Compound Interest Calculator

Doubling an interest rate does not double the ending balance — it bends the growth curve, and the difference between a 5% and a 10% annual rate on a 20-year investment is not a linear 2×. Compound Interest Calculator makes that non-linearity concrete: enter a principal, rate, and term and the tool shows exactly where compounding frequency changes the answer. Monthly compounding on a $10,000 deposit at 5% for 20 years with $200 per month added produces a final balance of $145,316 — $70,456 of which is interest the money earned on itself.

The optional Monthly Contribution field is the detail most basic compound interest calculators omit. Regular deposits into a savings account, pension, or index fund compound too, so their future value is a separate calculation added on top. Both paths are computed here with standard annuity formulas so the interest-on-interest effect is correctly captured for contributions as well as principal.

How to Use It

1. Enter your starting amount in the Principal field.
2. Set the Annual Interest Rate: use the nominal rate from your account or fund prospectus, not an already-compounded figure.
3. Enter a Time Period between 1 and 100 years.
4. Optionally, enter a Monthly Contribution to model regular deposits.
5. Pick a Compounding Frequency: daily compounding on the same nominal rate earns more than annual, and the effective annual rate line below the results names the exact difference.
6. Click Year-by-Year Table to expand the full growth schedule, or Copy Summary to paste the key figures.

The Formulas

For discrete compounding with n periods per year over t years: A = P × (1 + r/n)^(n·t). At continuous compounding (the mathematical limit as n → ∞): A = P × e^(r·t). The effective annual rate for any discrete frequency is (1 + r/n)^n − 1 — always higher than the nominal rate and shown in the results so you can compare quoted rates on equal footing. Monthly contributions use the standard annuity future-value formula with the effective annual rate applied per period.

Where Precision Has Limits

Interest is calculated using JavaScript Number (IEEE-754 double precision), adequate for all realistic planning figures up to 100 years. The time period is capped at 100 years; beyond that, floating-point precision on the exponentiation degrades noticeably. Annual rate accepts 0% (the zero-interest path divides principal evenly without compounding; a $10,000 deposit at 0% returns $10,000 unchanged after 20 years) up to 100%. A monthly contribution larger than 0 but smaller than $0.01 rounds to zero in the display but is still included in the underlying calculation, so the final balance is correct even when the monthly amount is very small.