Compound Interest Calculator
Enter a starting amount, a regular contribution, an interest rate and a number of years to see how your money grows. Add inflation, tax or a savings goal to plan with real numbers.
Free · No sign-up
Results are estimates for planning only and are not financial advice.
How to use it
- Enter your starting amount and how much you add each month or year.
- Enter the yearly interest rate or expected return, the number of years and how often interest is compounded.
- Open More options to add inflation, a tax on gains, a yearly increase in contributions or a savings goal.
- Read the final balance, the chart and the year-by-year table, and download the table as a CSV file.
Why use this tool
Regular contributions
Monthly or yearly, at the start or end of each period, with an optional yearly increase.
Any compounding
Yearly, twice a year, quarterly, monthly or daily, with the effective annual rate (APY).
Real value after inflation
See what the final amount is worth in today's money, not just the number on the statement.
Savings goal planner
Find out when you reach a target, and how much to save each month to reach it on time.
Rate range
Compare a lower and a higher return side by side, because investment returns are never certain.
Private and free
Everything is calculated in your browser, in any of 32 currencies. Nothing is sent or stored.
How compound interest is calculated
With compound interest, you earn interest on your interest. A single amount grows to A = P × (1 + r/n)^(n × t), where P is the starting amount, r the yearly rate, n the number of compounding periods per year and t the number of years.
Regular contributions add a second part. For a contribution C paid at the end of each period, the future value is C × ((1 + i)^N − 1) ÷ i, where i is the rate per period and N the number of periods. Paid at the start of each period, it is multiplied by (1 + i).
Example: $10,000 plus $500 a month at 7% a year, compounded yearly, grows to $292,561 in 20 years. You pay in $130,000, so $162,561 is interest.
How the compounding frequency changes the result
The more often interest is added, the faster it starts earning interest itself. The effect is real but smaller than most people expect: $10,000 at 5% for 10 years:
| Compounding | Effective annual rate | Balance after 10 years |
|---|---|---|
| Yearly | 5.000% | $16,288.95 |
| Twice a year | 5.062% | $16,386.16 |
| Quarterly | 5.095% | $16,436.19 |
| Monthly | 5.116% | $16,470.09 |
| Daily | 5.127% | $16,486.65 |
Time does most of the work
The longer your money stays invested, the larger the share of the balance that comes from interest. $500 a month at 7% a year:
| Years | Paid in | Balance | Share from interest |
|---|---|---|---|
| 10 | $60,000 | $85,558 | 30% |
| 20 | $120,000 | $253,865 | 53% |
| 30 | $180,000 | $584,948 | 69% |
| 40 | $240,000 | $1,236,240 | 81% |
Inflation and taxes
The number on your statement is not what you can buy with it. Use the options to see your result after both:
- Inflation: at 2.5% a year, the $292,561 from the example above is worth about $178,542 in today's money after 20 years.
- Real return: your rate minus inflation is roughly what your savings grow in buying power.
- Taxes: in many countries, interest and investment gains are taxed. In the US, savings interest is usually taxed as income, while accounts such as a 401(k) or an IRA defer or reduce the tax.
- The tax option in the calculator takes the rate you enter once from all gains at the end, which is a simple estimate. Check your country's rules for the exact amount.
The rule of 72
To estimate how long it takes to double your money, divide 72 by the yearly rate. The rule is close for typical rates:
| Rate per year | Rule of 72 | Exact (yearly compounding) |
|---|---|---|
| 2% | 36.0 years | 35.0 years |
| 4% | 18.0 years | 17.7 years |
| 6% | 12.0 years | 11.9 years |
| 8% | 9.0 years | 9.0 years |
| 10% | 7.2 years | 7.3 years |
| 12% | 6.0 years | 6.1 years |
Frequently asked questions
How much will $10,000 grow in 20 years?
With $500 added every month at 7% a year, it grows to $292,561. Without monthly contributions, $10,000 at 5% grows to $26,533 in 20 years. Real returns vary, so try a rate range.
What is the difference between simple and compound interest?
Simple interest is paid only on the starting amount, while compound interest is also paid on interest already earned. $10,000 at 5% for 20 years grows to $20,000 with simple interest and to $26,533 with yearly compounding.
Does monthly compounding make a big difference?
Less than you might think. At 5%, monthly compounding gives an effective rate of 5.116% instead of 5.000%. On $10,000 over 10 years, that is $181.15 more.
What is APY?
The annual percentage yield (APY) is the effective yearly rate after compounding. It lets you compare accounts that compound at different frequencies. The calculator shows it as the effective annual rate.
Does it matter if I contribute at the start or end of the month?
A little. Money paid at the start of each period earns interest for one extra period. In the example above, it adds $1,435 over 20 years.
What interest rate should I use?
For a savings account or a deposit, use the rate your bank pays. For investments, returns change from year to year and can be negative, so use a cautious estimate and the rate range option to see a lower and a higher outcome.
Are my numbers saved or sent anywhere?
No. The calculation runs in your browser. The page only remembers your currency and chosen options on this device, never your amounts.
How to Make Compound Interest Work for You
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