Compound interest is often called the most powerful force in personal finance, and the reason is simple: once interest is added to your balance, it starts earning interest of its own. Over enough time, that second layer of growth — interest on interest — can end up contributing more to your final balance than everything you put in.
How to Use This Compound Interest Calculator
Enter your starting deposit and the annual interest rate you expect to earn, then choose how often interest compounds — daily, monthly, quarterly, semiannually, or annually. If you plan to add money regularly, enter the contribution amount and how often you'll make it; the calculator handles a different compounding and contribution schedule at the same time (for example, a monthly deposit into an account that compounds quarterly).
How Compound Interest Is Calculated
Each time interest compounds, it's calculated on the current balance — including any interest already added in previous periods — and then added to that balance. The next compounding period earns interest on this new, larger total. This calculator steps through your chosen term period by period, applying interest and contributions in the order you specify, rather than using a single formula that would have to approximate irregular schedules.
Compound Interest Formula
For compounding with no contributions, the balance after t years is:
A = P × (1 + r/n)^(n×t)
P is the principal, r is the annual interest rate (as a decimal), n is the number of times interest compounds per year, and t is the number of years. When contributions are added, this calculator uses the step-by-step method described above rather than a single closed-form extension of this equation, since it needs to support contribution schedules that don't line up with the compounding schedule.
Example Calculation
$5,000 deposited at 6% annual interest, compounded monthly, with no further contributions, grows to about $16,470 after 20 years — more than triple the original deposit, with $11,470 of that being interest alone. Add a $200 monthly contribution to the same scenario, and the ending balance climbs to roughly $109,000, most of it from the contributions themselves compounding alongside the initial deposit.
What Affects Compound Growth
- Time — the single biggest driver. Compounding needs time to build momentum; the same deposit left in twice as long can produce far more than double the growth.
- Interest rate — a higher rate compounds faster, and the effect amplifies over long periods.
- Compounding frequency — more frequent compounding (daily versus annual) produces a modestly higher return at the same stated rate, since interest starts earning its own interest sooner.
- Contributions — regular deposits, even modest ones, often add more to your final balance over a long horizon than the interest rate itself.