Compound Interest Calculator
See how money grows when interest is added to the balance and earns interest itself. Enter a starting amount, a yearly rate and a time, choose how often interest is added, and optionally add a regular contribution. A table shows the balance year by year.
The nominal yearly rate.
Up to 100. Decimals such as 2.5 are fine.
Added every compounding period.
Result
Enter the starting amount, the rate and the time.
The calculation runs in this browser tab. Nothing you enter is uploaded, saved or logged.
What is compound interest?
Compound interest is interest earned on the principal and also on the interest already added. 10,000 at 5% a year grows to 10,500 after one year; in the second year the 5% applies to 10,500, not 10,000, and so on. The longer it runs, the bigger the effect, which is why starting early matters.
How often interest is added, the compounding frequency, also matters, though less. With 5% a year, 1,000 grows to 1,628.89 after 10 years if interest is added yearly, 1,647.01 if monthly and 1,648.66 if daily. The effective annual rate shown by the calculator is the single yearly rate that gives the same result as the chosen frequency.
How to use the Compound Interest Calculator
- Enter the starting amount, the yearly interest rate and the time in years.
- Choose how often interest is added: annually, semi-annually, quarterly, monthly, weekly or daily.
- Optionally enter a regular contribution, added every compounding period, and whether it is made at the start or the end of each period.
- Read the future value, the total put in, the interest earned and the effective annual rate, and look at the year-by-year table.
Formula
- Lump sum
- A = P × (1 + r ÷ n)^(n × t)
- Regular contributions
- A = P × (1 + r ÷ n)^(n × t) + C × ((1 + r ÷ n)^(n × t) − 1) ÷ (r ÷ n)
- Effective annual rate
- EAR = (1 + r ÷ n)^n − 1
- Rule of 72
- years to double ≈ 72 ÷ rate in %
Here P is the starting amount, r the yearly rate as a fraction, n the number of times interest is added per year, t the years and C the contribution per period. The calculator does not use the closed formulas: it simulates every period with exact arithmetic, which gives the same figures and handles contributions at the start or the end of a period.
The rule of 72 is a quick estimate: at 6% money doubles in about 72 ÷ 6 = 12 years (the exact figure is 11.9).
Worked example
10,000 invested at 5% a year with interest added monthly, for 10 years. These are the results the calculator gives:
| Question | Result |
|---|---|
| Input: Starting amount | 10000 |
| Input: Annual interest rate | 5 % |
| Input: Time in years | 10 |
| Input: Interest is added | Monthly (12 times a year) |
| Input: Contributions are made | At the end of each period |
| Future value | 16,470.09 |
| Total put in | 10,000.00 |
| Interest earned | 6,470.09 |
| Interest as a share of the final balance | 39.3% |
| Effective annual rate | ≈ 5.1162% |
Assumptions and limits
- The term must contain a whole number of compounding periods. For example, quarterly compounding needs a term that is a multiple of 3 months; the calculator tells you if it is not.
- The contribution is added every compounding period. With monthly compounding that means a monthly deposit; for a different pattern, use the savings calculator.
- The rate is assumed to be constant and the interest is reinvested. Real accounts and investments have changing rates, and returns on investments are not guaranteed.
- Taxes, fees and inflation are not included: an amount in 30 years buys less than the same amount today.
- Terms up to 100 years are supported. These results are a calculation from the numbers you enter, not financial advice.
Frequently asked questions
How do I calculate compound interest?
Multiply the principal by (1 + rate ÷ times per year) raised to the power of years × times per year. For 10,000 at 5% compounded monthly for 10 years: 10,000 × (1 + 0.05 ÷ 12)^120 = 16,470.09.
How much difference does the compounding frequency make?
A small one. At 5% for 10 years, 1,000 becomes 1,628.89 compounded yearly, 1,647.01 monthly and 1,648.66 daily. Going from yearly to monthly adds more than going from monthly to daily.
What is the effective annual rate?
The yearly rate that would give the same growth with annual compounding. A nominal 5% compounded monthly is an effective 5.1162% a year.
How do regular contributions change the result?
They add to the balance and also earn interest. 200 a month added to 10,000 for 10 years at 5% brings the final balance from 16,470.09 to 47,526.55, although only 24,000 more was put in.
What is the rule of 72?
A shortcut for how long money takes to double: divide 72 by the yearly rate in percent. At 8% it takes about 9 years. It is an approximation that works well for rates between about 5% and 10%.
Why does the term have to match the compounding period?
Interest is added at fixed times, so the term must end on one of them. With quarterly compounding, 2.5 years is 10 periods and works, but 2.3 years is not a whole number of quarters.