Compound Interest Calculator
See how your money grows over time with compounding interest. Includes optional regular contributions.
What is compound interest?
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. It's often called "interest on interest" — the engine behind long-term wealth building. The longer you let money compound, the more dramatic the growth becomes.
The compound interest formula
A = P(1 + r/n)nt
- A — final amount
- P — principal (initial deposit)
- r — annual interest rate (decimal)
- n — compounding periods per year
- t — time in years
If you also make regular contributions, our calculator adds the future value of an annuity on top.
The Rule of 72
A quick mental shortcut: divide 72 by your annual rate to estimate how long it takes for an investment to double. At 7% it doubles in roughly 10 years; at 10% in about 7 years.
Frequently Asked Questions
Is more frequent compounding always better?
Yes, but with diminishing returns. Daily compounding produces only marginally more than monthly compounding for most realistic rates. The bigger lever is time and consistent contributions.
Does this account for taxes or inflation?
No. The result is gross of taxes and in nominal dollars. To estimate real (inflation-adjusted) growth, subtract the long-run inflation rate (~2–3%) from your interest rate.
What's a realistic long-term return?
Historically, the S&P 500 has averaged around 7–10% annually before inflation. Bonds and savings accounts typically return less. Past performance is not a guarantee of future results.