How to use this calculator
- Enter your initial deposit — what you start with today (can be zero).
- Add a monthly contribution if you plan to save regularly.
- Enter an expected annual interest rate or investment return.
- Choose how many years to grow and how often interest compounds.
The table below the results shows your balance at the end of each year, split into what you put in and what interest added.
Why compound interest is so powerful
With simple interest you earn the same amount every year. With compound interest, each year's interest is added to the balance and then earns interest itself. The effect is small at first and enormous later. $10,000 at 7% earns $700 in the first year — but after 30 years the balance is over $76,000, and the interest earned in that final year alone is about $5,000.
This is why time matters more than amount. Someone who invests $200 a month from age 25 to 35 and then stops can end up with more at 65 than someone who invests $200 a month from 35 to 65, because the early money had three extra decades to compound.
The formula
For a single deposit: A = P(1 + r/n)nt, where P is the principal,
r the annual rate as a decimal, n the number of compounding periods per year and
t the number of years. Monthly contributions are added on top and each one compounds from the
month it is deposited. This calculator simulates every month so it handles any combination exactly.
Ways to make your money grow faster
- Start early — even small amounts benefit from decades of compounding.
- Automate contributions so saving happens before you can spend it.
- Reduce fees — a 1% annual fee can consume a quarter of your gains over 30 years.
- Reinvest returns rather than withdrawing interest or dividends.
Borrowing instead of saving? The Loan Calculator shows compound interest working against you.
Frequently asked questions
What is compound interest?
Compound interest is interest earned on both your original money and on the interest it has already earned. Over time this "interest on interest" makes savings grow faster and faster.
How often is interest compounded?
It depends on the account. Savings accounts often compound daily or monthly, bonds semi-annually, and long-term investment returns are usually quoted annually. More frequent compounding gives a slightly higher result.
What return should I assume?
High-yield savings accounts have recently paid around 3–5%. Broad stock market index funds have historically averaged around 7–10% per year before inflation, but returns vary a lot year to year and are not guaranteed.
What is the Rule of 72?
Divide 72 by the annual interest rate to estimate how many years it takes to double your money. At 8%, money doubles in about 9 years.
Does this account for inflation or taxes?
No. Results are nominal, before taxes and inflation. To estimate real growth, subtract expected inflation (for example 2–3%) from your interest rate.