Free Calculator
Compound Interest Calculator
Estimate how a starting amount plus regular monthly contributions grows over time. No signup, nothing saved or sent anywhere — all calculation happens in your browser.
What this calculator actually does
It projects a future account balance from a starting amount plus monthly contributions, assuming those contributions compound monthly at a constant annual rate you specify. It's a standard compound-growth calculation — the same math behind most retirement and savings projections — made interactive so you can see how changing one input (rate, timeline, contribution size) changes the outcome.
Assumptions built into the result
- The annual return rate you enter is treated as constant every year — real markets don't work this way; actual returns vary significantly year to year, even if long-run averages smooth out closer to a typical assumption.
- Contributions are assumed to happen on a fixed monthly schedule, with no gaps or increases over time.
- The result doesn't subtract fees, taxes, or inflation — it's a gross, pre-cost projection. See our fee drag calculator for how expense ratios specifically affect a similar projection.
A worked example
Using the default values above ($1,000 starting, $300/month, 7% assumed annual return, 25 years): the tool projects a future value of roughly $248,700, built from $91,000 in total contributions and roughly $157,700 in investment growth. Try lowering the assumed rate to 5% with the same contributions — the projected balance drops to roughly $182,100, illustrating how sensitive long-horizon projections are to the rate assumption, which is exactly the number nobody can know in advance.
How this is calculated
This tool compounds monthly: future value = (starting amount compounded monthly at your rate) + (monthly contributions compounded monthly at your rate). It assumes a constant rate of return every year, which real markets never actually deliver — returns vary year to year. Treat the result as a rough, simplified estimate, not a forecast. See our full calculator methodology.
Limitations
This is an educational estimate, not a financial projection you should rely on for a specific retirement or savings decision. It can't account for market volatility, sequence-of-returns risk, changes to your contribution ability, or fees and taxes. For a plan specific to your situation, consider a financial professional.
This calculator is for general education only and doesn't constitute personalized investment advice. See our full disclaimer.