Compound Interest Calculator
See how a lump sum plus regular monthly contributions grows with compound interest. Adjust the rate and timeline to compare scenarios instantly.
How it works
FV = P(1+r/n)^(nt) + PMT · [ ((1+r/n)^(nt) − 1) / (r/n) ] × (1+r/n)
- Compound interest pays interest on your interest. Compounding monthly at an annual rate r means each month grows your balance by r/12.
- The first term is your initial lump sum growing for the whole period; the second is the future value of every monthly contribution.
- Time matters more than rate: doubling years roughly squares the growth multiple on the lump sum.
FAQ
Is the contribution added at the start or end of each month? This calculator assumes contributions are made at the start of each month (annuity due), which is why the contribution formula includes the extra (1+r/n) factor.
Does it account for taxes or inflation? No — results are nominal. For real purchasing power, run the result through the Inflation Calculator.
What rate should I use? Historical US stock market returns average roughly 7–10% per year before inflation, but past performance never guarantees future results.