Compound Interest Calculator
See what time and steady contributions actually do to money — including the year your growth starts out-earning your paycheck. It's the math this company is named after.
Your Numbers
After 30 years
Projected balance
$386,158
Total contributed
$100,000
Growth earned
$286,158
Money doubles every
~10 yrs
The same money, parked differently
Checking account (0.45%)
$107,784
High-yield savings (4%)
$206,647
Invested at your 7%
$386,158
Year by year
| Year | You put in | Growth | Balance |
|---|---|---|---|
| 1 | $13,000 | $821 | $13,821 |
| 5 | $25,000 | $7,074 | $32,074 |
| 10 | $40,000 | $23,368 | $63,368 |
| 15 | $55,000 | $52,730 | $107,730 |
| 20 | $70,000 | $100,619 | $170,619 |
| 25 | $85,000 | $174,772 | $259,772 |
| 30 | $100,000 | $286,158 | $386,158 |
Results are nominal, pre-tax estimates for educational purposes — no fees, taxes, or inflation, and investment returns are not guaranteed.
Disclaimer: This calculator is provided for educational and informational purposes only and produces estimates based on the figures and assumptions you enter, including investment returns that are not guaranteed. It does not account for taxes, fees, or inflation. Nothing here constitutes financial, investment, tax, or legal advice. Consult a qualified professional before making financial decisions. See our full disclaimer.
The Three Levers (and Which One Actually Matters)
Every compound-growth projection comes down to three inputs: how much you add, what it earns, and how long you leave it alone. They are not equally powerful:
- Time is the heavyweight. Ten extra years usually beats a higher return or bigger contributions, because the curve is steepest at the end. Starting small today beats starting big later.
- Contributions are the workhorse.In the early years, almost all of your balance is money you put in. That's not a failure — it's the setup. Consistency is what gives compounding something to compound.
- Rate matters most at the extremes.The real cost isn't 7% vs 8% — it's money sitting at 0.45% in checking when it could be earning something. Use the “parked differently” comparison above to see that gap in dollars.
Watch for the highlighted crossover year in the table — the year your money starts earning more than you contribute. Everything before it is you building the engine; everything after it is the engine running.
Compound Interest, Answered
What is compound interest?
Compound interest is interest earned on both your original money and the interest it has already earned. Each period's growth gets added to the balance, so the next period's growth is calculated on a bigger number — which is why balances curve upward over time instead of growing in a straight line.
What annual return should I assume?
There's no guaranteed number. A diversified stock portfolio has historically averaged around 7–10% per year over long periods (before inflation), high-yield savings accounts currently pay around 4%, and checking accounts pay close to zero. The most useful way to use the calculator is to compare scenarios, not to predict a single future.
How is compound interest calculated?
The classic formula is A = P(1 + r/n)^(nt): P is your starting amount, r the annual rate, n how many times per year interest compounds, and t the years. This calculator runs that math month by month and also adds your monthly contribution at the end of each month, which the one-line formula can't capture.
Does compounding frequency matter much?
Less than most people expect. At the same nominal rate, daily compounding beats monthly by a fraction of a percent per year, and monthly beats annual by a similar sliver. The levers that actually move the result are time in the market, your contribution rate, and the return itself.
Does this calculator include taxes or inflation?
No — results are nominal, pre-tax estimates with no fees. Taxes depend on the account type (a 401(k), IRA, or taxable brokerage all differ), and inflation reduces what a future balance buys. Treat the output as a comparison tool, not a promise.
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