Compound Interest Calculator
Understand the power of compound interest with this comprehensive calculator. Enter your starting amount, monthly contributions, interest rate, and investment timeline to see how your wealth can grow. Compare different compounding frequencies (daily, monthly, annually), factor in inflation to see real purchasing power, and account for taxes on investment gains. Perfect for financial advisors, banks, and educational websites helping people plan for retirement or long-term savings goals.
Try the Calculator
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export function compoundInterestCalculator(form: FormTs) {
// Compounding frequency periods per year
const compoundingPeriods: Record<string, number> = {
'annually': 1,
'semi-annually': 2,
'quarterly': 4,
'monthly': 12,
'weekly': 52,
'daily': 365
};
form.addRow(row => {
row.addTextPanel('header', {
computedValue: () => 'Compound Interest Calculator',
customStyles: { 'font-size': '1.5rem', 'font-weight': '600', 'color': '#1e293b' }
});
});
form.addSpacer({ height: 20 });
// Initial Investment Section
const investmentSection = form.addSubform('investment', { title: '💵 Initial Investment' });
investmentSection.addRow(row => {
row.addDecimal('principal', {
label: 'Starting Amount',
min: 0,
max: 10000000,
defaultValue: 10000,
placeholder: 'e.g. 10000',
prefix: '$',
isRequired: true
}, '1fr');
row.addDecimal('monthlyContribution', {
label: 'Monthly Contribution',
min: 0,
max: 100000,
defaultValue: 500,
placeholder: 'e.g. 500',
prefix: '$',
tooltip: 'Additional amount added each month'
}, '1fr');
});
// Interest & Time Section
const interestSection = form.addSubform('interest', { title: '📈 Interest & Time' });
interestSection.addRow(row => {
row.addDecimal('interestRate', {
label: 'Annual Interest Rate',
min: 0,
max: 30,
defaultValue: 7,
placeholder: 'e.g. 7',
suffix: '%',
isRequired: true
}, '1fr');
row.addInteger('years', {
label: 'Investment Period',
min: 1,
max: 50,
defaultValue: 20,
suffix: 'years',
isRequired: true
}, '1fr');
});
interestSection.addRow(row => {
row.addDropdown('compoundingFrequency', {
label: 'Compounding Frequency',
options: [
{ id: 'annually', name: 'Annually (1x/year)' },
{ id: 'semi-annually', name: 'Semi-Annually (2x/year)' },
{ id: 'quarterly', name: 'Quarterly (4x/year)' },
{ id: 'monthly', name: 'Monthly (12x/year)' },
{ id: 'weekly', name: 'Weekly (52x/year)' },
{ id: 'daily', name: 'Daily (365x/year)' }
],
defaultValue: 'monthly',
isRequired: true
}, '1fr');
row.addDropdown('contributionTiming', {
label: 'Contribution Timing',
options: [
{ id: 'end', name: 'End of Period' },
{ id: 'beginning', name: 'Beginning of Period' }
],
defaultValue: 'end',
tooltip: 'When contributions are made each month'
}, '1fr');
});
// Inflation Adjustment (Optional)
const optionsSection = form.addSubform('options', { title: '⚙️ Advanced Options' });
optionsSection.addRow(row => {
row.addCheckbox('adjustForInflation', {
label: 'Adjust for Inflation',
defaultValue: false
}, '1fr');
row.addDecimal('inflationRate', {
label: 'Expected Inflation Rate',
min: 0,
max: 15,
defaultValue: 3,
suffix: '%',
isVisible: () => optionsSection.checkbox('adjustForInflation')?.value() === true
}, '1fr');
});
optionsSection.addRow(row => {
row.addCheckbox('accountForTaxes', {
label: 'Account for Taxes',
defaultValue: false
}, '1fr');
row.addDecimal('taxRate', {
label: 'Tax Rate on Gains',
min: 0,
max: 50,
defaultValue: 25,
suffix: '%',
isVisible: () => optionsSection.checkbox('accountForTaxes')?.value() === true,
tooltip: 'Applied to interest earnings only'
}, '1fr');
});
form.addSpacer({ height: 20, showLine: true, lineStyle: 'dashed' });
// Results Section
const summarySection = form.addSubform('summary', { title: '💰 Investment Growth', isCollapsible: false });
summarySection.addRow(row => {
row.addPriceDisplay('totalContributions', {
label: 'Total Contributions',
computedValue: () => {
const principal = investmentSection.decimal('principal')?.value() || 10000;
