Growth of a lump sum plus monthly contributions.
FV = P × (1 + r)^n + C × [((1 + r)^n − 1) / r]
The total future value (FV) is the sum of two components:
These two components are added together to give the total portfolio value at the end of the investment period.
Lump sum FV = 10,000 × (1.0058333)^240 = 10,000 × 4.03928 ≈ $40,393 Annuity FV = 200 × [((1.0058333)^240 − 1) / 0.0058333] = 200 × [(4.03928 − 1) / 0.0058333] = 200 × [3.03928 / 0.0058333] = 200 × 521.03 ≈ $104,206 Total FV = $40,393 + $104,206 = $144,599
Result: Approximately **$144,599** after 20 years. Total contributions = $10,000 + ($200 × 240) = $58,000. Total interest earned ≈ $86,599.
In this example, a $10,000 initial investment combined with $200 monthly contributions grows to roughly $144,599 over 20 years at a 7% annual return. Of that total, only $58,000 came from your own pocket — the remaining ~$86,600 is pure compound interest. This powerfully illustrates why starting early and contributing consistently dramatically amplifies long-term wealth.
Note: Results are estimates only. Actual returns vary based on market performance, fees, taxes, and inflation. This calculator does not account for investment fees or tax implications. Consult a qualified financial advisor before making investment decisions.
Compound interest means you earn returns not just on your original principal, but also on all previously accumulated interest. Over long periods, this "interest on interest" effect — often called compounding — creates exponential rather than linear growth.
The annual rate of return has an outsized effect on final value. Using the Rule of 72, you can estimate how long it takes to double your money: divide 72 by the annual rate. At 6%, money doubles roughly every 12 years; at 9%, every 8 years.
Time is the most powerful variable. Starting 10 years earlier can more than double your final portfolio, even with the same contributions, because early dollars compound the longest.
To work backwards from a target future value, you can rearrange the annuity formula: C = (FV − P × (1 + r)^n) × r / ((1 + r)^n − 1). For example, to reach $500,000 in 30 years starting with $5,000 at 7% annual return, you'd need to contribute roughly $370/month.
Research generally shows that lump-sum investing outperforms dollar-cost averaging (DCA) about two-thirds of the time over long horizons, because markets tend to rise over time and immediate exposure maximises compounding. However, DCA reduces the emotional risk of investing at a market peak and is the practical approach for most people investing from regular income.
More frequent compounding (daily vs. monthly vs. annually) produces slightly higher returns. For example, $10,000 at 7% compounded daily grows marginally more than at 7% compounded monthly. The difference is small in practice, but monthly compounding — used in this calculator — is the most common standard for investment accounts.
The Rule of 72 is a quick mental math shortcut: divide 72 by your annual return rate to estimate how many years it takes to double your money. At 7%, your money doubles roughly every 10.3 years (72 ÷ 7 ≈ 10.3). You can verify this with the calculator by entering a lump sum, no monthly contributions, and the expected doubling period.
A common benchmark for a diversified U.S. stock index fund is 7–10% nominal annually (the S&P 500 has averaged roughly 10% historically before inflation). For a balanced portfolio (stocks + bonds), 5–7% is a more conservative estimate. Always use a rate that reflects your actual investment mix and risk tolerance.
No — the calculator uses a nominal return rate and produces a nominal future value. To estimate inflation-adjusted (real) future value, subtract the expected inflation rate from your annual return before entering it (e.g., use 4–5% instead of 7% if inflation averages ~2–3%).
A lump sum begins compounding immediately for the entire investment duration, maximising its growth. Each monthly contribution only compounds from the month it is added, so later contributions have less time to grow. However, consistent monthly contributions harness dollar-cost averaging and allow you to invest income as you earn it.
Yes — you can model a retirement account by entering your current balance as the lump sum, your monthly contribution, an expected average annual return, and the number of years until retirement. Just note that this tool does not account for employer matching, contribution limits, or tax treatment inside the account.
This calculator assumes a fixed monthly contribution. If your contributions change over time, you can approximate by running multiple scenarios or using a more advanced financial planning tool. A common approach is to use an average monthly contribution figure.
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