Compound Interest Calculator
Compound interest is interest that also accrues on the interest already earned. The future value is M = C × (1 + i/n)^(n×t). Example: 10,000 at 7% a year with monthly compounding becomes 20,096.61 after 10 years, i.e. 10,096.61 in interest.
Growth Over Time
Method note: contributions are assumed at the end of each month and compounded at the monthly rate equivalent to the frequency you pick, (1 + i/n)^(n/12) − 1; all amounts are before tax and inflation.
| Year | Total paid in | Interest earned | End-of-year balance |
|---|---|---|---|
How do you calculate compound interest?
The future value is M = C × (1 + i/n)^(n×t), where C is the starting capital, i the annual rate as a decimal (7% → 0.07), n the number of compounding periods per year and t the number of years; interest earned is M − C. With C = 10,000, i = 0.07, n = 12 and t = 10 you get 10,000 × (1 + 0.07/12)^120 = 20,096.61, so 10,096.61 in interest. If you add a recurring payment PMT, the future value of that annuity is added on top: PMT × [((1 + i_m)^(12t) − 1) / i_m], where i_m = (1 + i/n)^(n/12) − 1 is the monthly rate equivalent to the selected frequency and equals i/12 under monthly compounding — exactly what the "Monthly contribution" field does above.
How much does 10,000 become with compound interest?
At 5% a year 10,000 becomes 16,289 after ten years and 43,219 after thirty; at 7% you reach 19,672 and 76,123. The table shows the future value for the most common rates and, in the last column, the exact number of years needed to double the capital.
| Annual rate | 5 years | 10 years | 20 years | 30 years | Years to double |
|---|---|---|---|---|---|
| 2% | $11,041 | $12,190 | $14,859 | $18,114 | 35.0 |
| 3% | $11,593 | $13,439 | $18,061 | $24,273 | 23.4 |
| 5% | $12,763 | $16,289 | $26,533 | $43,219 | 14.2 |
| 7% | $14,026 | $19,672 | $38,697 | $76,123 | 10.2 |
| 10% | $16,105 | $25,937 | $67,275 | $174,494 | 7.3 |
Does monthly or daily compounding change the result?
The more often interest is credited, the sooner it starts earning — but the effect is small. On 10,000 at 5% for one year you go from 10,500.00 with annual compounding to 10,511.62 monthly and 10,512.67 daily: a 12.67 difference, about 13 basis points of annual percentage yield. The APY, defined for US deposit accounts by Regulation DD, folds that gap into a single number and is the figure to compare across products.
| Compound Frequency | Value after 1 year | Annual percentage yield (APY) |
|---|---|---|
| Annually (n = 1) | $10,500.00 | 5.000% |
| Semi-annually (n = 2) | $10,506.25 | 5.062% |
| Quarterly (n = 4) | $10,509.45 | 5.095% |
| Monthly (n = 12) | $10,511.62 | 5.116% |
| Daily (n = 365) | $10,512.67 | 5.127% |
What is the difference between simple and compound interest?
Simple interest is always computed on the starting capital: 10,000 at 5% pays 500 a year, 5,000 over ten years, for a total of 15,000. Compound interest rolls the interest back into the capital so it earns too: the same 10,000 becomes 16,288.95, with 6,288.95 of interest. The gap is 1,288.95 after ten years but 18,219 after thirty. The same mechanism works against you on a revolving credit card or an overdraft.
What is the rule of 72 and when should you use it?
Divide 72 by the annual rate in percent and you get a close estimate of how many years it takes to double your money: 72/6 = 12 years at 6%, 72/9 = 8 years at 9%. The error stays under 3% between 4% and 12%; outside that band use the exact formula t = ln(2) / ln(1 + i), which gives 35.0 years at 2% instead of the 36 the rule suggests, and 7.3 years at 10% instead of 7.2.
Sources
- U.S. SEC (Investor.gov) — Compound Interest Calculator
- U.S. SEC (Investor.gov) — Compound interest, glossary definition
- Regulation DD, Appendix A to 12 CFR Part 1030 — Annual Percentage Yield calculation
- EUR-Lex — Directive 2008/48/EC on credit agreements for consumers (APR definition; replaced by Directive (EU) 2023/2225 from 20 November 2026)
About Compound Interest Calculator
This compound interest calculator projects what an investment grows to from a starting balance, a monthly contribution, an annual rate and a time horizon, compounding annually, semi-annually, quarterly, monthly or daily. It splits the future value into the money you put in and the interest earned, expresses that interest as a percentage of contributions, and plots year-by-year growth. Useful for comparing savings rates or seeing how much a modest monthly deposit matters over decades. Your figures stay in the browser and are never uploaded.
