Compound Interest Calculator
Compound interest pays interest on interest. Add monthly contributions to that engine and growth accelerates. Enter your starting amount, deposits, rate, and years to see the full curve.
Your results
Charts & visualization
Contributions vs compound growth
The earnings band widens as interest earns its own interest — that is compounding becoming visible.
Every chart’s underlying numbers are available as text — use the “view as table” control under each chart.
How it works
Each month the balance grows by the periodic rate, then your contribution is added. The calculator replays this loop for every month of your horizon and records yearly snapshots.
The stacked chart separates what you put in from what compounding produced. Early on, deposits dominate; given enough years, earnings dominate. That crossover is why time invested matters more than almost any other input.
Formula used
P— starting balanceC— monthly contributionr— rate per period (annual % ÷ periods per year ÷ 100)n— total number of periods
Example calculation
Starting with $10,000 and adding $500 monthly at 7% compounds to about $300,851 after 20 years. Your deposits account for $130,000; compounding supplies the remaining $170,851 — a 2.3× multiple on every dollar you put in. Extend the horizon to 30 years and the earnings share roughly triples.
Assumptions & limitations
- A constant average return replaces real-world volatility; actual sequences vary widely.
- Contributions arrive at month-end and grow at the stated rate.
- Taxes on gains and fees are excluded — tax-advantaged accounts behave closest to this model.
- Inflation is not applied; use the inflation calculator for real purchasing power.
Results are planning estimates based on your inputs and the stated assumptions — not financial advice or a loan offer. Verify important figures with your lender or advisor.
Frequently asked questions
What is the rule of 72?
Divide 72 by your annual rate to estimate doubling time in years — at 7%, about 10.3 years. It matches the full calculation within a few percent for typical rates.
Does starting early really matter that much?
Yes, because compounding needs time more than size. $5,000/year started at 25 versus 35 can produce hundreds of thousands more by retirement even though only ten extra years of deposits were made.
What return should I use?
For long-run stock index planning, 6%–8% nominal is a common conservative range; savings accounts currently earn less. Use modest numbers — optimism compounds into disappointment.
Is compound interest the same as APY?
APY already includes the effect of intra-year compounding on one year of growth. This calculator extends the same math across many years with ongoing deposits.