To estimate how much monthly ETF investments could be worth, choose a monthly contribution, a time horizon, an assumed return, and a deposit date. Then use the future-value formula for a series of monthly deposits. The result is a scenario—not a prediction or guaranteed balance—because ETF returns vary, fees reduce returns, and losses are possible.
What the estimate tells you—and what it does not
A future-value calculation answers a specific question: if you invest the same amount regularly and earn a constant assumed rate, what would the account balance be after a chosen number of months? It helps you compare savings plans and see how contribution size, time, and return assumptions affect a hypothetical result.
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It does not forecast what an ETF will earn. The calculation treats returns as steady, while real ETF returns rise and fall. The order of gains and losses can change an account’s actual path. ETF shares also trade at market prices that can differ from the fund’s net asset value (NAV). Investor.gov notes that ETFs are not FDIC-insured or guaranteed by another government agency, investors can lose money, and past performance does not predict future returns.
Use the result as a planning estimate. It is most useful when you make the assumptions visible, compare more than one plausible scenario, and update the estimate as your contribution plan or circumstances change.
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The monthly-investment formula
For equal deposits made at the end of each month—an ordinary annuity—the future value is:
FV = P × (((1 + r)^n − 1) / r)
- P is the amount invested each month.
- r is the assumed monthly return, written as a decimal.
- n is the number of monthly deposits.
- FV is the estimated value after the final deposit has had no additional time to grow.
For a monthly contribution over a whole number of years, n = 12 × years. The formula adds each deposit after allowing it to compound for the months that follow it. Earlier deposits have more time to grow than later ones.
If you deposit at the start of each month
When each contribution is made at the beginning of the month, it earns one extra month of assumed return. This is an annuity due:
FV_due = FV × (1 + r)
The difference is a timing effect, not a different contribution amount. If the assumed monthly return is zero, both formulas reduce to the total amount deposited, P × n.
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Convert the annual return into a monthly rate
Do not divide an annual return by 12 unless the assumption is a nominal annual rate compounded monthly. The conversion depends on what the annual rate means.
- Effective annual return: If
Ris the return over a full year after compounding, user = (1 + R)^(1/12) − 1. This conversion makes 12 months at raterequal to the stated effective annual return. - Nominal annual rate compounded monthly: Use
r = R/12. In this case, the effective return over a year is not exactly the nominal rate because monthly compounding applies.
For example, a 7% effective annual assumption converts to a monthly rate of about 0.5654%. A 7% nominal annual rate compounded monthly converts to about 0.5833% per month. Those interpretations produce different estimates, so label the convention whenever you report a result.
Investor.gov’s compound-interest calculator asks for a monthly contribution, saving period, estimated annual interest rate, and compounding frequency. Match your monthly contribution and period to the calculator’s inputs, and decide how its annual-rate and compounding settings correspond to the rate convention you intend to use. A calculator can do the arithmetic; it cannot determine which return assumption is appropriate for your investment.
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Suppose you invest $300 at the end of every month for 20 years. There are 12 × 20 = 240 deposits, and your own contributions total $300 × 240 = $72,000. To illustrate how sensitive the estimate is to the return assumption, the table uses effective annual total-return scenarios converted to monthly rates. The assumed rates are treated as net of fees and other costs; no separate charges are subtracted. They are examples, not ETF forecasts.
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| Assumed effective annual return | Monthly rate used | Estimated value after 20 years | Of which: contributions | Estimated growth above contributions |
|---|---|---|---|---|
| 4% | About 0.3274% | About $109,000 | $72,000 | About $37,000 |
| 7% | About 0.5654% | About $152,000 | $72,000 | About $80,000 |
| 10% | About 0.7974% | About $215,000 | $72,000 | About $143,000 |
These are rounded estimates for end-of-month deposits, using the formula above. The $72,000 contribution amount stays fixed; the difference among the estimated balances comes from the return assumption and compounding. A higher assumed return has a large effect over a long period, but the table does not show how an investor would experience the uneven gains and losses of an actual ETF.
Investor.gov’s investing introduction uses a 7% average annual return in illustrations of how much someone might need to contribute to reach a target by age 65. For example, its undated page accessed October 9, 2026, illustrates $209 per month for a 25-year-old targeting $500,000 and $418 per month for a $1 million target. Those are illustrations under that stated assumption, not promises, forecasts, or evidence that an ETF will earn 7%.
Calculate your own estimate step by step
- Choose the monthly contribution. Enter the amount you actually plan to invest, not an annual amount mistakenly treated as monthly. If your contribution changes over time, a constant-payment formula will not represent that schedule exactly.
- Set the time horizon. Convert years to deposits by multiplying by 12. For example, 15 years means 180 monthly deposits. Be consistent about whether the final contribution is made at the end or start of the final month.
- Choose and label the annual assumption. State whether it is an effective annual return or a nominal annual rate compounded monthly. Treat it as a scenario input rather than a prediction of what a particular ETF will earn.
