CalcEmperor

Compound interest calculator

See how a starting balance and regular deposits could grow over time when the interest earns interest.

What compound interest means

Simple interest is paid only on the money you put in. Compound interest is paid on your money and on the interest already earned, so growth accelerates over time. The classic formula for a single deposit is:

A = P × (1 + r ÷ n)n × t
P = starting balance   r = annual rate   n = compounding periods per year   t = years

This calculator extends that idea by adding a monthly contribution and stepping through every month, so you get a year-by-year table rather than a single number.

Worked example

Start with $10,000 and add $250 a month at a 7% annual return, compounded monthly, for 20 years. You will have contributed $70,000 in total. The projected balance is roughly $170,600, meaning about $100,600 came from interest rather than from your own deposits.

Time matters more than almost anything else

Compare two savers who each put in $250 a month at 7%. One invests for 30 years and ends with roughly $305,000. The other invests for 20 years and ends with roughly $130,000. Ten extra years cost only $30,000 more in deposits but add about $175,000 to the result, because the early money has longer to compound. That is why starting early usually beats contributing more later.

The Rule of 72

A quick mental shortcut: divide 72 by the annual return to estimate how many years it takes money to double. At 7%, that is about 10 years; at 4%, about 18 years. It is an approximation, but it gets close for typical rates.

Does compounding frequency matter?

Slightly. Monthly compounding beats annual compounding, and daily beats monthly, but the differences are small compared with the effect of the rate and the number of years. If you are comparing savings accounts, focus on the APY, which already includes the effect of compounding.

Important limits

Real investments do not earn a constant return, and results are not guaranteed. Fees, taxes and inflation reduce what you actually keep and what it can buy. Use a conservative rate for planning, and try a few different rates to see a range rather than a single answer. This tool is for education and is not financial advice.

Last updated September 19, 2026.

Frequently asked questions

How do I calculate compound interest?

Multiply your balance by (1 + rate ÷ periods per year) for each compounding period, and repeat for every period. With regular deposits, add each contribution as you go. This calculator handles the steps month by month.

What return should I assume?

It depends on where the money is held. Savings accounts pay a modest, variable rate; stock market returns fluctuate and have no guaranteed average. Try several values to see a realistic range.

Are contributions added at the start or end of the month?

At the end of each month. Depositing at the start would produce a slightly higher result.

Does this account for inflation or taxes?

No. The figures are nominal, before taxes, fees and inflation.

Related tools and guides