What is this calculator?
This calculator estimates what a series of equal monthly investments could grow to, given an assumed annual rate of return. It separates the total you contributed from the growth on top of it, which is the distinction that makes a projection useful.
Use it when:
- You are planning a monthly investment amount towards a goal and want a sense of scale.
- You want to see how much of a projected corpus is your own contribution rather than returns.
- You are comparing how the horizon (years invested) changes the outcome more than the rate does.
Intended for: Investors planning regular monthly contributions to a fund or savings plan, and anyone modelling long-term goal saving.
How is this calculated?
- The expected annual return you enter is converted to a monthly rate by dividing by 12 and by 100.
- The tenure is converted to a whole number of monthly contributions.
- Each contribution is compounded forward for the number of months remaining until the end of the term — the earliest contribution compounds the longest, the last one barely at all.
- Summing those future values gives the projected corpus; this is the future value of an annuity.
- If contributions are made at the start of each month rather than the end, every contribution earns one extra month of growth, and the whole series is scaled up by one period accordingly.
- Invested amount is simply the contribution multiplied by the number of months; estimated gains are the corpus minus that figure.
Formulas
Projected corpus
FV = M × ((1 + r)^n − 1) / r × (1 + r)^t
The future value of a series of equal contributions, each compounded for the time remaining after it was made.
- M
- Monthly investment amount
- r
- Monthly return = annual rate ÷ 12 ÷ 100
- n
- Number of monthly contributions
- t
- 1 for start-of-month contributions, 0 for end-of-month
Estimated gains
Gains = FV − (M × n)
The portion of the corpus that is growth rather than your own money.
Example
Investing 1,000 per month for 12 months at an expected 12% per year, contributing at the end of each month.
- The monthly return is 12 ÷ 12 ÷ 100 = 0.01.
- The first contribution compounds for 11 months, the second for 10, and so on; the last does not compound at all.
- Their total future value is approximately 12,682.50, against 12,000 contributed.
Result: Estimated corpus ≈ 12,682.50, invested 12,000, estimated gains ≈ 682.50.
Frequently asked questions
Is the projected corpus guaranteed?
No. It is a projection based on a constant rate of return that you supply. Real market returns vary year to year and can be negative; the actual outcome will differ, sometimes substantially.
Why does the same rate produce so much more over a longer horizon?
Because returns compound. Each year's growth earns growth of its own in later years, so the curve steepens over time. Extending the horizon usually affects the outcome more than a modest change in the rate.
What is the difference between start-of-month and end-of-month contributions?
A start-of-month contribution is invested one month earlier, so it earns one extra month of return. Over a long horizon this produces a modestly higher corpus for the same money.
Are taxes, exit loads, or fund charges included?
No. The projection is on the gross return you enter. Expense ratios, exit loads, and capital-gains tax all reduce the amount you actually receive.
Assumptions
What this calculator takes as given:
- The interest or return rate you enter is treated as fixed for the entire period.
- Every period is treated as equal in length; no calendar-day, leap-year, or day-count convention is applied.
- Contributions are equal, made every month without interruption, and compounded monthly.
- Returns are reinvested in full; nothing is withdrawn during the term.
Limitations
What this calculator cannot know or does not model:
- Market returns are not constant. A single assumed rate cannot capture volatility, sequence-of-returns risk, or a negative year.
- Fund expense ratios, exit loads, transaction costs, and taxes are not deducted.
- Inflation is not applied, so the projected corpus is in today's nominal currency, not in purchasing power.
- Contribution step-ups, pauses, and partial withdrawals are not modelled.