Compound Interest Calculator (Monthly)

📈 Compound Interest Calculator Tool

Principal Amount

Annual Interest Rate (%)

Time Period (Years)

Compounding Frequency

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Final Amount

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Principal

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Interest Earned

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Money Multiplied

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Years to Double (Rule of 72)

Principal—

Interest Earned—

Year-by-Year Growth

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Enter principal, rate and time period to see how your money grows with compound interest.

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How Do You Calculate Compound Interest?

Compound interest is interest earned on your original money and on the interest already added. Use A = P(1 + r/n)^(nt): P is the amount invested, r the yearly rate as a decimal, n how often interest is added each year and t the years. 100,000 at 10% compounded monthly grows to 732,807 in 20 years.

The Compound Interest Formula

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

A is the final amount, P the principal, r the annual rate as a decimal (10% = 0.10), n the number of compounding periods a year (12 for monthly, 365 for daily) and t the number of years. Interest earned is A − P.

Worked Example

Invest 100,000 at 10% for 20 years. With simple interest you would have 300,000. With monthly compounding it grows to 732,807, more than twice as much, because each month's interest starts earning interest too.

Monthly vs Daily vs Annual Compounding

100,000 at 10% for 10 years:

Compounding	Final amount	Interest earned
Annually	259,374	159,374
Semi-annually	265,330	165,330
Quarterly	268,506	168,506
Monthly	270,704	170,704
Daily	271,791	171,791

More frequent compounding always pays a little more, but the jump from monthly to daily is small. The rate and the number of years matter far more than the frequency.

How Money Grows Over Time

100,000 at 8% a year, compounded monthly:

Years	Final amount	Growth
5	148,985	1.49×
10	221,964	2.22×
20	492,680	4.93×
30	1,093,573	10.94×

Growth speeds up with time: the last 10 years add more than the first 20 put together. That's why starting early matters. 100,000 left for 35 years at 8% grows to about 1.63 million, while 200,000 invested 10 years later reaches only about 1.47 million by the same date.

Rule of 72: How Long to Double Your Money

Divide 72 by the annual rate to estimate the years it takes to double. It's close for everyday rates:

Rate	Rule of 72	Exact (annual compounding)
6%	12 years	11.9 years
8%	9 years	9.0 years
10%	7.2 years	7.3 years
12%	6 years	6.1 years

Lump Sum vs Monthly Contributions

This calculator grows a single lump sum invested once at the start. To add a fixed amount every month, use the SIP calculator, and to work out how much to save for a target, use the savings goal calculator. For the borrowing side of the same maths, see the EMI calculator or read how compound interest works. Results assume a fixed rate; real investment returns vary, so treat them as estimates rather than financial advice.

How to Use the Compound Interest Calculator

Enter the starting amount (principal).
Enter the annual interest or return rate.
Enter the number of years and choose how often interest is added: annually to daily.
Read the final balance, the interest earned and the year-by-year growth.

Who Uses the Compound Interest Calculator?

Savers: see how much a deposit grows if the interest is left in.
Comparing accounts: check whether monthly compounding beats a slightly higher annual rate.
Teaching: show why starting early matters more than adding more later.

How the Compound Interest Calculator Saves You Time

Compound growth needs repeated multiplication or a spreadsheet. The calculator applies the formula for any compounding frequency instantly.

Frequently Asked Questions

What is the compound interest formula?

A = P(1 + r/n)^(nt), where A is the final amount, P is the principal, r is the annual rate as a decimal, n is the number of compounding periods per year and t is the time in years. Subtract P from A to get the interest earned.

How do I calculate compound interest monthly?

Divide the annual rate by 12 and multiply the years by 12, then use A = P(1 + r/12)^(12t). For 100,000 at 10% for 10 years: 100,000 × (1 + 0.10/12)^120 = 270,704. Choose Monthly in the calculator above to do this automatically.

How does compound interest differ from simple interest?

Simple interest is paid only on the original amount, so 100,000 at 10% earns 10,000 every year. Compound interest is also paid on interest already earned, so the yearly amount keeps growing. Over 20 years, simple interest gives 300,000; monthly compounding gives 732,807.

Is daily compounding much better than monthly?

Only slightly. 100,000 at 10% for 10 years grows to 270,704 with monthly compounding and 271,791 with daily compounding, a difference of about 1,100. The interest rate and how long you stay invested make a much bigger difference than the compounding frequency.

What is the Rule of 72?

A quick way to estimate how long money takes to double: divide 72 by the annual interest rate. At 8%, money doubles in about 9 years; at 12%, about 6 years. It is an approximation that works well for rates between roughly 6% and 10%.

What is the difference between the interest rate and APY?

The rate you enter is the nominal annual rate. APY, or annual percentage yield, includes the effect of compounding, so it is higher whenever interest is added more than once a year. A 10% rate compounded monthly equals an APY of about 10.47%.

Can I add monthly deposits in this calculator?

No. This calculator models a single lump sum invested at the start. For a fixed amount added every month, use the SIP calculator, which is built for regular monthly investing, or the savings goal calculator to find the monthly amount needed to reach a target.

Last updated: September 2026