What it is and what it is for
Compound interest is calculated on your starting money and also on the interest you have already earned. That is why growth is not a straight line: it looks slow at first and speeds up over the years. This calculator shows how much you would have at the end of the term if you start with a lump sum and add a fixed contribution every month.
Use it to compare a deposit, a fund or a savings account, to see the effect of starting early and to set a realistic goal. The maths assumes a constant rate and monthly compounding, so treat the output as a reference and not as a promise of returns.
How to use it, step by step
- Type the money you start with. If you begin from zero, enter 0.
- Enter how much you will add each month. It can be 0 if you only want to see a lump sum grow.
- Enter the annual interest rate you expect. Use a cautious value, not the best-case scenario.
- Choose the number of years. Long terms are where compounding makes the difference.
- Press Calculate and compare the final balance with the total you paid in: the gap is the interest.
Worked example
With 1,000 to start, 100 a month, 8% a year and 10 years, you pay in 13,000 in total. The final balance is around 20,500, so about 7,500 is interest. Keep the same inputs for 20 years and you pay in 25,000 while the balance reaches about 63,800: time matters more than the monthly effort.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is always calculated on the original amount. Compound interest is also calculated on accumulated interest, so it grows faster over time.
Are contributions added at the start or the end of the month?
The calculator adds them at the end of each month. If you contribute at the start, the real result will be slightly higher.
Does it include fees, inflation or taxes?
No. They all lower the real return, so it is wise to subtract them from the rate you enter.
Can I use a 0% rate?
Yes. The result is then just your starting money plus all the contributions.