Compound Interest Calculator
Set your starting balance, monthly contribution, rate, and time horizon — the projection updates as you type, and the link in your address bar always reflects your numbers.
How this calculator works
Compound growth means each month's interest is calculated on the whole balance — including all the interest already earned. This tool compounds monthly: the annual rate is divided by 12, applied every month, and your contribution is added at the end of each month (so a deposit starts earning interest the month after you make it). The closed-form version of that loop is FV = P·(1 + i)n + C·((1 + i)n − 1) / i, where i is the monthly rate and n the number of months. Real investment returns aren't a smooth fixed rate, so treat the output as a planning estimate, not a prediction.
Worked example
Start with $10,000, add $200 a month, and assume 7% a year for 20 years. Over 240 months you contribute $48,000 on top of the initial $10,000 — $58,000 in total. The projected balance is about $144,572.72, meaning roughly $86,572.72 came from compounding rather than your deposits. Stretch the horizon to 25 years and the balance reaches about $219,268.52 — five extra years add nearly $74,696.
Frequently asked questions
What compounding frequency does this calculator use?
Monthly. The annual rate is divided by 12, applied to the balance every month, and each contribution is added at the end of the month it's made. Annual-compounding calculators will show slightly lower results for the same rate, because interest is credited less often.
Is 7% a realistic annual return to assume?
It's a common ballpark for broad stock-market index returns over long historical periods, measured before inflation. Real-world returns vary a lot year to year and can be negative, and past averages don't guarantee anything about the future. Try a range — say 4%, 7%, and 10% — to see how sensitive your plan is. This tool does the math; it isn't investment advice.
Why does growth look so slow in the first few years?
Because interest earns interest, the curve bends upward over time. With $10,000, $200/month, and 7%: after 5 years the balance is about $28,495, of which only $6,495 is interest — most is your own deposits. By year 20 interest earned ($86,573) has overtaken everything you put in ($58,000). The boring early years are what make the later ones dramatic.
Does starting 5 years earlier really matter that much?
Yes, more than most people expect. Using the defaults ($10,000 start, $200/month, 7%): 20 years grows to about $144,573, while 25 years grows to about $219,269. Those five extra years add roughly $74,696 — yet they only involve $12,000 of additional deposits. The rest is compounding working on a larger base for longer.