Compound Interest Calculator
See how your money grows with compounding and regular contributions.
Your details
Future value
$144,572.72
After 20 years at 7%
Balance over time
Principal vs interest
- Total contributed
- $58,000.00
- Interest earned
- $86,572.72
What does this mean?
Compound interest is interest earned on both your original money and the interest it has already earned. Over time this snowballs: the longer your money stays invested, the larger the share of your final balance that comes from interest rather than your own contributions.
The formula
For a lump sum, the future value is A = P × (1 + r/n)^(n·t), where P is the starting amount, r is the annual interest rate as a decimal, n is how many times interest compounds per year, and t is the number of years. When you add a regular monthly contribution, each deposit also grows, so we add the future value of that stream of payments on top of the lump-sum result.
A quick example
Put in $10,000 at 7% for 20 years and add $200 a month. You contribute $58,000 of your own money, but compounding turns it into far more — most of the growth happens in the final years, which is why starting early matters more than the exact rate.
Tip: small increases to your monthly contribution usually beat chasing a slightly higher rate, and they are fully in your control.
Frequently asked questions
What is compound interest in simple terms?+
It is interest that you earn on your interest. Each period, the interest you already earned is added to your balance, so the next round of interest is calculated on a larger amount.
Does compounding frequency really matter?+
It helps, but less than people expect. Going from annual to monthly compounding adds a little to your total; the bigger levers are the interest rate, how much you contribute, and how long you stay invested.
Are the monthly contributions added before or after interest?+
This calculator adds each monthly contribution at the end of the month, then compounds the balance going forward. That is the common, slightly conservative convention.
Is this calculator accurate for real investments?+
The math is exact, but real returns vary year to year and are not guaranteed. Treat the result as an estimate for planning, not a promise. It is for educational purposes only and is not financial advice.