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Compound Interest Calculator

See how money grows when interest earns interest. Enter your numbers and the results update as you type.

How compound interest works

Put $1,000 in an account paying 5% a year. After one year you have $1,050.

In year two you earn 5% on $1,050, not $1,000. That is $52.50, and your balance becomes $1,102.50.

The extra $2.50 is interest on interest. It looks tiny, but it is the whole idea.

Each year the interest is a little bigger than the year before. After 30 years that $1,000 is $4,322, and $3,322 of it is interest.

Why time matters most

Compounding is slow at first and fast later. The curve bends upward the longer you leave it.

At a 7% return, someone who invests $200 a month from age 25 to 35 and then stops ends up with more at 65 than someone who invests $200 a month from 35 to 65. The first person put in a third as much money.

That is why the best time to start is now. The second best time is also now.

The formula

The standard compound interest formula looks like this.

A = P(1 + r/n)^(nt)

A = final amount, P = starting amount, r = annual rate as a decimal, n = compounding periods a year, t = years.

For $10,000 at 6% compounded monthly for 10 years: A = 10,000 x (1 + 0.06/12)^120 = $18,194.

Regular deposits add a second term to the formula. The calculator above handles that for you.

Three things that drive the result

Compounding frequency is a distant fourth. Daily versus monthly barely moves the needle.

Compounding works against you too

Credit cards, loans and mortgages compound in reverse. Interest is charged on interest you have not paid.

That is why a $5,000 card balance at 25% can take decades to clear on minimum payments. The credit card calculator shows exactly how long.

A quick example

Years$10,000 at 4%$10,000 at 7%$10,000 at 10%
5$12,167$14,026$16,105
10$14,802$19,672$25,937
20$21,911$38,697$67,275
30$32,434$76,123$174,494
40$48,010$149,745$452,593

At 10% the money grows 45 times over in 40 years. At 4% it grows under 5 times.

The rate matters, but every column shows the curve bending upward.

More combinations are in the growth tables.

This is not financial advice. Nothing on this site is investment, tax, legal or financial advice of any kind.

The calculators give simplified estimates from the numbers you enter. They ignore taxes, fees, rate changes and real market returns, and they can be wrong.

Do not make financial decisions based on these results. Speak to a licensed financial adviser, accountant or lender before acting, and read the full disclaimer.

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