Compound Interest Calculator

See how much your money grows when interest earns interest. Add a starting amount and monthly deposits, then change any number to update the result instantly.

Compounding
Balance after 20 years
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Total deposits–
Interest earned–
Year 1Year 20

Year-by-year growth

YearTotal depositsInterestBalance

What is compound interest?

Compound interest means you earn interest on your original money and on the interest it has already earned. Put $1,000 in an account paying 10% a year and you have $1,100 after the first year. In the second year you earn 10% on $1,100, not on $1,000, so you finish with $1,210. That extra $10 is interest on interest, and over long periods it becomes the largest part of your balance.

Simple interest, by contrast, is only ever paid on the starting amount. The longer the time frame, the bigger the gap between the two.

How to use this calculator

  1. Initial deposit: the amount you start with today. Enter 0 if you are starting from nothing.
  2. Monthly contribution: how much you plan to add every month.
  3. Annual interest rate: the yearly rate or expected average return, as a percentage.
  4. Years: how long the money stays invested.
  5. Compounding: how often interest is added to the balance. Savings accounts often compound daily or monthly.

The result shows your final balance, how much of it came from your own deposits, and how much came from interest. The chart and table show how the balance builds each year.

The compound interest formula

For a single deposit, the future value is:

A = P × (1 + r / n)n × t

Here A is the final amount, P the starting deposit, r the yearly rate as a decimal (5% = 0.05), n the number of times interest is added each year, and t the number of years.

Regular monthly deposits are added with the future value of an annuity formula, PMT × ((1 + i)N − 1) ÷ i, where i is the monthly rate and N the number of months. This calculator runs the numbers month by month, which gives the same answer and also produces the yearly table.

Worked example

Say you start with $10,000, add $100 every month, and earn 5% a year compounded monthly for 10 years.

  • The $10,000 grows to $16,470.09.
  • The 120 monthly deposits of $100 grow to $15,528.23.
  • Your total balance is $31,998.32, of which $22,000 is your own money and $9,998.32 is interest.

Does compounding frequency matter?

Yes, but less than most people expect. Here is $10,000 at 5% for 10 years with no extra deposits:

CompoundingFinal balance
Yearly$16,288.95
Quarterly$16,436.19
Monthly$16,470.09
Daily$16,486.65

Moving from yearly to daily compounding adds about $200 here. The interest rate and the number of years have a far bigger effect.

Why starting early matters most

Time is the strongest lever in compounding. Saving $200 a month at a 7% average return for 40 years grows to roughly $525,000 from $96,000 of deposits. Start 10 years later and save for 30 years instead, and you end with about $244,000 from $72,000 of deposits. Ten fewer years of saving cuts the result by more than half.

Tips for realistic estimates

  • Use a conservative rate. High-yield savings accounts and CDs pay a fixed or variable rate, while stock returns change from year to year.
  • Fees reduce your effective return. A fund charging 1% a year turns a 7% return into roughly 6%.
  • Remember inflation. To see the result in today's money, lower the rate by your expected inflation rate.
  • Taxes can apply to interest each year unless the money is in a tax-advantaged account.

Last reviewed October 2026.

Frequently asked questions

What is the difference between APR and APY?

APR is the yearly rate before compounding. APY includes the effect of compounding, so it shows what you actually earn in a year. A 5% APR compounded monthly equals about 5.12% APY. If your bank quotes an APY, enter it with yearly compounding to get the closest match.

How often is interest compounded?

It depends on the account. Most savings accounts and CDs compound daily or monthly, many bonds compound twice a year, and stock market returns are usually described as a yearly average. Pick the option that matches your account, or use monthly as a sensible default.

How accurate is this compound interest calculator?

The math is exact for the numbers you enter. Real results will differ if your rate changes, if you skip or change deposits, or if fees and taxes apply. Treat the result as a planning estimate, not a guarantee.

When are my monthly contributions added?

Each contribution is added at the end of the month, then earns interest from the following month on. This is the standard assumption used for savings plans and matches the usual future value of an annuity formula.

Does the calculator include taxes or inflation?

No. The result is the balance before taxes and in future dollars. Interest in a regular savings account is usually taxable each year, while accounts such as a Roth IRA can grow tax-free. Inflation also reduces what the final amount can buy.

What is the rule of 72?

It is a quick way to estimate how long money takes to double: divide 72 by the yearly interest rate. At 6% a year, money doubles in about 12 years (72 ÷ 6). The calculator gives the exact answer, but the rule is handy for mental math.