Compound Interest Calculator
See what regular contributions turn into, and how much of the total is growth rather than deposits.
Nothing you type here is sent anywhere. The maths runs in your browser.
Growth is not a straight line
Compound interest pays return on your return. In year one, $10,000 at 7% earns $700. In year two the same 7% is charged against $10,700, so it earns $749. The increment looks trivial and stays trivial for years — and then it does not. The curve is exponential, which means almost all of the visible action happens in the final third of the term.
This is why the results panel separates total contributed from interest earned. Over 25 years at 7% with $500 a month, you contribute $160,000 and end with roughly $460,000 — meaning about two thirds of the final balance is money you never deposited. Over 10 years with the same inputs, growth is well under half. The variable doing that work is time, not the rate.
Choosing a return rate honestly
The US stock market's long-run average is often quoted at about 10% nominal, but that number is misleading for planning in two ways. It ignores inflation, and it ignores sequence. Subtracting a long-run 3% inflation gives roughly 7% real, which is what this calculator's default represents: purchasing power, not headline dollars.
Sequence matters too. That average is composed of years like +28% and years like −37%. A plan that only works if returns arrive smoothly is not a plan. If the result at 7% is uncomfortably tight, run it again at 5% and see whether the outcome still works.
Fees compound too
A 1% annual management fee does not reduce your final balance by 1%. It reduces your compounding rate by 1 percentage point every year, and the effect compounds against you exactly as returns compound for you. On the default 25-year scenario, moving from 7% to 6% costs roughly $70,000 — for a fee that presents itself as a rounding error on a statement.
The cost of waiting
Because the last years contribute the most growth, delay is unusually expensive. Start at 25 and contribute $500 a month to 65 and you finish far ahead of someone who starts at 35 and contributes $1,000 — despite the second person depositing more money in total. Set the time field to 40 and then to 30 to see the size of that gap for yourself. Time in the market is the one input you cannot buy back later.
Common questions
When are contributions added?
At the end of each month, and the balance compounds monthly. Contributing at the start of each month instead produces a slightly higher result, by roughly one month of return.
Does this account for inflation?
Not explicitly. Enter a real (inflation-adjusted) rate — around 7% for a diversified stock portfolio historically — and the result is in today's purchasing power. Enter a nominal rate such as 10% and the result is in future dollars worth less than today's.
Does it account for tax?
No. Returns in a taxable account are reduced by tax on dividends and realised gains; tax-advantaged accounts such as a 401(k), IRA or ISA defer or eliminate that drag. The difference over decades is large.
Is any of this stored?
No. Everything is computed in your browser and nothing you enter is transmitted or saved.
These results are estimates for planning purposes, not financial advice. Your lender sets your actual terms. See our terms of use.