Compound Interest Calculator
See how your investments grow with compound interest. Includes monthly contribution support and yearly breakdown.
What this calculator shows
The compound interest calculator projects how a sum of money grows when interest is added to the balance and then earns interest of its own. You can add a regular monthly contribution, pick how often interest compounds, and see a summary plus a yearly balance table. It is useful for savings goals and for seeing how much the time period matters.
The formulas
Without contributions the tool uses the standard compound interest formula:
A = P (1 + r/n)^(nt)
P is the starting principal, r is the annual rate as a decimal, n is the number of compounding periods per year, and t is the number of years. Monthly contributions are added with the matching future-value formula, so each deposit earns interest from the period it is paid in. The monthly amount is spread evenly across the compounding periods of each year. The total you contribute is therefore the same whatever frequency you pick, and only the compounding changes.
How to use it
- Enter the Initial Principal in dollars.
- Enter the Annual Interest Rate as a percentage. A rate of 0 is accepted.
- Enter the Time Period in whole years.
- Choose a Compound Frequency: Daily, Monthly, Quarterly or Annually.
- Optionally enter a Monthly Contribution. Leave it empty or at 0 for a lump sum only.
- Press Calculate Growth. Read Total Contributions, Total Interest Earned and Final Balance, then scroll the Yearly Growth list.
Worked example
Start with $10,000 at 8% a year, compounded monthly, for 10 years, with no contributions. The monthly rate is 0.08 divided by 12, and there are 120 periods, so A = 10,000 × (1.006667)^120, which is about $22,196. The interest earned is about $12,196.
Now add $100 a month. Total contributions are $12,000 plus the $10,000 starting principal, or $22,000. Those $12,000 of deposits grow to about $18,295 by year 10. The final balance is about $40,491, which leaves about $18,491 of interest earned.
Limits and tips
- The projection assumes the same rate every year. Real investments vary, and returns are never guaranteed.
- Taxes, fees and inflation are not subtracted, so the balance shown is before those costs.
- The time period should be whole years. The yearly table only includes complete years.
- Compare daily and annual compounding to see how small the frequency effect is next to the rate and the time period.
Compound Interest Calculator – Frequently Asked Questions
What is compound interest?
Compound interest is interest calculated on both the initial principal and the interest already added to the balance. Unlike simple interest, it grows faster and faster over time.
What is compounding frequency?
How often interest is calculated and added to your balance. Daily compounding gives slightly more than monthly, which gives more than annual, at the same annual rate.
How do monthly contributions change the result?
Each contribution is added to the balance and then earns interest for the rest of the period. For example, $100 a month at 8% for 10 years grows to about $18,300, of which $12,000 is the money you paid in.
What is the Rule of 72?
Divide 72 by your annual interest rate to estimate how many years it takes to double your money. At 8% interest, money doubles in about 9 years. It is a quick estimate, not an exact figure.