Use Cases
How to use
Enter your starting amount in "Initial investment", for example 10,000.
Type how much you will add each month in "Monthly contribution", or leave 0 for a single lump sum.
Set the "Annual interest rate" in percent (use an after-tax rate if you want a net projection) and the horizon in "Time period (years)".
Pick the "Compound frequency" — annually, semi-annually, quarterly, monthly or daily — to match how your account credits interest.
Read the four boxes that update as you type: Future Value, Total Contributions, Total Interest Earned and Interest Percentage.
Open "Show year-by-year plan" for the yearly detail, then download the CSV or copy the table to the clipboard.
Pro Tips
- Enter the net rate, not the gross one: in most of the EU interest from deposit accounts and bonds is taxed at source, and the tax changes the compounding base.
- For an honest comparison, subtract expected inflation from the rate: 7% nominal with 2% inflation is about 5% real, and over 30 years the gap is huge.
- Compounding frequency barely matters: annual versus daily at 5% is 13 basis points. Focus on the rate and the number of years instead.
- Use the rule of 72 as a sanity check: if the calculator says 10,000 doubles in about 10 years, the implied rate is close to 7%.
- Download the CSV before closing the page: nothing is stored on the server and the fields reset to their defaults on reload.
Frequently Asked Questions
Apply the future-value formula M = C × (1 + i/n)^(n×t) and subtract the capital: interest is M − C. With 5,000 at 4% compounded quarterly (n = 4) for 8 years you get 5,000 × 1.01^32 = 6,874.70, so 1,874.70 in interest. The calculator runs the same arithmetic as you type and, if you add a monthly deposit, adds the future value of that annuity on top.
Simple interest is always computed on the starting capital; compound interest is added to the capital and earns interest itself. On 20,000 at 3% for 15 years simple interest pays 600 a year — 9,000 in total, so 29,000 — while compounding takes the balance to 31,159.35, i.e. 2,159.35 more. The gap widens with every extra year.
Divide 72 by the annual rate in percent to get the years needed to double your money. The constant that is exactly right is 69.3, that is ln(2) × 100, but only for continuous compounding; with discrete compounding no single constant is exact at every rate. 72 is used instead because it divides evenly by 2, 3, 4, 6, 8, 9 and 12. At 8% the rule says 9 years and the exact figure is 9.0; at 15% it says 4.8 against a real 5.0.
It means the balance grows by 7% a year and next year's 7% is computed on the already increased balance. With annual compounding 10,000 becomes 10,700 after one year, 11,449 after two and 19,672 after ten. Treat 7% as an assumption: type it into the "Annual interest rate" field and compare it with other values, because no future return is guaranteed.
Add the future value of the annuity to the future value of the capital: PMT × [((1 + i/12)^(12t) − 1) / (i/12)] with monthly compounding, or the same expression with the equivalent monthly rate i_m = (1 + i/n)^(n/12) − 1 for any other frequency. With 10,000 up front, 100 a month, 7% a year and 10 years you get 20,096.61 from the capital plus 17,308.48 from the contributions, i.e. 37,405.09 on 22,000 paid in.
Barely, even over long horizons. On 10,000 at 3% for 20 years annual compounding gives 18,061.11 and daily compounding 18,220.74 — 159.63 apart after two decades. The frequency you pick drives both the starting capital and the monthly deposits, converted into the equivalent monthly rate; the rate itself matters far more.
It depends on the gross rate. At 3% compounded annually, 200,000 becomes 206,000 after one year and 268,783 after ten. Tax treatment differs by country: US banks pay interest gross and report it on Form 1099-INT, UK banks have paid interest gross since April 2016, and most euro-area countries withhold at source. Enter an after-tax rate if you want a net projection.
With compound interest alone the exact time is ln(2)/ln(1 + i): 14.2 years at 5%, 10.2 at 7% and 7.3 at 10% (the rule of 72 estimates 14.4, 10.3 and 7.2). Contributions speed it up a lot: 10,000 plus 100 a month at 7% with monthly compounding passes 20,000 in 54 months, about four and a half years. No market return is guaranteed.
Any product that reinvests interest instead of paying it away: fixed-term deposit accounts with periodic compounding, accumulating funds and ETFs, and pension plans. Always compare the annual percentage yield in the product disclosure, because that is the figure that already includes the compounding frequency.
No. The whole calculation runs in your browser in JavaScript: amounts, rate and duration never leave your device and are not logged. The CSV export and the copy-to-clipboard table are also generated locally, so you can use real figures without any privacy concern.