Your result
What your result means
Starting with $10,000 and adding $300 each month for 20 years at 5% with monthly compounding, you would have $150,436.50. You put in $82,000; the other $68,437 is interest, much of it earned on earlier interest. The effective annual rate after compounding is 5.116%.
The math
Formula and sources
i = (1 + r ÷ k)^(k ÷ 12) − 1 (monthly rate equivalent) FV = P × (1 + i)^n + D × ((1 + i)^n − 1) ÷ i × (1 + i if deposits are at the start)
- P
- initial deposit
- D
- monthly deposit
- r
- annual rate
- k
- compounding periods per year
- n
- months
This matches Excel’s FV function. Taxes, fees and changing rates are not included.
Sources we checked this against
- 1How does compound interest work?Consumer Financial Protection Bureau · consumerfinance.gov
- 2Compound Interest Calculator and explanationU.S. Securities and Exchange Commission, Investor.gov · investor.gov
- 3FV function (future value formula and examples)Microsoft Support · support.microsoft.com
Formula last checked 2026-11-04. Found an error? Tell us and we will fix it.
Step by step
Two worked examples
CFPB: $1,000 at 5%, compounded yearly
Total deposited
$1,000
- Initial deposit
- $1,000
- Monthly deposit
- $0
- Annual interest rate
- 5%
- Years
- 2 years
- Compounding
- yearly
- Deposits made at
- End of each month
Year 1 earns $50 (balance $1,050); year 2 earns 5% of $1,050 = $52.50, for $1,102.50. The extra $2.50 is interest on interest.
Monthly saver for 20 years
Total deposited
$82,000
- Initial deposit
- $10,000
- Monthly deposit
- $300
- Annual interest rate
- 5%
- Years
- 20 years
- Compounding
- monthly
- Deposits made at
- End of each month
Deposits total $82,000. Compounding monthly at 5% turns that into $150,436.50, so interest contributes $68,437.
| Result | Example 1 | Example 2 |
|---|---|---|
| Total deposited | $1,000 | $82,000 |
| Interest earned | $103 | $68,437 |
| Future balance | $1,102.50 | $150,436.50 |
Questions
Frequently asked questions
How often should interest compound?
More frequent compounding helps a little: 5% compounded daily is about 5.127% a year versus 5.116% monthly. The rate itself and the time you leave money invested matter far more.
What is the Rule of 72?
Divide 72 by the annual rate to estimate how many years money takes to double. At 6% that is about 12 years. It is a mental shortcut; this calculator gives the exact figure.
Can I use this for stock market returns?
You can enter an average return, but markets do not grow at a steady rate. Treat the result as one scenario and try lower rates to stress-test your plan.
Does it include inflation?
No. To see today’s purchasing power, use a real rate (return minus expected inflation), or try the retirement savings calculator, which adjusts for inflation.