Compound Interest Calculator
What a balance becomes when it grows on itself, with contributions added along the way — and how much of the final figure is growth rather than money you paid in.
The address carries whatever you changed, so anyone opening it sees these numbers. Nothing is saved on our side.
Balance after 25 years
$462,290
You pay in $160,000 and the growth adds $302,290 on top — 65% of the final balance.
- $160,000
- Starting balance and contributions
- $302,290
- $249,355
- After 2.5% inflation
- 65%
- Of the final balance
Where the balance comes from
Each column is the balance at the end of that year, split into the money you paid in and the growth sitting on top of it.
Year
From year 17 onward the account holds more growth than money paid into it.
What another decade is worth
The same contributions, left alone for longer. Compounding is not linear in time, which is why the gap between the rows widens rather than staying even.
| Horizon | Balance | Growth | vs yours |
|---|---|---|---|
| 5 years$40,000 paid in | $49,973 | −$292,317 less in total | |
| 10 years$70,000 paid in | $106,639 | −$265,651 less in total | |
| 20 years$130,000 paid in | $300,851 | −$131,439 less in total | |
| 25 yearsyours$160,000 paid in | $462,290 | — | |
| 30 years$190,000 paid in | $691,150 | +$198,860 more in total | |
| 40 years$250,000 paid in | $1,475,521 | +$923,231 more in total |
How this calculator works
One formula for the balance, and one convention that decides whether the answer is right.
A = P (1 + r/n)n·t
with P the starting balance, r the annual rate, n the number of times a year it compounds, and t the years.
Contributions arrive monthly, so the projection advances a month at a time using a per-month factor of (1 + r/n)n/12. With nothing added this reproduces the formula above exactly, so the result agrees with a bank’s own arithmetic; with contributions it spreads each period’s growth across the months inside it, which is the only sensible way to credit growth on money that arrived part-way through.
What is assumed
- The return is the same every year. No real investment does this, and the average is not the experience.
- Contributions land at the end of each month, and every month.
- No tax and no fees. Subtract fees from the rate you enter.
- Nothing is withdrawn along the way.
A worked example
$10,000 to start, $500 a month, 7% a year compounded monthly, left for 25 years.
| Starting balance | Paid in on day one |
|---|---|
| Contributions | $500 × 300 months |
| Total paid in | Starting balance plus contributions |
| Growth | Everything above what was paid in |
| Balance after 25 years |
Growth ends up nearly twice what was paid in, but it does not arrive evenly. For the first sixteen years the contributions are the larger half of the balance; only in year 17 does the growth overtake them, and from there the gap widens quickly. That crossover is the whole argument for starting early, and it is marked on the chart above.
Questions people ask
That depends entirely on what the money is in, and nobody can tell you what the next thirty years will do. A savings account pays whatever it currently pays. A diversified stock portfolio has historically averaged more than that over long periods, with years that were far worse and far better. Whatever figure you choose, enter it after fees — a 7% return in a fund charging 1% is 6% to you.
Less than people expect. At 8%, moving from annual to daily compounding on $10,000 over twenty years takes the balance from $46,610 to $49,522 — $2,912 more, which is real money. But one extra percentage point of return over the same period is worth $9,435, more than three times as much. The frequency is worth getting right; it is not worth optimising for.
Because inflation is doing what inflation does. A dollar in thirty years buys less than a dollar now, so a balance quoted in future dollars overstates what it will actually buy. The figure in today's money is the one to judge against your current life. Both are shown because both are true.
At the end. A contribution made at the end of a month has not been invested during that month, so it earns nothing until the next one. Calculators that add at the start report slightly higher balances for identical inputs — about 1% more at 12% over a year. Ending-of-month is the more conservative and more common convention.
No. Interest, dividends and capital gains are taxed differently, and the account wrapper changes everything — a Roth IRA and a taxable brokerage account holding identical investments end up in very different places. Enter an after-tax return if the account is taxable, or treat the result as a pre-tax figure.
Growing what you save
Investment ReturnWhat a portfolio returned, annualizedRelated calculators
A projection at a constant rate, not a forecast. Real returns vary year to year, and the order they arrive in matters for anyone drawing the money down.