What the compound interest calculator does
The compound interest calculator projects how a starting balance plus optional regular contributions will grow over time when interest is paid on interest. Pick a rate, a time horizon and how often the account compounds (daily, monthly, quarterly, semi-annually or annually) and the result updates live. The year-by-year table shows you how much of your final balance came from contributions and how much from pure growth — a number that often surprises first-time users.
The formula
For a lump-sum deposit:
A = P × (1 + r ÷ n)n × t
where P is the starting principal, r is the annual rate as a decimal, n is the number of compounding periods per year, and t is the number of years. For regular contributions on top of that, the calculator adds the future value of an ordinary annuity — contributions multiplied by ((1 + r/n)nt − 1) ÷ (r/n). Both pieces are summed for the final balance.
Why compound interest is so powerful
Albert Einstein supposedly called compound interest "the most powerful force in the universe" — apocryphal, but it captures the math. A linear (simple-interest) account paying 7% on $10,000 earns $700 every year, forever. A compound account paying 7% earns $700 in year one, then $749 in year two (7% of $10,700), then $801, and so on. After 30 years the simple account is at $31,000; the compound account is over $76,000. The longer the time horizon, the more dramatic the gap.
Contributions matter even more than the rate
People obsess over rate. They should obsess over contributions. Putting $300 a month into a 7% account for 30 years grows to about $367,000 — and only $108,000 of that came from you. The other $259,000 is compound growth on those steady deposits. Doubling the rate to 14% (unrealistic over the long run) gets you to $1.6M; doubling the contribution to $600/month at the realistic 7% gets you to $735,000 with similar effort.
The rule of 72
A useful mental shortcut: divide 72 by the annual percentage rate to estimate years to double. At 6% money doubles every 12 years; at 9% every 8; at 12% every 6. This calculator gives the exact answer, but the rule is gold for mental sanity-checking ad copy that promises "your money doubles in 5 years!" — that would require a 14.4% return, every year, without fail.
Practical assumptions
The math here assumes a constant rate, regular contributions, no taxes and no withdrawals. Real-world investments don't earn a smooth 7% every year — they bounce up and down, average out over decades, and pay taxes along the way. Use the result as a planning model, not a guarantee, and pair it with our sales tax calculator or federal income tax percentage calculator for the bill that eventually comes due.