Compound Interest Calculator
Example: $10,000 plus $200/month at 7% compounded monthly grows to $144,573 in 20 years. You contribute $58,000. The other $86,573 is interest.
| Year | Total contributed | Interest earned | Balance |
|---|---|---|---|
| 1 | $12,400 | $801 | $13,201 |
| 2 | $14,800 | $1,834 | $16,634 |
| 3 | $17,200 | $3,115 | $20,315 |
| 4 | $19,600 | $4,662 | $24,262 |
| 5 | $22,000 | $6,495 | $28,495 |
| 6 | $24,400 | $8,633 | $33,033 |
| 7 | $26,800 | $11,100 | $37,900 |
| 8 | $29,200 | $13,918 | $43,118 |
| 9 | $31,600 | $17,114 | $48,714 |
| 10 | $34,000 | $20,714 | $54,714 |
| 11 | $36,400 | $24,747 | $61,147 |
| 12 | $38,800 | $29,246 | $68,046 |
| 13 | $41,200 | $34,244 | $75,444 |
| 14 | $43,600 | $39,776 | $83,376 |
| 15 | $46,000 | $45,882 | $91,882 |
| 16 | $48,400 | $52,603 | $101,003 |
| 17 | $50,800 | $59,983 | $110,783 |
| 18 | $53,200 | $68,070 | $121,270 |
| 19 | $55,600 | $76,915 | $132,515 |
| 20 | $58,000 | $86,573 | $144,573 |
The compound interest formula
A balance compounds n times a year at annual rate r for t years. It grows to P × (1 + r/n)n·t. Monthly deposits add the future value of an annuity: PMT × [(1 + i)m − 1] / i. Here i is the monthly rate and m is the number of deposits. The calculator puts each deposit at the end of the month.
Step by step with the example
- Monthly rate: 7% ÷ 12 = 0.5833%.
- Number of months: 20 × 12 = 240.
- Growth factor: (1 + 0.005833)240 = 4.0387.
- Initial deposit: $10,000 × 4.0387 = $40,387.39.
- Monthly deposits: $200 × (4.0387 − 1) ÷ 0.005833 = $104,185.33.
- Total: $40,387.39 + $104,185.33 = $144,572.72.
Does compounding frequency matter?
Less than most people think. Here is $10,000 at 7% for 20 years with no deposits.
| Compounding | Effective annual yield | Balance after 20 years |
|---|---|---|
| Annually | 7% | $38,697 |
| Quarterly | 7.186% | $40,064 |
| Monthly | 7.229% | $40,387 |
| Daily | 7.25% | $40,547 |
Daily beats annually by $1,850 over 20 years. That's not a small amount. But it's small next to the rate and the years. Put another way, frequency is the last thing to tune. Banks advertise APY (annual percentage yield) because APY already includes compounding. The CFPB explains APY vs. APR. Stocks have no fixed rate. The S&P 500's compound annual return from 1928 to 2025 was about 10% with dividends reinvested (NYU Stern, Damodaran). Single years swung hard.
Lump-sum growth tables
Each page shows one amount at 4% to 10%. Each page also shows monthly additions and the value after inflation.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is paid only on the original deposit. Compound interest is also paid on interest you already earned. $10,000 at 7% simple interest for 20 years earns $14,000. Compounded annually, it earns $28,697.
Are deposits made at the beginning or end of the month?
The end of each month. That is the standard ordinary-annuity convention. A deposit at the start of the month gets one extra month of growth. So your real result would be a little higher.
What rate should I enter?
For savings accounts and CDs, use the account's APY. For a diversified stock portfolio, many people use 6-8%. That is below the roughly 10% historical S&P 500 average. Stock returns are not guaranteed, so a lower number is safer.
Does this include taxes and fees?
No. Interest in a taxable account is usually taxed each year. Fund expense ratios also cut returns. Subtract your fund's expense ratio from the rate to account for fees.
Not investment advice. These results are estimates. They use the numbers you enter and a constant rate of return. Real returns change every year. They can be negative. They are not guaranteed. Taxes, fees and inflation will change your real results. Talk with a licensed financial professional before you decide.
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Updated: October 2, 2026 · Sources and methodology