const monthly = investmentSection.decimal('monthlyContribution')?.value() || 500;
const years = interestSection.integer('years')?.value() || 20;
return Math.round(principal + (monthly * 12 * years));
},
variant: 'default'
}, '1fr');
row.addPriceDisplay('totalInterest', {
label: 'Interest Earned',
computedValue: () => {
const principal = investmentSection.decimal('principal')?.value() || 10000;
const monthly = investmentSection.decimal('monthlyContribution')?.value() || 500;
const rate = (interestSection.decimal('interestRate')?.value() || 7) / 100;
const years = interestSection.integer('years')?.value() || 20;
const frequency = interestSection.dropdown('compoundingFrequency')?.value() || 'monthly';
const timing = interestSection.dropdown('contributionTiming')?.value() || 'end';
const n = compoundingPeriods[frequency] || 12;
const r = rate / n;
const t = years * n;
// Future value of principal
const fvPrincipal = principal * Math.pow(1 + r, t);
// Future value of contributions (annuity)
const monthlyRate = rate / 12;
const totalMonths = years * 12;
let fvContributions = 0;
if (monthly > 0 && monthlyRate > 0) {
fvContributions = monthly * ((Math.pow(1 + monthlyRate, totalMonths) - 1) / monthlyRate);
if (timing === 'beginning') {
fvContributions *= (1 + monthlyRate);
}
} else if (monthly > 0) {
fvContributions = monthly * totalMonths;
}
const totalValue = fvPrincipal + fvContributions;
const totalContributions = principal + (monthly * 12 * years);
return Math.round(totalValue - totalContributions);
},
variant: 'success',
prefix: '+'
}, '1fr');
});
summarySection.addRow(row => {
row.addTextPanel('growthMultiple', {
computedValue: () => {
const principal = investmentSection.decimal('principal')?.value() || 10000;
const monthly = investmentSection.decimal('monthlyContribution')?.value() || 500;
const rate = (interestSection.decimal('interestRate')?.value() || 7) / 100;
const years = interestSection.integer('years')?.value() || 20;
const frequency = interestSection.dropdown('compoundingFrequency')?.value() || 'monthly';
const n = compoundingPeriods[frequency] || 12;
const r = rate / n;
const t = years * n;
const fvPrincipal = principal * Math.pow(1 + r, t);
const monthlyRate = rate / 12;
const totalMonths = years * 12;
let fvContributions = 0;
if (monthly > 0 && monthlyRate > 0) {
fvContributions = monthly * ((Math.pow(1 + monthlyRate, totalMonths) - 1) / monthlyRate);
} else if (monthly > 0) {
fvContributions = monthly * totalMonths;
}
const totalValue = fvPrincipal + fvContributions;
const totalContributions = principal + (monthly * 12 * years);
const multiple = totalValue / totalContributions;
return `Your money grows ${(Number(multiple) || 0).toFixed(2)}x over ${years} years`;
},
customStyles: { 'font-size': '0.95rem', 'color': '#475569', 'text-align': 'center', 'font-weight': '500' }
});
});
// Year-by-Year Breakdown hint
summarySection.addSpacer({ height: 15 });
summarySection.addRow(row => {
row.addTextPanel('yearlyBreakdown', {
computedValue: () => {
const principal = investmentSection.decimal('principal')?.value() || 10000;
const monthly = investmentSection.decimal('monthlyContribution')?.value() || 500;
const rate = (interestSection.decimal('interestRate')?.value() || 7) / 100;
const years = interestSection.integer('years')?.value() || 20;
const milestones = [5, 10, 15, 20, 25, 30].filter(y => y <= years);
if (milestones.length === 0) return '';
let result = 'Milestones: ';
milestones.forEach((year, index) => {
const monthlyRate = rate / 12;
const months = year * 12;
const fvPrincipal = principal * Math.pow(1 + monthlyRate, months);
let fvContributions = 0;
if (monthly > 0 && monthlyRate > 0) {
fvContributions = monthly * ((Math.pow(1 + monthlyRate, months) - 1) / monthlyRate);
}
const value = Math.round(fvPrincipal + fvContributions);
result += `Year ${year}: $${value.toLocaleString()}`;
if (index < milestones.length - 1) result += ' | ';
});
return result;
