Compound interest calculator
Type in a starting sum, a monthly top-up, a rate and a time span. The page shows the answer and every step behind it.
On this page
- Compound interest is interest earned on your deposits and on interest already added to the balance.
- With the defaults, $1,000 plus $100 a month at 4% for 10 years ends at $16,215.81, including $3,215.81 of interest.
- The rate and the number of years move the result far more than how often interest is compounded.
- The output is arithmetic on the rate you type in, not a forecast; real rates change and investments can fall.
This calculator grows a balance month by month, adding interest on deposits and on interest already earned. With its defaults — $1,000 to start, $100 a month, 4% a year compounded monthly, 10 years — the balance reaches $16,215.81, of which $3,215.81 is interest.
How do you use the compound interest calculator?
Starting amount. The money already in the account on day one.
Added each month. A regular deposit. The calculator adds it at the end of every month, after that month's interest. Type 0 to see a lump sum on its own.
Annual interest rate. The yearly rate before compounding, as a percentage.
Years. Whole years, from 1 to 60.
Compounding. How often interest is added: yearly, quarterly, monthly or daily.
Press Calculate. Three numbers come back: the final balance, the total deposited (your own money) and the interest earned (the difference). Everything runs in your browser; nothing you type is sent anywhere.
How is it calculated?
Investor.gov, the SEC's investor education site, defines compound interest as interest paid on the original principal and on the interest that has already accumulated. The calculator applies that idea one month at a time.
Step 1 — turn the yearly rate into a monthly growth factor. With r the annual rate as a decimal and m the compounding periods per year:
g = (1 + r / m)m/12
For monthly compounding (m = 12) this is simply 1 + r/12.
Step 2 — repeat for every month. Start with balance = P (the starting amount) and, for each of the n = 12 × years months:
balance = balance × g + C
where C is the monthly deposit. The same result in one line is B = P × gn + C × (gn − 1) / (g − 1).
Step 3 — split the result. Total deposited = P + C × n and Interest = B − Total deposited.
Worked example: where does $16,215.81 come from?
These are the same figures the calculator shows before you change anything, so you can confirm the tool and the formula agree.
How do rate, time and compounding change the result?
Each row changes one input from the defaults ($1,000 start, $100 a month, 4%, 10 years, monthly compounding).
| Scenario | Final balance | Interest earned |
|---|---|---|
| Defaults | $16,215.81 | $3,215.81 |
| Rate 2% | $14,493.17 | $1,493.17 |
| Rate 6% | $18,207.33 | $5,207.33 |
| 20 years | $38,900.04 | $13,900.04 |
| 30 years | $72,718.44 | $35,718.44 |
| Yearly compounding | $16,149.84 | $3,149.84 |
| Daily compounding | $16,221.78 | $3,221.78 |
| No monthly deposits | $1,490.83 | $490.83 |
Two patterns stand out. Doubling the time from 10 to 20 years more than quadruples the interest, because later interest is earned on a much larger balance. Compounding frequency matters far less: daily instead of yearly compounding adds $71.94 over the whole decade.
What does this calculator not tell you?
- Whether the rate will last. The tool applies one fixed rate every month for the whole period. Savings rates can change (see how interest rates affect markets), and investment returns are not fixed at all.
- What the money will buy. $16,215.81 in ten years is not worth $16,215.81 today. Run the result through the inflation calculator to see its purchasing power.
- Taxes and fees. Neither is deducted. Both reduce what compounds.
- Exact bank timing. Banks may credit interest on daily balances and on different dates. Daily and quarterly options here use an equivalent monthly rate, so a statement can differ by a few cents or dollars.
What mistakes do people make with compound interest figures?
- Comparing an APR with an APY. One excludes compounding, the other includes it. Compare like with like.
- Counting deposits as growth. In the default example, $13,000 of the $16,215.81 is your own money; only $3,215.81 is interest.
- Treating a long-run projection as a plan. A 30-year result depends on a rate holding for 30 years.
Questions readers ask
Does daily compounding make a big difference?
Not much at ordinary rates. With the default inputs, daily compounding gives $16,221.78 after 10 years against $16,215.81 for monthly — $5.97 more.
What is the difference between compound and simple interest?
Simple interest is paid only on the original amount. $1,000 at 4% simple interest earns $40 a year, or $400 over 10 years. Compounded monthly with no deposits, the same $1,000 earns $490.83, because interest also earns interest.
Can I use this for crypto lending or staking yields?
The arithmetic works for any fixed rate, but those yields are usually variable and the underlying asset can fall in price, so the result is only an illustration. See DeFi lending explained for the risks.
Why does my bank statement show a slightly different number?
Banks may calculate interest on daily balances, credit it on set dates and round each step. This tool assumes deposits arrive at the end of each month and the rate never changes.
Compound growth comes from three things: the rate, the time and the money you keep adding. The calculator shows how they combine under one fixed assumption. Use it to compare scenarios, then check the rate's terms, taxes and inflation before relying on any figure.
Sources
- Investor.gov, US Securities and Exchange Commission, Compound interest (glossary) (2026)Primary source
- Consumer Financial Protection Bureau / Electronic Code of Federal Regulations, 12 CFR Part 1030 (Regulation DD, Truth in Savings), Appendix A: Annual Percentage Yield Calculation (2026)Primary source
Educational content only — not financial, investment, legal or tax advice. Crypto-assets are high-risk and you could lose all the money you put in. Rules differ by country; check with your national regulator. See our risk disclosure and editorial policy.
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