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Compound Interest Calculator

A compound interest calculator shows how a starting amount and monthly additions grow when interest is paid on earlier interest too.

How much could my money grow?

Compounding frequency

How often the interest is added to the balance. Monthly adds it twelve times a year, daily adds it 365 times (we assume 365 days), and yearly adds it once. The choice changes the answer only a little.

How much goes in on day one. Type a plain number like 100, with no dollar sign or commas. The most we accept is $10,000,000.

How much you add at the end of every month. Type 0 for none. The most we accept is $100,000 a month.

The yearly interest rate you want to try, as a plain number like 7. It is an assumption, not a forecast. We accept 0 to 30.

How many whole years the money grows. We accept 1 to 80.

How Does a Compound Interest Calculator Work?

A compound interest calculator grows a balance one month at a time, adding interest to the balance each time so the next round of interest is figured on a larger number. In the formula, A is the final amount, P the starting amount, r the yearly rate as a decimal, n how many times a year interest is added, and t the years.

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

With a monthly addition M: balance = P × (1 + i)^m + M × ((1 + i)^m - 1) ÷ i, where i is the growth for one month and m is the number of months.

  1. Choose a starting amount, a monthly addition, a yearly rate and a number of years.
  2. We turn the yearly rate into a rate for one month. With monthly compounding that is the yearly rate divided by 12. Daily compounding adds interest 365 times a year, and we assume 365 days with no leap years. Yearly compounding adds it once.
  3. The starting amount is there from day one. Each monthly addition goes in at the end of its month, so the first one earns nothing until the second month.
  4. Total put in is the starting amount plus 12 monthly additions for every year. Interest earned is the balance minus the total put in.
  5. Interest share of balance is the interest earned divided by the final balance.
  6. The table shows the same plan every 5 years, and always the last year.
$100 to start and $25 a month at 7% a year, compounded monthly, shown every 5 years. The rate is an assumption, and history is not a forecast.
YearTotal put inInterest earnedBalance
0$100$0$100
5$1,600$332$1,932
10$3,100$1,428$4,528
15$4,600$3,609$8,209
20$6,100$7,327$13,427
25$7,600$13,224$20,824
30$9,100$22,211$31,311
35$10,600$35,577$46,177
40$12,100$55,151$67,251

The Math Behind Compound Interest

Why does starting early matter so much?

Starting early matters because compound interest pays interest on earlier interest, so the last years of growth are the biggest ones. Rapunzl's calculator shows it with the default plan stopped 10 years sooner: after 30 years it reaches $31,311, and after 40 years it reaches $67,251. Those last 10 years add only $3,000 of deposits but about $35,941 to the balance. The 7% is an assumption, and history is not a forecast.

How much of my balance is interest, and when does it pass what I put in?

For the default plan, interest is 82% of the final balance, and it passes the money put in during year 18. Rapunzl's chart stacks the two layers so you can watch the interest layer start thin and end up taller than the deposits underneath it. After 5 years the interest is only $332 on $1,600 put in. By year 40 it is $55,151 on $12,100. Raise the rate or add years and the interest layer grows faster, and a rate of 0 leaves it flat.

Does the compounding choice change the answer?

The compounding choice changes the final balance by a small amount compared with the rate and the years. For the default plan, Rapunzl's calculator ends at $63,286 with yearly compounding, $67,251 with monthly and $67,630 with daily. Deposits still go in once a month in all three, and daily compounding assumes 365 days with no leap years. A bank's own rules decide which one applies to a real account, so the choice here is a way to see how little the frequency matters next to the rate.

Using the Calculator in Class

Project it and ask students to guess the balance for the default plan before you press go. Then change only the years, from 40 down to 30, and ask what losing 10 years takes off the balance, then compare that with the $3,000 of deposits those years would have added. Close with the rate: try 4% and 10% and ask what changes more, the rate or the years. The modules below cover exponents and saving versus investing, and the posts have worksheets for the same idea.

Questions about the compound interest calculator

At an assumed 7% a year, compounded monthly, $100 to start and $25 a month grows to about $67,251 after 40 years. Of that, $12,100 is money put in and $55,151 is interest, which is 82% of the balance. The rate matters a lot: the same plan ends at about $30,043 at 4% and about $163,472 at 10%. These are assumed rates, and history is not a forecast.

The compound interest formula is A = P(1 + r/n)^(nt), where A is the final amount, P the starting amount, r the yearly rate as a decimal, n the number of times a year interest is added and t the years. With no monthly additions, $100 at 7% compounded monthly for 40 years becomes about $1,631. Monthly additions need an extra term, and this calculator does that sum for you, which is how the default plan with $25 a month reaches $67,251.

Simple interest is paid only on the starting amount, while compound interest is also paid on the interest already earned. Take $100 at 7% a year for 40 years with no additions. Simple interest earns $280 for a total of $380. Compounded monthly, the same $100 earns $1,531 for a total of $1,631. The gap grows every year because the interest keeps joining the balance.

Any yearly rate from 0 to 30 works, and it is an assumption rather than a quote from a bank. For the default plan, the FDIC's national average savings rate of 0.37% (September 2026) ends at about $13,047, 4% ends at about $30,043, 7% at about $67,251 and 10% at about $163,472. What an account or investment really pays changes from year to year, so history is not a forecast.

No. The balance is arithmetic on the numbers you enter, not a prediction. It assumes the same rate every year and a deposit at the end of every month, and it leaves out taxes, account fees and inflation. Prices rise over time, so $67,251 after 40 years buys less than $67,251 does today. Real accounts and investments rise and fall, and history is not a forecast.

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Sources. Investor.gov (SEC): Compound Interest: Compound interest is interest paid on principal and on accumulated interest. FDIC, National Rates and Rate Caps (savings, national deposit rate): as of 2026-09-21 (FDIC data date; checked 2026-10-12). Figures last checked October 12 2026.

Each monthly addition goes in at the end of its month. Daily compounding uses 365 days a year with no leap years. The rate is an assumption, results are before taxes, fees and inflation, and history is not a forecast.

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