(0,0,0,0.05);
}
.ci-calculator-container h2, .ci-calculator-container h3 {
margin-top: 0;
color: #333;
}
.ci-calc-row {
display: flex;
flex-wrap: wrap;
margin-bottom: 15px;
gap: 15px;
}
.ci-calc-col {
flex: 1 1 45%;
min-width: 200px;
}
.ci-calculator-container label {
display: block;
margin-bottom: 5px;
font-weight: 600;
color: #555;
}
.ci-input-group {
position: relative;
}
.ci-input-group span {
position: absolute;
left: 10px;
top: 50%;
transform: translateY(-50%);
color: #777;
}
.ci-input-group input[type="number"] {
width: 100%;
padding: 10px 10px 10px 25px;
border: 1px solid #ccc;
border-radius: 4px;
box-sizing: border-box; /* Important for WordPress themes */
}
.ci-input-group.percent-group input[type="number"] {
padding: 10px 25px 10px 10px;
}
.ci-input-group.percent-group span {
left: auto;
right: 10px;
}
.ci-calc-btn {
background-color: #0073aa;
color: white;
border: none;
padding: 12px 25px;
font-size: 16px;
font-weight: bold;
border-radius: 4px;
cursor: pointer;
width: 100%;
transition: background-color 0.2s;
}
.ci-calc-btn:hover {
background-color: #005177;
}
#ci-results {
display: none;
background-color: #fff;
border: 1px solid #0073aa;
padding: 20px;
border-radius: 6px;
margin-top: 20px;
}
.ci-result-row {
display: flex;
justify-content: space-between;
padding: 10px 0;
border-bottom: 1px solid #eee;
}
.ci-result-row.final {
border-bottom: none;
font-size: 1.2em;
font-weight: bold;
color: #0073aa;
}
.ci-article-content {
margin-top: 40px;
padding-top: 20px;
border-top: 2px solid #eee;
line-height: 1.6;
color: #444;
}
.ci-article-content h3 {
margin-top: 25px;
}
function calculateCompoundGrowth() {
// 1. Get inputs and handle edge cases (NaN or negative values)
var principalInput = document.getElementById('ci-initial').value;
var monthlyInput = document.getElementById('ci-monthly').value;
var rateInput = document.getElementById('ci-rate').value;
var yearsInput = document.getElementById('ci-years').value;
var P = parseFloat(principalInput) || 0; // Initial Principal
if (P < 0) P = 0;
var PMT = parseFloat(monthlyInput) || 0; // Monthly Payment
if (PMT < 0) PMT = 0;
var rPercent = parseFloat(rateInput) || 0; // Annual Rate Percentage
if (rPercent < 0) rPercent = 0;
var t = parseFloat(yearsInput) || 0; // Years
if (t < 0) t = 0;
// Basic validation to ensure there is something to calculate
if (t === 0 || (P === 0 && PMT === 0)) {
alert("Please enter an investment period and either an initial investment or monthly contributions.");
return;
}
// 2. Prepare variables for formula
var r = rPercent / 100; // Decimal rate
var n = 12; // Compounding frequency (monthly)
var totalMonths = n * t;
var futureValue = 0;
var totalContributed = P + (PMT * totalMonths);
// 3. The Calculation Logic
// Handle zero interest rate case explicitly to avoid division by zero in annuity formula
if (r === 0) {
futureValue = totalContributed;
} else {
// Formula part 1: Future value of the initial lump sum
// FV = P * (1 + r/n)^(nt)
var fvLumpSum = P * Math.pow((1 + (r / n)), totalMonths);
// Formula part 2: Future value of a series of monthly deposits (Annuity)
// FV = PMT * [ ((1 + r/n)^(nt) – 1) / (r/n) ]
// We use the rate per period (r/n) frequently, let's define it.
var ratePerPeriod = r / n;
var growthFactor = Math.pow((1 + ratePerPeriod), totalMonths);
var fvAnnuity = PMT * ((growthFactor – 1) / ratePerPeriod);
futureValue = fvLumpSum + fvAnnuity;
}
var totalInterest = futureValue – totalContributed;
// 4. Format and Display Results
var currencyFormatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2,
maximumFractionDigits: 2
});
document.getElementById('ci-total-principal').textContent = currencyFormatter.format(totalContributed);
document.getElementById('ci-total-interest').textContent = currencyFormatter.format(totalInterest);
document.getElementById('ci-future-value').textContent = currencyFormatter.format(futureValue);
// Show results container
document.getElementById('ci-results').style.display = 'block';
}
Compound Interest Calculator with Monthly Deposits
Calculate how your investments grow over time with the power of compounding and regular contributions.
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Investment Projection Results
Total Principal Invested:
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Total Interest Earned:
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Future Investment Value:
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Understanding Compound Interest and Monthly Deposits
This calculator demonstrates one of the most powerful concepts in finance: compound interest combined with regular investing. Unlike simple interest, which is only calculated on your initial principal, compound interest is calculated on your principal plus any accumulated interest. It's essentially "interest on interest."
Why Monthly Contributions Matter
While a large initial investment is helpful, the true magic happens when you consistently add to your investment over time. This approach offers several benefits:
- Accelerated Compounding: Every new monthly deposit immediately starts generating its own interest, increasing the base upon which future interest is calculated.
- Dollar-Cost Averaging: By investing the same amount regularly, you buy more shares when prices are low and fewer when prices are high, potentially lowering your average cost per share over the long run.
- Habit Formation: Treating investments like a monthly bill ensures you prioritize saving for your future self.
Real-World Example: The Power of Consistency
Let's look at a realistic scenario using the calculator above:
- You start with an Initial Investment of $1,000.
- You commit to a Monthly Contribution of $300.
- You achieve an average annual return of 8% (historical average of stock market indices).
- You invest for 25 Years.
Over those 25 years, you would have physically invested a total of $91,000 ($1k initial + $300/mo x 300 months).
However, thanks to compound interest, your investment would grow to approximately $285,300. That means you earned over $194,000 purely in interest. The calculator above allows you to adjust these variables to see how small changes in contributions or time horizons affect your long-term wealth.