UtilibyWritten by the Utiliby editorial team · Last updated October 23, 2026 · Rules and formulas reviewed October 2026
What this free compound interest calculator does
Albert Einstein is often credited with calling compound interest the eighth wonder of the world, although there is no evidence he said it. What is certain is that interest earning interest is the engine behind most long-term savings, retirement accounts and investment growth. A compound interest calculator lets you see that effect in numbers instead of guessing.
This free online compound interest calculator shows the future value of a starting amount with optional monthly or yearly deposits, any compounding frequency from annual to continuous, and a yearly deposit increase to model pay rises. It also shows the value in today's money after inflation, a stacked chart of deposits versus interest, a year-by-year table you can download, a goal planner that tells you how long you need or how much to save, a frequency comparison and a Rule of 72 tool. Everything runs in your browser.
How to use the compound interest calculator
- Enter the starting amount, rate and time.
- Choose the compounding frequency. Monthly is a common default for savings accounts.
- Add regular deposits. Pick monthly or yearly, and start or end of period.
- Optionally add a yearly deposit increase and inflation.
- Read the chart. Blue shows what you put in and green shows interest.
- Use the other tabs to plan a goal, compare frequencies or apply the Rule of 72.
The compound interest formula
The classic formula is A = P (1 + r/n)^(nt). P is the principal, r the annual rate as a decimal, n the number of times interest compounds each year and t the number of years. For $10,000 at 7 percent compounded monthly for 10 years, A = 10,000 x (1 + 0.07/12)^120 = $20,096.61. With annual compounding the answer is $19,671.51, and with continuous compounding, A = P e^(rt), it is $20,137.53.
Regular deposits are handled by stepping the balance forward month by month at the equivalent monthly rate, adding each deposit as it is made. With $500 deposited at the end of every month, the same 7 percent and 10 years gives $106,639.02, of which $70,000 is the start plus deposits and $36,639 is interest.
| Years | No deposits | $500 a month |
|---|---|---|
| 10 | $20,097 | $106,639 |
| 20 | $40,387 | $300,851 |
| 30 | $81,165 | $691,150 |
Does compounding frequency matter?
Slightly. The more often interest compounds, the faster the balance grows, because interest is added to the balance sooner. Yet the effect is modest. $10,000 at 7 percent for 10 years ends at $19,672 compounded annually, $20,097 monthly, $20,136 daily and $20,138 continuously. Moving from annual to monthly adds about $425, while moving from monthly to continuous adds only $41. A one percentage point higher rate or five more years does far more than any compounding choice.
Compound interest tables: lump sums and monthly deposits
These tables give quick answers without a calculation. Both assume interest compounds monthly at a constant rate. In the deposit table, money is added at the end of each month and the starting balance is zero. Figures are rounded to the nearest dollar. For your own numbers, use the calculator above.
What $10,000 grows to, with no further deposits
| Annual rate | 5 years | 10 years | 20 years | 30 years |
|---|---|---|---|---|
| 3% | $11,616 | $13,494 | $18,208 | $24,568 |
| 4% | $12,210 | $14,908 | $22,226 | $33,135 |
| 5% | $12,834 | $16,470 | $27,126 | $44,677 |
| 6% | $13,489 | $18,194 | $33,102 | $60,226 |
| 7% | $14,176 | $20,097 | $40,387 | $81,165 |
| 8% | $14,898 | $22,196 | $49,268 | $109,357 |
| 10% | $16,453 | $27,070 | $73,281 | $198,374 |
The table is linear in the starting amount, so $5,000 grows to half of each figure and $20,000 to double. For example, $10,000 at 7% for 20 years becomes $40,387, so $25,000 becomes about $100,968.
What regular monthly deposits grow to at 7% a year
| Monthly deposit | 10 years | 20 years | 30 years | 40 years |
|---|---|---|---|---|
| $100 | $17,308 | $52,093 | $121,997 | $262,481 |
| $250 | $43,271 | $130,232 | $304,993 | $656,203 |
| $500 | $86,542 | $260,463 | $609,985 | $1,312,407 |
| $1,000 | $173,085 | $520,927 | $1,219,971 | $2,624,813 |
Saving $500 a month for 30 years means putting in $180,000, yet the balance reaches $609,985, so interest supplies about $429,985. Doubling the deposit doubles the result, and starting ten years earlier adds far more than the extra ten years of deposits.
