About the Compound Interest Calculator
The eighth wonder of the world, computed live: see how a starting balance plus monthly contributions grows with compound interest over any number of years.
Your figures are computed in the browser and never transmitted. No account, no email capture, and no salesperson at the end of it — this is a calculator, not a lead form.
Results are estimates for general information only and are not financial advice. Actual loan terms, returns, and taxes vary — confirm figures with your lender or advisor.
How to use it
- Enter your starting amount.
- Enter the annual interest rate and how many times a year it compounds.
- Set the number of years, and add a regular contribution if you make one.
- Read the final balance and the interest earned.
Why the exponent changes everything
The formula is A = P(1 + r/n)nt. The part that matters is that nt sits in the exponent rather than beside it — that is the mathematical signature of growth feeding on itself.
Simple interest always pays on your original deposit. Compound interest pays on the deposit plus every bit of interest already added, so each year starts from a slightly higher base.
A worked example
$5,000 at 6% for 4 years:
- Simple interest: $1,200.00
- Compounded annually: $1,312.38
- Compounded monthly: $1,352.45
Four years is barely long enough to see the effect — the gap is about $150. Run the same deposit for thirty years and simple interest pays $9,000 while annual compounding pays $23,717. The mechanism is identical; only the time changed.
Common mistakes
- Comparing an APR to an APY. APY already includes compounding, so it is the higher number for the same rate.
- Chasing compounding frequency instead of the rate. Daily versus monthly is worth a rounding error; 0.6% versus 4% is worth thousands.
- Forgetting that inflation compounds too, so a 4% return during 3% inflation is roughly a 1% real gain.
Questions people ask
What is the Rule of 72?
Divide 72 by the interest rate for a quick estimate of the years to double. At 7%, that is about 10.3 years — the exact answer is 10.24, so the shortcut is close enough for mental arithmetic.
Does it handle monthly contributions?
Yes. Regular deposits are compounded alongside the starting balance, which is how most retirement accounts actually behave.
Is this financial advice?
No. It is arithmetic on the numbers you supply. Real returns vary, are not guaranteed, and tax and fees are not modelled here.