The Compound Interest Calculator shows how your investments grow over time when both your principal and your accumulated interest earn returns. Albert Einstein reportedly called compound interest the eighth wonder of the world — this tool makes it visual.
Use it free in your browser — no account or download required.
Key Features
Compound interest calculation with regular contributions
Save up to 2 scenarios (Pro) or 5 scenarios (Pro+)
Side-by-side scenario comparison with charts (Pro+)
Export results to PDF or Excel (Pro+)
How to Use Compound Interest Calculator
Step 1: Enter your starting principal This is the amount you're investing today. It can be as small as $100 — the most important variable is time, not the starting amount.
Step 2: Set a monthly contribution Enter how much you'll add each month. Regular contributions (even small ones) dramatically accelerate growth. $200/month invested consistently for 30 years produces more than a $50,000 lump sum invested once.
Step 3: Enter the annual interest rate Use 6–8% for a diversified stock market index fund, 4–5% for bonds or high-yield savings, or the exact rate for your specific account.
Start as early as possible — even 5 years earlier has a massive impact thanks to compound growth.
Increasing your monthly contribution by just $50 can add tens of thousands to your final balance.
To get inflation-adjusted projections, subtract the inflation rate (typically 2–3%) from your return rate.
Frequently Asked Questions
What compounding frequency should I use?
Monthly is most accurate for savings accounts and regular investment accounts. Annual compounding is simpler but slightly understates returns. The difference between monthly and daily compounding over 30 years is typically less than 1%.
Does this account for taxes?
No. For after-tax projections, reduce your return rate by your marginal tax rate on investment gains, or use 70% of the gross rate as a rough approximation.
Available Plans
Free — Core features available instantly, no account needed
Pro — Unlock AI features, unlimited saves, and PDF/DOCX export
Pro+ — Everything in Pro, plus no ads and advanced AI features