Compound Interest Calculator
See how your savings grow with compound interest and regular deposits, work out simple interest, or find what a CD or term deposit pays at maturity.
Growth over time
Year-by-year schedule
| Period | Added | Interest | Balance |
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How to use the compound interest calculator
- Choose a mode: Compound for long-term savings growth, Simple for interest on the principal only, or Savings / CD for what a term deposit pays at maturity.
- Enter your starting amount and the annual interest rate (APR) your bank or investment pays.
- For compound interest, set the time period, how often interest is compounded and an optional monthly or yearly contribution.
- Open Contribution timing, tax and inflation to model deposits at the start of each period, tax on interest or the real value of your money.
- Hover or tap the chart to see any year, open the year-by-year schedule, and copy the result or the full breakdown.
How compound interest works
Interest is what you earn for leaving money in a savings account, CD or investment, usually quoted as an annual percentage rate. The same rate can produce very different results depending on how it is applied. With simple interest you only earn on the original principal. With compound interest the interest is added to your balance and starts earning interest itself, so growth speeds up over time.
The compound interest formula
A = P × (1 + r ÷ n)n × t
P is the principal, r the annual rate as a decimal, n the number of times interest is compounded per year and t the number of years. $10,000 at 5% compounded monthly grows to about $16,470 in 10 years. Try it in the calculator.
Adding regular contributions
Regular deposits make the biggest difference. Add $200 every month to the same account and after 10 years you have about $47,527: $34,000 you paid in and $13,527 of interest. See the growth chart. Contributing at the start of each period instead of the end earns slightly more, because each deposit has one extra period to grow.
APR vs APY
The APR is the nominal annual rate. The APY (annual percentage yield, or effective annual rate) includes the effect of compounding: 5% compounded monthly is an APY of about 5.12%, and daily compounding adds a little more. Use APY to compare accounts that compound at different frequencies.
Simple interest and CDs
Interest = P × r × t
Simple interest is common for short-term loans between people, some bonds and term deposits that pay out at maturity. $50,000 at 4% for 1 year earns $2,000. The Savings / CD tab counts the term in days (on a 365-day year) and can deduct tax from the interest.
The rule of 72 and inflation
To estimate how long it takes to double your money, divide 72 by the interest rate: at 8% it takes about 9 years. Also keep inflation in mind. If your rate is below inflation, your balance grows but its purchasing power shrinks. Enter an inflation rate to see the result in today’s money.
Everything is calculated in your browser and no numbers are sent anywhere. Results are estimates; your bank’s day count and rounding may differ slightly.
Frequently asked questions
How do I calculate compound interest?
Use A = P × (1 + r/n)^(n × t), where P is the principal, r the annual rate as a decimal, n the compounding periods per year and t the years. The interest earned is A minus P. This calculator also handles regular contributions, tax and inflation.
What is the difference between simple and compound interest?
Simple interest is paid only on the original principal. Compound interest is paid on the principal plus the interest already earned, so the balance grows faster every year.
Does compounding frequency matter?
Yes, but less than the rate and the time. At 5%, compounding monthly instead of yearly raises the effective rate from 5.00% to about 5.12%, and daily compounding to about 5.13%.
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 6% it takes about 12 years; the exact answer is 11.9 years.
Is my data saved or sent anywhere?
No. All calculations run in your browser. Only your last inputs are remembered on your own device so they are there next time.