Compound Interest Calculator
Project a balance from a starting amount, rate, and regular contributions.
Contributions win early, compounding wins late
Over the first several years of a savings plan, almost all growth comes from money you deposited. The returns on a small balance are small in absolute terms no matter how good the rate is. Somewhere past the ten-year mark — the exact point depends on the rate and the contribution size — the balance becomes large enough that a year of returns exceeds a year of contributions, and from then on the curve does the heavy lifting.
This matters for behaviour. In the early years, the contribution rate is the only lever that meaningfully changes the outcome, so chasing a slightly higher return is usually less productive than saving slightly more. In the later years the reverse is true, and fees start to matter enormously.
Why a single rate is a fiction
The projection assumes a constant return credited every period. No real investment behaves that way. Two portfolios that both average 7% will end at very different balances if one delivered it smoothly and the other alternated between +25% and −10%, because contributions made before a crash buy in at different prices than contributions made after one. The order of returns — sequence risk — is invisible in this kind of calculation and matters most in the years right before you start withdrawing.
The practical response is not to abandon the estimate but to bracket it. Run a pessimistic rate, a middling one, and an optimistic one. The spread between them is a more honest answer than any single figure.
Fees and inflation quietly reduce the result
A 1% annual fee does not reduce a 30-year outcome by 1%. Because the fee is charged on the whole balance every year, it compounds against you; over long horizons a one-percentage-point difference in fees commonly removes a fifth or more of the final balance. If you want the result in today's purchasing power, subtract expected inflation from the rate you enter and read the output as real money rather than nominal money.
How to use it
- Enter the amount you already have set aside as the starting balance.
- Enter the annual rate — use a real rate (nominal minus inflation) if you want the result in today's money.
- Enter your regular contribution and the number of years.
- Repeat with a rate two or three points lower to see the pessimistic case.
Before you rely on this
Is the projected balance guaranteed?
No. It assumes a fixed return credited every period, which no market investment delivers. Treat it as a scenario comparison tool rather than a forecast, and always run a lower rate alongside your main assumption.
Should I enter a nominal or a real rate?
Either, as long as you know which. A nominal rate gives a future balance in future money; subtracting expected inflation first gives an answer in today's purchasing power, which is usually more useful for goal setting.
How do fees change the result?
Subtract the annual fee percentage from your assumed return before entering it. Because the fee applies to the whole balance every year, its effect compounds — over decades, one percentage point of fees typically costs far more than most savers expect.
Does contribution timing matter?
Yes, though less than people assume. Contributing at the start of each period rather than the end earns one extra period of return on every deposit, which adds a small but consistent amount over a long horizon.
Disclaimer: These calculators produce planning estimates, not financial advice. Results exclude fees, taxes, insurance, and rate changes, and no output here constitutes an offer or a recommendation. Speak to a qualified, regulated adviser before making a financial commitment.