Finocturne
Investment

Compound Interest Calculator

Enter a starting amount, monthly contributions, expected annual return and time to see how much could accumulate.

Nominal annual return · month-end contributions · constant return assumption

Inputs

Compounding frequency

Results

Start with your numbers

Review or adjust your inputs, then calculate to see your results and interpretation.

Understand this calculation

Final value equals contributions plus net investment return; a loss makes it lower than contributions. The input is a nominal annual rate converted through the chosen compounding frequency.

See calculation method and assumptions

Formula

Compound Interest

Final value equals the compounded initial principal plus the accumulated value of each monthly contribution.

Variables, rates and periods

PV / FV
PV is the amount today; FV is the amount at the end of the stated time.
r
Nominal annual interest or discount rate. Enter a percentage; formulas use decimals (5% = 0.05).
t
Time in years. Period counts and periodic rates must use matching time units.
m
Compoundings per year. The periodic rate is r / m and period count is m × t.
PMT / j
PMT is the month-end contribution; j counts months up to 12 × t. Other compounding frequencies are converted to an equivalent monthly rate.

Rate and period assumptions

  • r is the nominal annual return and m is compounding periods per year.
  • The periodic rate r ÷ m is converted to an equivalent monthly rate before each month-end contribution.
  • The period t accepts whole years only, with 12 contributions per year.
  • Scenario Comparison instead uses an effective annual return, so the inputs are not interchangeable.

Estimate final value over whole years using a nominal annual return, compounding frequency, and month-end contributions.

Worked example

  1. Enter an initial investment of 100,000 and a monthly contribution of 5,000.
  2. Set a 7% nominal annual return, 10 whole years, and monthly compounding.
  3. The result separates 700,000 of contributions from the modeled investment gain and final value.

Frequently asked questions

When are monthly contributions added?

At month end, so the new contribution earns no interest for that month.

Is the nominal annual return guaranteed?

No. The calculator uses a constant assumption and cannot predict market fluctuations.

What changes with compounding frequency?

At the same nominal annual rate, more frequent compounding generally raises the effective annual return.

Why must the period be a whole number of years?

This tool adds one contribution at each month end and shows complete 12-month years. Whole years avoid ambiguity in partial-month contribution counts and yearly charts.

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