Compound interest calculator
Enter a starting balance, a monthly contribution, and an assumed annual return. The calculator grows the balance month by month and shows the ending balance, how much of it you put in, how much of it is growth, and the year-by-year path.
Your growth plan
Monthly compounding, contribution added at the end of each month.
Year by year
The balance at the end of each year, split into what you put in and what growth added.
| Year | Contributions to date | Growth to date | Balance |
|---|---|---|---|
| 1 | $13,600 | $841 | $14,441 |
| 2 | $17,200 | $2,002 | $19,202 |
| 3 | $20,800 | $3,508 | $24,308 |
| 4 | $24,400 | $5,383 | $29,783 |
| 5 | $28,000 | $7,654 | $35,654 |
| 6 | $31,600 | $10,349 | $41,949 |
| 7 | $35,200 | $13,500 | $48,700 |
| 8 | $38,800 | $17,138 | $55,938 |
| 9 | $42,400 | $21,299 | $63,699 |
| 10 | $46,000 | $26,022 | $72,022 |
| 11 | $49,600 | $31,346 | $80,946 |
| 12 | $53,200 | $37,316 | $90,516 |
| 13 | $56,800 | $43,977 | $100,777 |
| 14 | $60,400 | $51,380 | $111,780 |
| 15 | $64,000 | $59,578 | $123,578 |
| 16 | $67,600 | $68,629 | $136,229 |
| 17 | $71,200 | $78,595 | $149,795 |
| 18 | $74,800 | $89,542 | $164,342 |
| 19 | $78,400 | $101,540 | $179,940 |
| 20 | $82,000 | $114,665 | $196,665 |
How the growth is calculated
The calculator steps month by month: each month the balance is multiplied by one plus one-twelfth of the annual return, then your contribution is added at the end of the month. That monthly process is exactly the standard compound interest formula:
Final balance = P(1 + r/12)12t + C × [((1 + r/12)12t − 1) ÷ (r/12)]
where P is the starting balance, C the monthly contribution, r the annual return, and t the number of years. The first term is the starting money compounding on its own; the second is every monthly contribution compounding for the months it has been invested. Contributions are the part you control directly; the return is an assumption you should stress-test, which is why the guide below treats 7% as an example, not an expectation.
Start with $10,000, add $300 a month for 20 years at 7%: the starting balance grows to about $40,387 on its own, and the stream of $300 contributions grows to about $156,278 — a final balance of $196,665. Money you put in totals $82,000 (the $10,000 start plus $72,000 of contributions), so $114,665 of the ending balance is growth. More than half the final amount was never deposited; it was earned by earlier money staying invested.
Why the early years feel slow
Compounding is back-loaded. In the example above, the balance after five years is about $28,500 — growth of roughly $3,500 on $25,000 contributed. In the final five years the balance grows by about $74,000, most of it interest on a balance that is already large. Two practical consequences follow. First, the contribution matters most early, when it has the longest time to compound; the 50-30-20 budget calculator is one way to find a monthly amount you can sustain. Second, a plan judged at year three looks disappointing by design — the curve is supposed to look flat before it bends. What the savings eventually have to fund is a spending target; the 4% rule turns that target into a portfolio number, and the Coast FIRE guide shows the point where growth alone can carry the rest.
Pick the contribution you can sustain for the whole period, then check the result at a return two points lower. If the plan only works at the optimistic rate, raise the contribution or extend the years — do not raise the assumed return.
Frequently asked questions
How does compound interest grow money?
Each month the balance earns interest, and the next month interest is charged on the larger balance — the original deposit, every contribution, and all interest earned so far. That is why the curve bends upward: growth itself starts producing growth. This calculator compounds monthly and adds your contribution at the end of each month.
What return should I enter?
Enter a planning assumption, not a prediction. Long-run U.S. stock market returns have averaged roughly 7% a year after inflation over very long periods, and less for bonds and cash — but any single decade can land far above or below an average. Re-run the numbers at a lower rate, such as 5%, and treat anything that only works at the optimistic rate as not yet a plan.
Does this calculator account for inflation?
Turn on the inflation adjustment and enter an inflation rate; the final balance is then also shown in today’s dollars — what the ending amount would buy at today’s prices. Contributions and growth are still shown in nominal dollars, so the today’s-dollars figure is the one to compare against your current spending.
Why does starting earlier matter so much?
Because the last years do the heavy lifting. In the worked example on this page, the balance grows more in the final five years than in the first ten — interest on a large balance outruns the monthly contribution. Starting ten years sooner at the same monthly amount can beat doubling the contribution later.
What does this calculator leave out?
Taxes, investment fees, and market ups and downs. Real returns arrive unevenly — some years negative — and a fund’s expense ratio quietly subtracts from the rate every year. Enter a return net of the fees you actually pay, and treat the smooth path as the average of a bumpy one, not a promise.