Compound Interest Calculator
See how savings and investments can grow over time. Add monthly deposits, choose how often interest compounds, and view the result in future and in today's money.
Projected growth
Enter your details above to see results.
In today's money
Total contributed
Interest earned
Growth multiple
Effective annual rate
Growth over time
Year-by-year
| Year | Contributed | Interest | Balance |
|---|
Projections assume a constant rate of return and are for illustration only. Real returns vary and are not guaranteed. This is not financial advice.
How compound interest works
Compound interest means you earn interest on your original money and on the interest it has already earned. Each period the balance gets a little bigger, so the next period's interest is a little bigger too. Over many years this snowball effect does most of the work.
A = P × (1 + r/n)n×t
P is the starting amount, r the annual rate as a decimal, n the number of compounding periods per year and t the number of years. With regular deposits, this calculator converts your rate to its effective annual rate, then applies the matching monthly growth to your balance and deposits each month. That keeps the result consistent whichever compounding frequency you pick.
An example of time doing the work
The table starts with a $5,000 deposit and adds $300 a month at an assumed 7% return, compounded monthly. The rate is an example, not a forecast.
| Time | Contributed | Interest earned | Balance |
|---|---|---|---|
| 10 years | $41,000 | $20,974 | $61,974 |
| 20 years | $77,000 | $99,472 | $176,472 |
| 30 years | $113,000 | $293,574 | $406,574 |
Notice that in the later years the interest earned is larger than everything you put in. Starting earlier matters more than most people expect.
The Rule of 72
A quick way to estimate doubling time is to divide 72 by the annual rate. At 6% your money roughly doubles in 12 years, and at 9% in about 8 years. It is an approximation, but it works well for typical rates.
Why inflation matters
A balance of $100,000 in 30 years will buy less than $100,000 does today. The "in today's money" figure above divides your result by cumulative inflation so you can compare it with today's prices. Investment returns are also not guaranteed and can vary year to year, so use this as a planning tool, not a promise.
Simple vs Compound Interest
The gap between the two grows fast. With simple interest you earn the same amount every year. With compound interest each year’s interest is added to the balance, so the next year earns interest on a bigger number.
| Time | Simple interest | Compound interest |
|---|---|---|
| 10 years | $15,000 | $16,289 |
| 20 years | $20,000 | $26,533 |
| 30 years | $25,000 | $43,219 |
How Much Does a Lump Sum Grow?
These values use monthly compounding, the same as the calculator’s default. The rates are examples, not forecasts.
| Rate | 10 years | 20 years | 30 years |
|---|---|---|---|
| 3% | $13,494 | $18,208 | $24,568 |
| 5% | $16,470 | $27,126 | $44,677 |
| 7% | $20,097 | $40,387 | $81,165 |
| 10% | $27,070 | $73,281 | $198,374 |
Monthly Deposits at 7%
Regular deposits are where compounding becomes noticeable. These figures assume you start from zero and deposit at the end of each month at an assumed 7% a year.
| Monthly deposit | 10 years | 20 years | 30 years |
|---|---|---|---|
| $100 | $17,308 | $52,093 | $121,997 |
| $300 | $51,925 | $156,278 | $365,991 |
| $500 | $86,542 | $260,463 | $609,985 |
| $1,000 | $173,085 | $520,927 | $1,219,971 |
How Much to Save Each Month to Reach $1 Million
Time does more than the monthly amount. Starting 10 years earlier cuts the required deposit by more than half in the example below.
| Time | At 5% | At 7% | At 9% |
|---|---|---|---|
| 20 years | $2,433 | $1,920 | $1,497 |
| 30 years | $1,202 | $820 | $546 |
| 40 years | $655 | $381 | $214 |
Compounding Frequency and the Effective Annual Rate
The more often interest is added, the higher the effective yearly return, but the difference is small. Switching from yearly to daily compounding at 7% adds only about a quarter of a percentage point.
| Nominal rate | Yearly | Quarterly | Monthly | Daily |
|---|---|---|---|---|
| 5% | 5.00% | 5.09% | 5.12% | 5.13% |
| 7% | 7.00% | 7.19% | 7.23% | 7.25% |
| 10% | 10.00% | 10.38% | 10.47% | 10.52% |
Rule of 72 Compared With the Exact Answer
| Rate | Rule of 72 | Exact (yearly compounding) |
|---|---|---|
| 3% | 24.0 years | 23.4 years |
| 5% | 14.4 years | 14.2 years |
| 6% | 12.0 years | 11.9 years |
| 7% | 10.3 years | 10.2 years |
| 8% | 9.0 years | 9.0 years |
| 10% | 7.2 years | 7.3 years |
| 12% | 6.0 years | 6.1 years |
The rule is close for rates between 5% and 12%. At very low or very high rates the exact figure drifts away from it.
Real Return After Inflation
If your money grows 7% a year and prices rise 3%, your real return is not 4% but (1.07 / 1.03) − 1 = 3.88%. Over long periods this difference matters, which is why the calculator offers an inflation-adjusted figure.
Compound Interest in Excel or Google Sheets
The same result in a spreadsheet: =FV(rate/12, years*12, -monthly_deposit, -starting_amount). For example, =FV(0.07/12, 10*12, -300, -5000) returns about 61,974, matching the example above.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is calculated only on the original amount. Compound interest is calculated on the original amount plus the interest already earned, so growth accelerates over time.
Does compounding more often make a big difference?
It helps, but less than people think. At 7%, annual compounding gives an effective rate of 7.00%, monthly gives about 7.23% and daily about 7.25%. The rate, time and deposits matter far more.
What is the effective annual rate?
It is the real yearly growth after compounding is included. A nominal 7% rate compounded monthly grows your money by about 7.23% over a year.
Should I deposit at the start or the end of the month?
Depositing at the start of the month earns slightly more because each deposit gets an extra month of interest. The difference is small but grows over long periods.
Can I use this for debt?
Yes, the same math shows how a balance grows when no payments are made. For loans with regular repayments, a loan or mortgage calculator is a better fit.
Is my data saved or shared?
No. The numbers you enter stay in your browser and are not uploaded or stored by this page.
How do I calculate compound interest on $10,000?
Use A = P (1 + r/n)^(nt). At 5% compounded yearly, $10,000 becomes $16,289 after 10 years and $43,219 after 30 years. The calculator above does this for any rate, period and deposit.
How long does it take to double my money?
Divide 72 by the annual rate for a quick estimate: 6% takes about 12 years and 9% about 8 years. The exact figure at 7% is 10.2 years.
How much do I need to save each month to reach $1 million?
At an assumed 7% return, you would need about $820 a month for 30 years, or about $1,920 a month for 20 years. Returns are not guaranteed, so treat these as planning figures.
Is daily compounding much better than monthly?
Not by much. At 7%, monthly compounding gives an effective rate of about 7.23% and daily about 7.25%. Saving more or starting earlier changes the result far more.
What is the difference between APR and APY?
APR is the stated yearly rate without compounding. APY, also called the effective annual rate, includes compounding during the year, so it is slightly higher for the same nominal rate.
Can I calculate compound interest in Excel?
Yes. Use the FV function, for example =FV(0.07/12, 120, -300, -5000) for a 5,000 starting balance with 300 deposited each month for 10 years at 7%, compounded monthly.
Sources and further reading
- U.S. Securities and Exchange Commission, Investor.gov. Compound Interest Calculator.
- Investor.gov. Introduction to investing and saving.
- Effective annual rate and compound growth formulas, as used in standard finance references.