Enter a starting amount, interest rate, and number of years to see how your money grows with compound interest, with an optional monthly contribution.
Starting amount ($):
Annual interest rate (%):
Years:
Compounds per year:
Monthly contribution ($, optional):
How It Works
The calculator applies interest period by period based on how often it compounds, adding any monthly contribution along the way, and totals the final balance, contributions, and interest earned.
How to Use This Tool
- Enter your starting amount, interest rate, and number of years.
- Leave compounds per year at 12 for monthly compounding, or change it if you know your account compounds differently.
- Optionally add a monthly contribution, then click Calculate.
This assumes a constant interest rate for the whole period, which is a simplification; real investment returns vary year to year.
The Compound Interest Formula
Without contributions, the balance follows a simple formula:
A = P × (1 + r ÷ n)n × t
- A is the final balance.
- P is the starting amount.
- r is the annual rate as a decimal (7% is 0.07).
- n is how many times interest compounds per year.
- t is the number of years.
Regular monthly contributions add a second piece: each deposit starts earning interest as soon as it lands in the account, so early contributions grow the most.
Worked example: put 10,000 into an account earning 7% a year, compounded monthly, and leave it for 10 years. The balance grows to roughly 20,100, so about 10,100 of that is interest you earned on your interest. Now add 100 a month. Over the same 10 years you contribute 12,000 of your own money, and the total balance lands near 37,400. Your 22,000 of deposits (the starting amount plus contributions) produced about 15,400 in interest. Small results will differ by a few dollars depending on when in the month deposits are counted.
The Rule of 72
A quick mental shortcut: divide 72 by the annual rate to estimate how many years it takes money to double. At 7%, that is about 10 years, which matches the example above. At 4%, it takes about 18 years; at 10%, about 7. The rule is an approximation, but it makes the power of time and rate easy to see at a glance.
Why Time Matters More Than Amount
Try running the same calculation for 10, 20, and 30 years. The balance does not grow in a straight line; each extra decade adds more than the last, because a larger balance earns more interest. That is why financial educators often say the best time to start saving is early, even with small amounts. The calculator also works in reverse as a reality check: compare what you pay in deposits to what the account earns, and you can see how much of your final balance came from time rather than effort.
Did You Know?
- Compound interest means you earn interest on your interest, not just on your original deposit, which is why growth accelerates over time.
- Starting even small regular contributions early can outperform larger contributions started later, thanks to the extra time compounding has to work.
Frequently Asked Questions
Is this financial advice?
No, this is a general math calculator, not personalized financial advice. Talk with a financial advisor for guidance specific to your situation.
What does “compounds per year” mean?
It is how often interest is calculated and added to your balance each year, commonly 12 (monthly) for savings accounts, but sometimes daily, quarterly, or annually.
What is the difference between simple and compound interest?
Simple interest is calculated only on the original amount, so growth is a straight line. Compound interest is calculated on the original amount plus the interest already earned, so growth speeds up over time. Savings accounts, most investments, and loans generally use compound interest.
Does compounding more often make a big difference?
Only a small one. Moving from annual to monthly compounding at 7% adds a modest amount over ten years, and moving from monthly to daily adds very little. The rate and the length of time matter far more than the compounding frequency.
Does the result account for taxes, fees, or inflation?
No. The figures are before taxes, account fees, and inflation, all of which reduce what your money can actually buy. Treat the result as a projection of growth under a steady rate, not a guarantee.