Enter a starting amount, a monthly contribution, an expected annual return, and a time horizon, and this calculator projects your final balance, total interest earned, and a year-by-year growth chart that shows the compounding curve taking off. Switch between daily, monthly, quarterly, and annual compounding to see how little the schedule matters next to rate and time. Everything runs in your browser; nothing you type is uploaded, and there is no sign-up.
What compound interest is
Compound interest is interest calculated on your whole balance, including the interest that has already been credited to it. Simple interest pays only on the original deposit, so it adds the same dollar amount every year forever. Compounding folds each payment back into the balance, and the larger balance earns more the next period, so the growth curve bends upward instead of running in a straight line. That feedback loop is the entire mechanism, and everything else on this site is a consequence of it. One honest caveat about the word "return": in a savings account or a certificate of deposit, the rate is contractual, so the compounding is arithmetic you can count on. A market return typed into the box above is an assumption you have chosen to test, not a promise anyone has made. The math below is exact. The input never is.
What this calculator computes
There are five inputs: a starting amount, a fixed monthly contribution, an annual rate, a number of years, and a compounding frequency (daily, monthly, quarterly, or annually). The engine steps forward one month at a time. Interest is credited at whatever frequency you pick: every month for monthly, every third month for quarterly, once every twelve months for annually, and a month's worth of daily accrual for daily. Your contribution is added at the end of each month, after that month's interest is posted, which is the standard ordinary-annuity convention. In plain terms, this month's deposit starts earning at the next crediting date rather than retroactively, so the projection stays on the conservative side of the estimate.
From that simulation you get the final balance, a breakdown splitting it into your starting amount, your contributions, and the interest earned, plus total invested, total interest, and a growth multiple (final balance divided by everything you put in). The chart draws one bar per year, with the amber portion showing money you contributed and the green showing the balance, so the widening green gap is compounding made visible. The year-by-year table underneath lists contributions, interest, and closing balance for every single year. A few limits worth knowing: the tool works in whole years, caps the rate at 100% and the horizon at 100 years, assumes the rate and the contribution never change, and ignores taxes and fees. To account for either, enter a net rate instead of a gross one.
A worked example
Take the reference case used throughout this site: a $10,000 starting amount, $500 a month, a 7% annual rate, monthly compounding, 20 years. The calculator returns a final balance of $300,851. You contributed $130,000 of that ($10,000 up front plus $120,000 in monthly deposits), so $170,851 is interest, and the growth multiple is 2.31 times what you put in. More than half the ending balance is money you never deposited.
| End of year | Balance | Interest earned that year |
|---|
| Year 1 | $16,919 | $919 |
| Year 5 | $49,973 | $3,148 |
| Year 10 | $106,639 | $6,968 |
| Year 15 | $186,971 | $12,383 |
| Year 20 | $300,851 | $20,061 |
The contribution is identical in all twenty years, but year 20 earns $20,061 in interest against year 1's $919. Nothing changed except the size of the balance doing the earning. That is why the last third of any long compounding run does so much of the work, and why the chart looks flat before it looks steep.
Does compounding frequency matter?
Far less than most people expect. Hold everything else fixed at $10,000 with no contributions, 7%, and 20 years, and switch only the schedule: annual compounding ends at $38,697, quarterly at $40,064, monthly at $40,387, and daily at $40,547. Moving from annual to daily is worth about 4.8% over two decades, while a single extra percentage point of rate, or a few more years of time, dwarfs it. The practical shortcut is to compare accounts by APY, which already folds the schedule into one number. The full breakdown lives in daily vs monthly compounding, and the deposit-versus-borrowing distinction is in APY vs APR.
Read more
Start with the guide to how compound interest works, which states the formula, works through it, and covers the common mistakes. Simple vs compound interest isolates the difference the two methods make on the same deposit, and the compound interest glossary defines the vocabulary (principal, nominal rate, APY, period, real return) in one place. For the levers you actually control: monthly contributions shows what regular deposits do to the curve, starting early vs starting late prices a delay in dollars, and how fees compound does the same for a percentage skimmed off every year.
On rates and time: the Rule of 72 gets you a doubling time with one division, daily vs monthly compounding settles the frequency question with real numbers, APY vs APR explains which rate to compare when saving and which when borrowing, and inflation and compound interest restates any projection in today's buying power. Saving in a certificate of deposit? The CD calculator is this same engine preset for a lump sum with daily compounding. Quick answers to specific questions are in the FAQ, and about this site explains who builds it and how the numbers are checked.