How to use the Compound Interest Calculator
- 1 Enter your starting balance, even if it is zero.
- 2 Enter how much you will add each month.
- 3 Enter the annual interest rate or expected return and the number of years.
- 4 Choose the compounding frequency, then read the final balance and the interest earned.
Worked example
$10,000 plus $200 a month at 5% for 10 years, compounded monthly. You put in $10,000 plus $200 × 120 months = $34,000. With compounding, the balance grows to about $47,526.55, so $13,526.55 of it is interest earned on your money and on earlier interest.
Features
Monthly deposits
Regular contributions are added every month, the way most people save.
Any compounding
Yearly, quarterly, monthly or daily compounding, converted to an exact monthly growth rate.
Clear split
See your own money and the interest separately, so the effect of compounding is obvious.
When to use it
Retirement saving
See what $300 a month could become over 30 years at different returns.
Saving for a child
Estimate a college fund from a starting gift and monthly deposits.
High-yield savings
Check what a savings account at 4% to 5% earns over a few years.
Starting early
Compare starting at 25 and at 35 to see how much time adds.
Frequently asked questions
What is compound interest?
Interest earned on both your original money and the interest already added. Over long periods it grows much faster than simple interest.
Does compounding frequency matter much?
A little. Daily compounding earns slightly more than monthly or yearly at the same rate, but the rate and the number of years matter far more.
Are investment returns guaranteed?
No. Savings accounts and CDs pay a stated rate, but stock and fund returns vary year to year. Use a conservative rate for planning.
Does this include taxes or inflation?
No. Interest in a taxable account is taxed, and inflation reduces what money buys. Use a lower rate to allow for both.
When are deposits added?
At the end of each month, after that month's interest, which is the usual assumption for savings plans.
How much will $100 a month grow to?
At 7% a year compounded monthly, $100 a month grows to about $17,400 in 10 years and about $121,000 in 30 years, of which you deposited only $36,000. Time does most of the work, which is why starting early matters so much.
What is a realistic rate to use?
High-yield savings and CDs pay their stated rate. For a diversified stock fund, many planners use 5% to 7% a year after inflation for long periods, but returns can be negative in any single year.
Can I see the effect of starting later?
Yes. Run the calculator twice, once with more years and once with fewer, keeping everything else the same. The gap shows the cost of waiting.
Results are estimates based on the official rates linked above. Confirm important figures with the issuing authority or a qualified adviser before you act on them.
Reviewed by ToolsUnlocked Team · Last updated