},
customStyles: { 'font-size': '0.8rem', 'color': '#64748b', 'text-align': 'center' }
});
});
const finalSection = form.addSubform('final', {
title: '🧾 Summary',
isCollapsible: false,
sticky: 'bottom'
});
finalSection.addRow(row => {
row.addPriceDisplay('futureValue', {
label: 'Future Value',
computedValue: () => {
const principal = investmentSection.decimal('principal')?.value() || 10000;
const monthly = investmentSection.decimal('monthlyContribution')?.value() || 500;
const rate = (interestSection.decimal('interestRate')?.value() || 7) / 100;
const years = interestSection.integer('years')?.value() || 20;
const frequency = interestSection.dropdown('compoundingFrequency')?.value() || 'monthly';
const timing = interestSection.dropdown('contributionTiming')?.value() || 'end';
const adjustInflation = optionsSection.checkbox('adjustForInflation')?.value() || false;
const inflationRate = (optionsSection.decimal('inflationRate')?.value() || 3) / 100;
const accountTaxes = optionsSection.checkbox('accountForTaxes')?.value() || false;
const taxRate = (optionsSection.decimal('taxRate')?.value() || 25) / 100;
const n = compoundingPeriods[frequency] || 12;
const r = rate / n;
const t = years * n;
// Future value of principal
const fvPrincipal = principal * Math.pow(1 + r, t);
// Future value of contributions
const monthlyRate = rate / 12;
const totalMonths = years * 12;
let fvContributions = 0;
if (monthly > 0 && monthlyRate > 0) {
fvContributions = monthly * ((Math.pow(1 + monthlyRate, totalMonths) - 1) / monthlyRate);
if (timing === 'beginning') {
fvContributions *= (1 + monthlyRate);
}
} else if (monthly > 0) {
fvContributions = monthly * totalMonths;
}
let totalValue = fvPrincipal + fvContributions;
const totalContributions = principal + (monthly * 12 * years);
const interestEarned = totalValue - totalContributions;
// Apply taxes to interest only
if (accountTaxes && interestEarned > 0) {
const taxAmount = interestEarned * taxRate;
totalValue = totalValue - taxAmount;
}
// Adjust for inflation
if (adjustInflation) {
totalValue = totalValue / Math.pow(1 + inflationRate, years);
}
return Math.round(totalValue);
},
variant: 'large'
});
});
finalSection.addRow(row => {
row.addTextPanel('inflationNote', {
computedValue: () => {
const adjustInflation = optionsSection.checkbox('adjustForInflation')?.value() || false;
if (adjustInflation) {
return "Value shown in today's dollars (adjusted for inflation)";
}
return 'Estimates for educational purposes. Actual returns may vary.';
},
customStyles: { 'font-size': '0.85rem', 'color': '#64748b', 'text-align': 'center' }
});
});
form.configureSubmitButton({
label: 'Save My Calculation'
});
}
Frequently Asked Questions
What is compound interest?
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest, your money grows faster because you earn interest on your interest.
How does compounding frequency affect my returns?
More frequent compounding results in slightly higher returns. Daily compounding earns more than monthly, which earns more than annually. However, the difference is relatively small - the interest rate and time invested matter more.
Why should I adjust for inflation?
Inflation reduces the purchasing power of money over time. A dollar today buys more than a dollar in 20 years. Adjusting for inflation shows your future value in today's dollars, giving you a realistic picture of what that money will actually be worth.
Can I customize this calculator for my website?
Yes. You can adjust all parameters, add your branding, change default values, and embed it on your financial services website. It's great for engaging visitors and helping them understand investment concepts.
Is this suitable for retirement planning?
This calculator provides a good starting point for understanding long-term investment growth. For comprehensive retirement planning, consider additional factors like Social Security, different account types (401k, IRA), and changing income needs.