Simple interest versus compound interest
Simple interest is paid only on the original amount, so it adds the same dollars every year. Compound interest is paid on the amount plus the interest already earned, so it adds more each year. The gap is small at first and large over decades. Here is $10,000 at 7 percent a year.
| Years | Simple interest | Compounded yearly | Compounded monthly |
|---|---|---|---|
| 5 | $13,500 | $14,026 | $14,176 |
| 10 | $17,000 | $19,672 | $20,097 |
| 20 | $24,000 | $38,697 | $40,387 |
| 30 | $31,000 | $76,123 | $81,165 |
After 30 years simple interest has added $21,000, while yearly compounding has added $66,123 and monthly compounding $71,165. Borrowers meet the same effect in reverse: interest that compounds on a debt grows the balance owed.
The power of regular deposits and time
Two levers drive compound growth: how much you put in and how long it has to work. With 30 years at 7 percent, $500 a month grows to about $691,000 from just $190,000 of deposits and the $10,000 start, so interest provides most of the final balance. Starting ten years earlier usually beats saving much more later. The chart makes that visible, with the green interest area overtaking the blue deposit area in later years.
Real returns and inflation
A balance of $106,639 in ten years will buy less than $106,639 buys today. If inflation averages 3 percent a year, the purchasing power equals about $79,349 in today's money. Enter an inflation estimate to see both figures. Looking at the real value is a more honest way to judge whether a savings plan meets a goal.
The Rule of 72 and doubling time
The Rule of 72 estimates how long money takes to double: divide 72 by the annual return. At 7 percent that is 72 / 7 = 10.3 years, close to the exact 10.24 years. It works the other way too: to double in 8 years you need about 9 percent. The fourth tab gives the estimate, the exact figure and a table of common rates.
Using the goal planner
Enter a target such as 1,000,000 and the planner steps through your inputs to find how long it takes. With $10,000 to start, $500 a month and 7 percent, the balance passes $1,000,000 after about 34 years and 10 months. It also solves for the deposit needed to reach the target within your chosen time frame, which is useful for retirement or savings goals.
How much to save each month to reach $1 million
The table shows the monthly deposit needed to reach $1,000,000 starting from zero, with monthly compounding and deposits at the end of each month.
| Annual return | 20 years | 25 years | 30 years | 35 years | 40 years |
|---|---|---|---|---|---|
| 4% | $2,726 | $1,945 | $1,441 | $1,094 | $846 |
| 5% | $2,433 | $1,679 | $1,202 | $880 | $655 |
| 6% | $2,164 | $1,443 | $996 | $702 | $502 |
| 7% | $1,920 | $1,234 | $820 | $555 | $381 |
| 8% | $1,698 | $1,051 | $671 | $436 | $286 |
At 7 percent, reaching $1,000,000 takes about $820 a month over 30 years or $381 a month over 40 years. Waiting ten years, from 40 years of saving to 30, more than doubles the monthly amount. The goal planner on the fourth tab solves the same problem for any target, deposit or starting balance.
Calculate compound interest in Excel or Google Sheets
Both programs have a future value function. =FV(rate/12, months, -deposit, -start) returns the balance with monthly compounding. For $10,000 to start, $500 deposited each month, 7 percent and 10 years, =FV(0.07/12, 120, -500, -10000) returns 106,639.02, which matches the calculator. For a lump sum alone, =10000*(1+0.07/12)^120 gives 20,096.61. Enter the deposit and start as negative numbers so that the result is positive. Download the yearly CSV from the calculator to chart or extend it in a spreadsheet.
Limits, privacy and responsible use
The calculator assumes a constant rate. Real investment returns vary, can be negative in some years and are reduced by fees and taxes. It is an illustration, not a forecast, and not investment or tax advice. Nothing you enter leaves your browser.
Related free tools
Use these free online tools together with this one:
- See the other side of interest, borrowing costs, with the loan calculator.
- Convert rates and returns with the percentage calculator.
- Model how a raise changes what you can save with the pay raise calculator.
- Find money to invest each month with the budget planner calculator.
- Plan how long cash lasts with the startup runway calculator.
- Turn an hourly wage into a yearly income to plan deposits with the hourly to salary calculator.
- Compare a property return with a savings return using the rental yield calculator.
- Estimate what a business could be worth with the business valuation calculator.
Frequently asked questions
What is a compound interest calculator?
