Compound Interest Calculator

How to use the compound interest calculator

1. Enter your starting amount and how much you will add each month (enter 0 for none).
2. Enter the annual interest rate or expected return, and how many years the money will grow.
3. Choose how often interest compounds. Savings accounts usually compound daily or monthly.

The result shows your future balance, how much of it came from your deposits and how much from interest, plus a table for every year.

Compound interest formula

A = P × (1 + r/n)^(n×t)

P is the starting principal, r the annual rate as a decimal, n the number of times interest compounds per year, and t the number of years. With regular contributions, each deposit grows in the same way from the month it is added. The calculator adds them month by month.

Worked example

$10,000 at 7% compounded monthly for 20 years grows to $10,000 × (1 + 0.07/12)^240 = $40,387. Adding $200 a month as well brings the total to about $144,573, of which $58,000 is your own money and the rest is interest.

Why compounding matters

With simple interest you earn interest only on your original deposit. With compound interest you also earn interest on past interest, so growth speeds up over time. That is why starting early matters: in the example above, more than half of the final balance is interest.

The Rule of 72

To estimate how long it takes money to double, divide 72 by the interest rate. At 7%, money doubles in about 72 ÷ 7 ≈ 10.3 years. At 10%, it takes about 7.2 years.

This calculator assumes a fixed rate. Investment returns vary from year to year and can be negative, and it does not include taxes, fees or inflation. Use it for estimates, not guarantees.

Frequently asked questions

What is compound interest?

It is interest calculated on both your original deposit and the interest you have already earned, so your balance grows faster over time than with simple interest.

How often should interest compound?

More frequent compounding earns slightly more. The difference between monthly and daily compounding is small. The APY row shows the effective yearly rate for each option.

What is the difference between APR and APY?

APR is the stated yearly rate before compounding. APY includes compounding, so it shows what you actually earn in a year.

How much will $10,000 be worth in 10 years?

At 5% compounded monthly it grows to about $16,470. At 7% it reaches about $20,097. Try your own rate in the calculator.

Does this include taxes or inflation?

No. To see the real (inflation-adjusted) value, subtract the expected inflation rate from your interest rate.