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Monthly deposits are added at the beginning of each month, starting today. Enter a nominal annual rate and a duration in years equal to whole months (for example, 1.5 years = 18 months). The selected compounding frequency is converted to an equivalent monthly growth rate, including partial-period growth. Daily compounding uses a 365-day year and equal monthly intervals.
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Understand how compound interest can grow an investment over time, how monthly contributions affect future value, and how interest rates, investment periods and compounding frequency influence projected investment growth.
Compound interest allows an investment balance to grow by earning interest on both the money invested and interest accumulated over time. As the balance increases, future growth can be calculated on a larger amount, which is why time can have a significant effect on long-term investment growth.
For a starting principal without additional contributions, compound growth is commonly represented by the following formula:
In this formula, A is the future value, P is the initial principal, r is the nominal annual interest rate expressed as a decimal (not an effective annual yield or APY), n is the number of compounding periods per year, and t is the number of years.
For monthly deposits, let C be the deposit amount and M = 12t the whole number of months. The equivalent monthly growth factor is q = (1 + r/n)n/12. Starting with B0 = P, each month is calculated as follows:
Add the monthly deposit first, then apply one month of growth. Equivalently, future value is PqM + C(q + q2 + … + qM). When the interest rate is zero, future value is simply P + CM.
Simple interest is generally calculated using the original principal, while compound interest allows previously accumulated interest to become part of the balance used for future growth. The difference may become more noticeable as the investment period increases.
Compounding has more opportunities to affect an investment when the money remains invested for a longer period. Even when the interest rate stays the same, extending the investment period can substantially change the projected future value.
If you are planning specifically for long-term retirement savings, use ToolXone's Retirement Calculator to explore savings based on your current age and planned retirement age.
Contributions are added at the beginning of each month. The first deposit is added alongside the initial investment at time zero, and the final deposit earns one month of growth. An M-month investment receives exactly M deposits, with no extra deposit at the ending valuation instant. Each deposit earns growth only for the time it is invested.
For example, an initial investment of 1,000 with deposits of 100 per month, a 12% nominal annual rate and yearly compounding grows to approximately 2,396.65 after one year. Total contributions are 2,200 and interest earned is approximately 196.65. Later deposits do not receive a full year's interest.
If you are working toward a particular savings amount, the Savings Goal Calculator can help you explore a target from a goal-based perspective.
The nominal annual rate is quoted with a selected compounding frequency: yearly (n = 1), quarterly (n = 4), monthly (n = 12) or daily (n = 365). The calculator converts that rate to an equivalent monthly growth factor so that monthly deposits can be valued consistently. Changing the selected frequency still changes the effective annual growth; it does not simply use r/12 for every option.
Incomplete compounding periods use fractional-exponent compound growth. For example, 1,000 at 12% with yearly compounding for 1.5 years grows to 1,000 × 1.121.5, approximately 1,185.30, without deposits. The projection includes growth within the final partial period rather than stopping at the last annual or quarterly boundary. It is an accumulated-value estimate, not a statement of when a bank credits interest.
Daily compounding assumes 365 days per year and equal one-twelfth-year months (365/12 days each). No start date is collected, so actual calendar month lengths and leap years are not modeled.
Total contributions represent the money added by you rather than the growth generated by interest. In this calculator, total contributions include the initial investment plus all monthly contributions made during the selected investment period.
Interest earned represents the difference between the projected future value and total contributions. It shows how much of the estimated final balance comes from growth rather than from the money contributed.
Growth percentage is 100 × interest earned / total contributions. It is not an annualized or money-weighted return. Calculations retain full internal floating-point precision; displayed amounts are rounded to two decimal places.
To compare a gain or loss with an amount invested from another perspective, explore ToolXone's ROI Calculator .
A future investment value may be larger in numerical terms, but inflation can reduce the purchasing power of money over time. This calculator estimates compound growth and does not automatically adjust the projected balance for future inflation.
Use the Inflation Calculator to explore how purchasing power may change over time.
Enter your initial investment, optional monthly contribution, nominal annual interest rate and investment period in years. The duration must equal a whole number of months: for example, 1.5 years is 18 months. Decimal approximations extremely close to whole months are accepted; genuine fractional months, such as 1.1 years (13.2 months), are not rounded. Select a compounding frequency and choose Calculate Growth to estimate future value, total contributions, interest earned and growth percentage.
Try several scenarios by changing the interest rate, monthly contribution, investment period or compounding frequency. Comparing different assumptions can help illustrate which inputs have the greatest effect on the projection.
The result is an estimate based on the values entered and the calculator's growth assumptions. Actual savings or investment results may differ because rates can change and real financial products may apply different contribution timing, compounding methods, fees, taxes, rounding rules or other conditions.
The calculator is useful for exploring mathematical growth scenarios, but its projected future value should not be treated as a guaranteed investment outcome.