A tool that shows how money grows when interest is added to the balance and then earns interest itself. Enter a starting amount, rate, time and optional deposits to see the future value.
What is the compound interest formula?
A = P (1 + r/n)^(nt), where P is the starting amount, r the annual rate, n the compounding periods per year and t the years. With regular deposits the future value of the deposits is added. For continuous compounding the formula is A = P e^(rt).
How much will $10,000 grow at 7% in 10 years?
Compounded monthly it becomes about $20,097. Compounded annually it becomes about $19,672. Adding $500 a month for the ten years takes it to about $106,639.
What is the difference between simple and compound interest?
Simple interest is paid only on the original amount. Compound interest is paid on the original amount plus interest already earned, so growth accelerates over time.
Does compounding frequency matter?
A little. More frequent compounding gives slightly more, but the gap between daily and monthly is small. The rate, the time and the deposits matter far more. The frequency tab shows the difference for your numbers.
What is continuous compounding?
The mathematical limit as compounding becomes infinitely frequent, given by P e^(rt). At 7 percent for 10 years, $10,000 grows to $20,138 continuously versus $20,097 monthly.
How do regular deposits change the result?
Each deposit starts earning interest as soon as it is made, so steady saving usually adds far more than the starting amount over long periods. You can set monthly or yearly deposits and choose start or end of period.
Can the deposit increase every year?
Yes. Set a yearly deposit increase, for example 3 percent, to model pay rises. Each year's deposits are that much larger than the last.
How do I see the effect of inflation?
Enter an expected inflation rate. The tool shows the final balance in today's money by dividing by (1 + inflation) raised to the number of years.
How long will it take to reach my goal?
Use the goal planner. It steps through your inputs month by month until the balance reaches the target, and also solves for the deposit needed to get there within your chosen time.
What is the Rule of 72?
A shortcut: divide 72 by the annual return to estimate years to double. At 7 percent it is about 10.3 years. The exact figure is 10.24 years.
Is the Rule of 72 accurate?
It is close for rates between about 6 and 10 percent. At very low or very high rates the exact formula, ln 2 divided by ln(1 + r), is better. The tool shows both.
How much is $1,000 worth in 10 years at 5%?
About $1,647.01 with monthly compounding and no further deposits, so interest adds about $647.01.
How much will $10,000 be worth in 20 or 30 years at 7%?
With monthly compounding and no deposits, $10,000 grows to about $40,387 in 20 years and $81,165 in 30 years.
How much will $100 a month grow to in 30 years at 7%?
About $121,997 with monthly compounding and deposits at the end of each month. You would have deposited $36,000, so interest supplies about $85,997.
How much do I need to save each month to reach $1 million?
At 7% with monthly compounding it takes about $1,920 a month for 20 years, $820 for 30 years or $381 for 40 years, starting from zero. Starting with a balance or earning a higher return lowers the amount.
How do I calculate compound interest in Excel or Google Sheets?
Use =FV(rate/12, months, -deposit, -start). For $10,000 to start, $500 a month, 7% and 10 years, =FV(0.07/12, 120, -500, -10000) returns 106,639.02. For a lump sum alone, use =start*(1+rate/12)^months.
Is compound interest good or bad?
It depends on which side you are on. It works for you on savings and investments, where interest earns interest. It works against you on debt such as credit cards, where unpaid interest is added to the balance and then charged interest itself.
Is compound interest the same as APY?
APY (annual percentage yield) is the effective yearly rate after compounding. A 7 percent rate compounded monthly has an APY of about 7.23 percent.
Can I use it for debt?
The same maths applies to debt that compounds, such as credit cards. For fixed loans with payments, use a loan calculator, which handles repayment schedules.
Does it include taxes and fees?
No. It shows the pre-tax, pre-fee growth at a constant rate. Fees and taxes reduce real returns, so model a lower rate to be cautious.
Is my data stored or sent anywhere?
No. Everything runs in your browser and nothing is stored or uploaded.
Is this compound interest calculator free?
Yes. It is free to use online with no sign-up.
Sources and further reading
- U.S. Securities and Exchange Commission, Investor.gov compound interest calculator.
- Investor.gov, Compound interest.
- Consumer Financial Protection Bureau, Consumer tools.
- Investopedia, Rule of 72.
- Microsoft Support, FV function.
Sources were consulted in October 2026. Standards and rules can change, so always check the official page for your situation.