Understanding your investment projection
Historical return references give context for the assumption entered; none of them predicts future performance.
| Assumption | Historical context (long-run US data) |
|---|---|
| 9–10% nominal | Approximate long-run average of US large-cap equities (S&P 500, since 1926) |
| 8–9% nominal | Approximate long-run average of a 60/40 stock-bond portfolio |
| 5–7% nominal | Common conservative planning assumption after fees and caution |
| 2–3% inflation | Federal Reserve 2% target; long-run US CPI average near 3% |
- The model assumes a constant return with monthly compounding, constant contributions, and no taxes, fees or fund expenses. Fees of 1% per year compound into a large drag over decades.
- Actual market returns are volatile; a portfolio earning 7% on average does not earn 7% each year, and the timing of good and bad years changes outcomes when contributions are ongoing (sequence-of-returns effect).
- Nothing here is investment advice or a product recommendation; asset allocation and suitability depend on personal circumstances and are matters for a licensed adviser.
What is an investment calculator?
An investment calculator applies compound-growth mathematics to project what an initial amount plus regular contributions could become at an assumed constant rate of return. It is the same future-value model used for savings, applied with return assumptions typical of investment portfolios rather than deposit accounts. The output separates what was paid in from what the assumed growth added.
The real-value result addresses inflation: dividing the nominal projection by (1 + inflation)^years expresses it in today's purchasing power. Over 20 years at 2.5% inflation, a dollar loses about 39% of its purchasing power, so the real value is markedly lower than the nominal figure — a distinction the US Securities and Exchange Commission's investor-education materials emphasize for long-horizon planning.
A constant-return model smooths over the defining feature of real markets: volatility. Historical long-run US equity returns have averaged roughly 10% per year nominally (S&P 500, 1926 onward), but individual years have ranged from deep losses to large gains, and the sequence of returns affects outcomes when money is added or withdrawn along the way. Projections at a smooth average are illustrations, not forecasts.
How to use this investment calculator
- Enter your starting investment amount and planned monthly contribution.
- Enter an assumed annual return. Long-run historical references: US equities roughly 10% nominal, balanced 60/40 portfolios roughly 8–9%; many planners use 5–7% as a conservative assumption.
- Enter the investment period in years and an assumed inflation rate (the US Federal Reserve targets 2%).
- Read the projected value, total contributions, investment gains, and the inflation-adjusted value in today's money.
- Re-run with lower and higher return assumptions to see how sensitive the outcome is — the range matters more than any single number.
The investment growth formula
The nominal projection combines the future value of the initial lump sum with the future value of the contribution stream, compounded monthly at the assumed return. The real value deflates the result by the assumed inflation rate.
Worked example: $10,000 initial plus $500 per month at 7% for 20 years (r ≈ 0.005833, n = 240). The lump sum grows to about $40,387 and the contributions to about $260,460, for a projected value near $300,850. Contributions total $130,000, so assumed gains are about $170,850. Deflated at 2.5% inflation over 20 years, the real value is roughly $183,600 in today's money.
Common mistakes
- Treating the projected value as a promise; it is a constant-rate illustration of an assumed return.
- Using a gross historical return without subtracting fund fees and expenses, which reduce compounding.
- Reading the nominal figure as purchasing power — the real (inflation-adjusted) value is the meaningful one for long horizons.
- Assuming contributions can always continue; income interruptions change the projection materially.
- Comparing the projection with a savings account without acknowledging the market risk the higher assumed return carries.
Часто задаваемые вопросы
What return should I assume for an investment projection?
There is no single correct number. Long-run historical references are roughly 10% nominal for US large-cap equities and 8–9% for a 60/40 stock-bond mix, before fees; many planners use 5–7% to be conservative. Running the projection at several rates — for example 4%, 6% and 8% — shows the range of outcomes rather than a false point estimate. Historical averages do not predict future returns.
What does the value in today's money mean?
It is the nominal projection deflated by the assumed inflation rate: real value = FV ÷ (1 + inflation)^years. A projected $300,850 in 20 years at 2.5% inflation equals about $183,600 of today's purchasing power. The real figure answers the practical question — what could that future balance actually buy — which the nominal figure overstates.
How much could $500 a month become in 20 years?
At an assumed 7% annual return compounded monthly, $500 per month plus a $10,000 starting amount projects to roughly $300,850 after 20 years, of which $130,000 is contributions and about $170,850 is assumed growth. At 5% the projection is materially lower and at 9% materially higher — the assumption drives the result, and actual returns are not guaranteed.
Why do small fee differences matter so much?
Fees reduce the effective compounding rate every year. The SEC's investor-education materials illustrate that a 1% annual fee on a portfolio earning 7% cuts the 20-year outcome by tens of thousands of dollars on a six-figure portfolio, because the forgone amounts would themselves have compounded. Comparing expense ratios is one of the few return levers an investor controls directly.
Does this calculator model market crashes or volatility?
No. It assumes the same return every month, which no real market delivers. Volatility means actual paths can end far above or below a smooth-average projection, and the order of good and bad years matters when money is being added over time. Monte Carlo simulations and historical backtests are the standard tools for exploring that uncertainty; this tool provides the deterministic baseline.
Are investment gains taxed?
Generally yes, though timing and rates depend on the account type and jurisdiction. In the US, gains in taxable accounts incur capital gains tax when realized and dividends are taxed in the year received, while retirement accounts such as 401(k)s and IRAs defer or modify taxation. This calculator is pre-tax; tax treatment for a specific situation is a matter for a qualified tax professional.
Источники
- US Securities and Exchange Commission (SEC). Investor.gov compound interest calculator and investing basics. investor.gov.
- Federal Reserve Bank of St. Louis. S&P 500 and CPI historical data. FRED Economic Data (fred.stlouisfed.org).
- US Securities and Exchange Commission (SEC). How fees and expenses affect your investment portfolio (Investor Bulletin). sec.gov.
- Bodie Z, Kane A, Marcus AJ. Investments (12th ed.). McGraw-Hill, 2021 — risk, return and history of interest rates.
- Dimson E, Marsh P, Staunton M. Triumph of the Optimists: 101 Years of Global Investment Returns. Princeton University Press, 2002.