Compound Interest Calculator
See how investments grow over time. Visualize compound growth with a chart and yearly table.
☕ This tool is free forever. If it saved you time, buy me a coffee.
- Verified: results checked against independent formulas on every release
- Last reviewed:
- Runs in your browser: nothing is uploaded
- Found an error?
How this is calculated
The calculator compounds monthly. Each month adds interest on the current balance, then your monthly contribution.
Formulas
The monthly rate is the annual rate divided by 12.
i = rate ÷ 100 ÷ 12
Each month the balance grows by interest and then receives the deposit (an ordinary annuity).
balance ← balance × (1 + i) + deposit
After n months this has the closed form:
FV = P × (1 + i)ⁿ + deposit × ((1 + i)ⁿ − 1) ÷ i
At 0% interest nothing compounds.
FV = P + deposit × n
Interest earned is the final balance minus everything you put in.
earned = FV − (P + deposit × n)
Sources
- Investor.gov (SEC), Compound interest calculator. The SEC's investor education calculator for the same compounding model.
- Investor.gov (SEC), Compound interest glossary entry. Defines compound interest: interest earned on both principal and previously earned interest.
How we test this
This tool is checked against independent formulas every time the site is released, and a release does not ship if a check fails. Example from the test suite: $500 a month for 30 years at 7%, starting from $0 gives $609,985. The test evaluates the closed-form formula above on its own and checks the calculator's month-by-month simulation agrees to the cent.
The test is “compound-interest: $500 a month for 30 years at 7% is $609,985” in tests/golden-calculators.test.mjs. More on how all tools are tested: our testing approach.
Assumptions and limits
- A constant rate of return. Real investments rise and fall, and past returns do not predict future ones.
- Contributions are made at the end of each month, after that month's interest.
- No taxes, fees or inflation. Results are in nominal dollars.
- This is a projection for planning, not financial advice.
Last reviewed , the date of the latest change to this tool's calculation or content.
Worked examples
Compound interest is easiest to understand with numbers. These examples use the same monthly compounding as the calculator above, so you can reproduce each result by entering the same inputs.
A single deposit left alone
$10,000 at 7% a year for 20 years, with no further deposits, grows to $40,387. About $30.4k of that is interest earned on interest.
Saving $500 a month
Starting from zero at 7%, $500 a month grows to $86,542 after 10 years, $260,463 after 20 years and $609,985 after 30 years. You contribute $60,000, $120,000 and $180,000 respectively, so the interest share rises with time.
The rule of 72
Divide 72 by the annual rate to estimate how many years it takes to double. At 7%, 72 ÷ 7 ≈ 10.3 years. It is an approximation, but it works well for rates between about 4% and 12%.
Why starting earlier matters
The 30 year result above is 2.3 times the 20 year result, even though you only contributed 1.5 times as much money. The extra decade of growth does most of the work.
Helpful answers
Frequently asked questions
- What is the compound interest formula?
- With monthly compounding, balance = P × (1 + r/12)^(12t) + PMT × ((1 + r/12)^(12t) − 1) ÷ (r/12), where P is the starting amount, PMT the monthly deposit, r the annual rate as a decimal and t the years.
- How often does the calculator compound?
- Monthly, with each month's deposit added after that month's interest. Annual or daily compounding gives slightly different totals.
- Does the result account for inflation, taxes or fees?
- No. It assumes a constant rate and no withdrawals, taxes or fees, so treat the result as an illustration of the maths and not a forecast of what an investment will return.
- What is the difference between simple and compound interest?
- Simple interest is paid only on the original amount. Compound interest is paid on the original amount plus the interest already earned, which is why balances accelerate over time.
Navigation
When to use this
You want to know what happens if you invest $500/month for 25 years. Or you're trying to show a teenager why starting a Roth IRA at 18 instead of 30 means retiring with twice the money despite contributing less. Compound interest is the single most important concept in personal finance, and this calculator makes it visual.
It's also the tool that settles debates. "Should I invest a lump sum or dollar-cost average?" "Does compounding frequency actually matter?" "How much does a 1% fee drag on returns over 30 years?" Plug in the numbers and see for yourself — the chart makes the answer obvious in a way that formulas don't.
The growth chart is the key feature. Watching the gap widen between your contributions (the money you put in) and your total balance (what compounding grew it to) is the most powerful financial visualization there is.
Good to know
Starting early beats investing more. $200/month from age 22 to 65 at 8% grows to about $895,000. $400/month from age 32 to 65 (more contributed: about $158,000 vs about $103,000) grows to only about $773,000. The 10 extra years of compounding are worth more than doubling your contribution.
Compounding frequency barely matters. Monthly vs. daily vs. continuous compounding on a $10,000 investment at 7% over 30 years differs by only about $497 between monthly and continuous compounding. Don't let anyone sell you a product based on "daily compounding" — the edge is trivial.
The Rule of 72 is your mental shortcut. Divide 72 by the annual return rate to estimate how many years it takes to double your money. At 8%, money doubles every 9 years. At 6%, every 12 years. At 10%, every 7.2 years.
Fees compound too — in reverse. A 1% annual fee doesn't sound like much, but on a $500,000 portfolio over 30 years at 8%, it costs you roughly $1,410,000 in lost growth compared with the same portfolio earning the full 8% (no new deposits in either case). Compound interest giveth, and compound fees taketh away.
Quick Reference
| Monthly Investment | Years | Return | Total Contributed | Final Balance |
|---|---|---|---|---|
| $200 | 10 | 8% | $24,000 | $36,589 |
| $200 | 20 | 8% | $48,000 | $117,804 |
| $200 | 30 | 8% | $72,000 | $298,072 |
| $500 | 20 | 7% | $120,000 | $260,463 |
| $500 | 30 | 7% | $180,000 | $609,985 |
| $1,000 | 30 | 8% | $360,000 | $1,490,359 |