Compound Interest Calculator with Monthly Contributions
See how your savings grow with compound interest. Add a starting amount and monthly contributions to get the future value, total interest and a year-by-year chart.
Compound interest is interest earned on both your deposits and the interest already added. With $10,000 invested at 7% a year, compounded monthly, plus $200 a month for 20 years, you end up with about $144,600 — of which roughly $86,600 is interest.
How to use Compound Interest Calculator
- Enter your initial deposit and how much you add each month (use 0 for none).
- Enter the expected annual interest rate and the number of years.
- Choose how often interest is compounded (monthly is most common) and whether you deposit at the start or end of each month.
- Read the future value, total contributions and total interest. Open the year-by-year table for the full schedule.
The compound interest formula
For a single deposit the future value is A = P × (1 + r/n)^(n×t), where P is the starting amount, r the annual rate as a decimal, n the number of compounding periods per year and t the number of years. With regular monthly contributions, each deposit grows for a shorter time, so the calculator adds them month by month. That is the same method banks and brokers use, and it is why the result matches a real savings account more closely than a simple formula.
Example: saving $200 a month
| Years | Total deposited | Interest earned | Balance at 7% |
|---|---|---|---|
| 10 | $34,000 | $20,700 | $54,700 |
| 20 | $58,000 | $86,600 | $144,600 |
| 30 | $82,000 | $243,200 | $325,200 |
Starting with $10,000, monthly compounding, deposits at the end of each month. Figures rounded.
After about 20 years the interest is larger than everything you paid in. That is the snowball effect of compounding: the longer the money stays invested, the faster it grows.
Effective annual rate
A 7% rate compounded monthly gives an effective annual rate (APY) of about 7.23%, because interest is added twelve times a year. Compounding daily raises it only slightly. When you compare savings accounts, compare the effective rates.
The rule of 72
A quick mental check: divide 72 by the interest rate to estimate how many years it takes to double your money. At 6% that is about 12 years; at 9% about 8 years.
Keep in mind
Investment returns are not guaranteed and vary from year to year, and inflation reduces what the money will buy. Starting early matters more than starting big: ten extra years of compounding can double the result. Use a lower rate for a more careful plan, and remember that taxes and fees reduce the final amount. For bills and running costs, try the Electricity Cost Calculator or the Fuel Cost Calculator.
You may also find these free tools useful: Fuel Cost Calculator, Electricity Cost Calculator and Loan Calculator.
Frequently asked questions
How do you calculate compound interest?
Multiply the balance by (1 + rate ÷ periods per year) once for every period. For a single deposit the formula is A = P × (1 + r/n)^(n×t). The calculator does this month by month and also adds your contributions.
What is the difference between simple and compound interest?
Simple interest is paid only on the original amount. Compound interest is also paid on the interest already earned, so the balance grows faster every year.
How much will $10,000 grow in 10 years?
At 7% a year compounded monthly, $10,000 grows to about $20,100 in 10 years without extra deposits. Adding $200 a month brings it to about $54,700.
Does compounding frequency matter much?
A little. Monthly compounding gives slightly more than yearly compounding at the same rate; daily adds only a tiny bit more than monthly.
Is the result guaranteed?
No. The calculator assumes a constant rate. Real investments go up and down, and taxes, fees and inflation are not included.
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