Compound Interest Calculator
Want to see how your money grows? Enter principal, rate, and time and you will see the future value with compounding. Small differences in rate add up fast.
TL;DR
- Interest is added to your balance, then that new balance earns interest
- Future Value Formula: A = P × (1 + r ÷ n)^(n × t)
- Example 1: Investing $10,000 at 6% compounded monthly for 10 years
How it works
Interest is added to your balance, then that new balance earns interest. The more often it compounds, the faster it grows.
The math
Multiply the principal by one plus rate divided by periods, raised to total periods.
A is the future value of the investment, P is the initial principal, r is the annual interest rate (decimal), n is the compounding frequency per year, and t is the time in years.
Examples
Example 1: Investing $10,000 at 6% compounded monthly for 10 years
- Principal (P): $10,000, Rate (r): 0.06, Frequency (n): 12, Years (t): 10
- Calculate rate factor: r ÷ n = 0.06 ÷ 12 = 0.005
- Calculate exponents: n × t = 12 × 10 = 120 months
- Apply growth: A = $10,000 × (1.005)^120 = $18,193.97
- Total Compound Interest earned: $8,193.97
When to use it
✦ Retirement Account Projections
Model long-term growth of 401(k) and IRA contributions over several decades.
✦ Savings Account and CD Yields
Compare certificates of deposit offering different interest rates and compounding frequencies.
✦ Assessing Credit Card Debt Cost
Understand how credit card interest compounds daily, rapidly increasing unpaid debt balances.
⚠ Common mistakes
- Confusing Simple and Compound Growth: Underestimating how compounding increases savings over long-term intervals.
- Underestimating Inflation: Forgetting that inflation reduces the purchasing power of your money over time.
FAQ (6)
What is the Rule of 72?
A rule of thumb to estimate when an investment will double. Divide 72 by your annual interest rate. (e.g. at 6%, doubling takes 12 years).
What compounding frequency is best?
The higher the compounding frequency (e.g. daily instead of annually), the faster your interest grows.
If I type my salary or prices here, do you store them?
No — and that's deliberate. Everything runs locally in your browser, nothing leaves your device. We don't see your net salary, your margins, or the VAT numbers you test. For a finance tool that matters: you can run a quick check on a payslip or an invoice while on public Wi-Fi and close the tab knowing no trace is kept server-side.
Can I use it on the train without signal to check an invoice?
Yes. Load the page once, then it works offline. VAT, discount and mortgage math are all pure JavaScript — no API call. I often test it in airplane mode: open the mortgage page, type 250,000 at 4.5% for 25 years, you get the schedule instantly, even in a tunnel.
Can I file taxes with these results?
Use it as a fast sanity check, not as the final filing. Formulas are standard (e.g. Net = Gross / 1+VAT) and verified against official sources listed at the bottom, but rates and thresholds change — Italy's 22% vs Germany's 19%, or IRPEF brackets. For the actual declaration, double-check with your accountant or Agenzia delle Entrate / HMRC / IRS.
How do I send a calculation to my accountant?
Hit 'Share Link' under the results. It encodes the inputs in the URL — e.g. ?price=1200&vat=22 — so your accountant opens the exact same view, no screenshot needed. You can also copy the result with one tap and paste it into an email.
Sources & References
⚠ Financial Disclaimer
This tool provides estimates for informational purposes only and does not constitute financial, tax, or legal advice. Results may vary based on local regulations, rates, and individual circumstances. Always consult a qualified professional before making financial decisions.