Compound Interest Calculator
See how your savings and investments grow over time. Model contributions, withdrawals, or both to plan for saving, retirement drawdowns, or any scenario in between.
Your numbers
How much you have right now
S&P 500 averages ~10% historically
Amount you'll add each month
Amount you'll take out each month (e.g. retirement income)
Results
Growth Over Time
Year-by-Year Breakdown
| Year | Start Balance | Contributions | Interest | End Balance |
|---|---|---|---|---|
| 1 | $10,000 | $6,000 | $919 | $16,919 |
| 2 | $16,919 | $6,000 | $1,419 | $24,339 |
| 3 | $24,339 | $6,000 | $1,956 | $32,294 |
| 4 | $32,294 | $6,000 | $2,531 | $40,825 |
| 5 | $40,825 | $6,000 | $3,148 | $49,973 |
| 6 | $49,973 | $6,000 | $3,809 | $59,782 |
| 7 | $59,782 | $6,000 | $4,518 | $70,299 |
| 8 | $70,299 | $6,000 | $5,278 | $81,578 |
| 9 | $81,578 | $6,000 | $6,094 | $93,671 |
| 10 | $93,671 | $6,000 | $6,968 | $106,639 |
How compound interest works
Compound interest is interest earned on both your original investment and on previously earned interest. Unlike simple interest, which only grows based on your principal, compounding creates a snowball effect — your money earns money, and then that money earns money too.
The formula is A = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) - 1) / (r/n)] where P is your principal, r is the annual rate, n is compounding frequency, t is time in years, and PMT is your periodic contribution.
The three most powerful levers are time, consistency, and rate of return. Even small monthly contributions can grow dramatically over decades thanks to compounding.
This calculator also supports withdrawals for modeling retirement drawdowns or other scenarios where you're taking money out while your balance continues to earn interest. You can set both contributions and withdrawals to see the net effect on your balance over time.