- Convert the annual assumption to a monthly rate. For an effective annual return, calculate
(1 + R)^(1/12) − 1. For a nominal annual rate compounded monthly, calculateR/12. Use decimals in the formula: for example, 7% is0.07. - Choose deposit timing. Use the ordinary-annuity formula for end-of-month deposits. For beginning-of-month deposits, multiply that result by
1 + r. - Calculate the future value and separate deposits from growth. Work out
P × nseparately. Subtract that total from the estimated future value to show the modeled growth above your contributions. - Check what the return includes. Decide whether your assumption is before or after fund expenses and transaction costs, and whether it includes reinvested distributions. Do not add a separate dividend estimate if distributions are already included in the total-return assumption.
- Compare scenarios on equal terms. Keep the contribution, horizon, deposit timing, and distribution treatment fixed while changing the return assumption. If you also change a fee assumption, identify that change so the difference is interpretable.
Account for ETF fees and trading costs
The return assumption should reflect the costs that reduce what you keep. ETF operating expenses lower fund returns. Brokerage commissions and other trading costs can also matter, especially when a flat charge is applied to a small recurring purchase. The SEC notes that ETF transaction charges may not appear in the prospectus fee table, so checking that table alone may not reveal every cost of buying or selling shares.
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You can handle costs in either of two ways: use an assumed return that is already net of the relevant costs, or model costs separately. Do not do both for the same expense, or you will count it twice. If you use a net return, state which costs it is intended to include. If a commission is charged on each purchase, account for it as a recurring deduction from the amount invested or as a separate cash cost rather than silently treating the full contribution as invested.
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The scale of fee drag is illustrated by a hypothetical example published by the SEC on July 23, 2025. Starting with $100,000, assuming 4% annual growth for 20 years, the SEC gives approximate ending values of $208,000 with a 0.25% annual fee, $198,000 with a 0.50% fee, and $179,000 with a 1.00% fee. These are approximate figures for that one-time $100,000 illustration—not an ETF forecast and not a calculation for monthly contributions. The SEC’s point is that costs compound too: money paid in fees is no longer invested and earning returns.
When comparing ETFs, review each fund’s prospectus for its objective, costs, and risks. The formula estimates the outcome of a set of assumptions; it does not identify an appropriate ETF or account for differences in holdings, objectives, or risk.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Decide how to treat distributions
ETF distributions are not guaranteed. Reinvesting them can help an investment grow over time, but a distribution payment also reduces the fund’s NAV when it is paid. A projection should make clear whether its return assumption is a total-return assumption that includes reinvested distributions.
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- If the assumption excludes distributions, the formula estimates growth without them. Any separate distribution model would need its own assumptions about amount, timing, tax treatment, and reinvestment.
The first approach is simpler for a high-level estimate, provided the assumption is labeled. Mixing a total-return assumption with a separately added dividend estimate overstates the modeled result by counting distributions twice.
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Why the actual ETF balance can differ
Returns do not arrive at a constant rate
The formula applies the same monthly rate to every period. Markets do not. An actual investment may gain in some months and lose in others, and the sequence of returns affects the path of an account receiving regular deposits. A constant-rate calculation smooths that path into one number; it cannot show the range of outcomes or the timing of a loss.
Trading prices can differ from NAV
ETF shares are bought and sold at market prices, which can differ from the value of the fund’s underlying holdings per share. The price at which recurring purchases execute can therefore affect the number of shares acquired and the account value. A formula using a smooth assumed return does not model those trades.
Costs, taxes, and contribution changes may not be represented
Unless they are explicitly included, a simple estimate does not separately account for commissions, trading costs, taxes, changing contributions, or withdrawals. The article’s scenario table assumes fixed end-of-month deposits and a net return that already reflects costs; it does not make assumptions about an individual investor’s taxes or account type. Adjust the assumptions to match the question you are trying to answer, and avoid presenting a simplified result as a precise account forecast.
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Common calculation mistakes to avoid
- Using an annual rate as if it were monthly. Convert the annual assumption using the stated effective or nominal convention before applying a monthly formula.
- Mixing annual and monthly periods. For monthly contributions, use the monthly rate and number of monthly deposits together.
- Ignoring deposit timing. Beginning-of-month contributions receive an additional compounding period compared with end-of-month contributions.
- Calling a scenario a forecast. A fixed rate is an assumption, not a prediction, and an ETF can lose value.
- Counting distributions twice. Do not add dividends separately to a total-return assumption that already includes reinvested distributions.
- Leaving costs out or subtracting them twice. Make clear whether the return is net of expenses and trading costs, then account for each cost only once.
- Reporting only the ending balance. Show total contributions separately so readers can distinguish their deposits from modeled growth.
How to make the estimate useful for a decision
Start with the contribution and timeline you can realistically sustain, then examine how the result changes under multiple explicitly labeled return assumptions. A scenario table should keep contribution, duration, deposit timing, and distribution treatment constant unless one of those is the factor being tested. This reveals which assumptions drive the difference instead of making several changes at once.
Use the result to answer practical planning questions: whether a contribution amount is in the right range for a target, how much more time or monthly saving could change the estimate, or how sensitive the projection is to fees. If a goal appears achievable only under the highest return assumption, that is a sign the plan depends heavily on an uncertain outcome, not proof that the outcome will occur.
For ETF-specific decisions, go beyond the projection. Read the prospectus, understand the fund’s objective, costs, and risks, and remember that the future-value formula does not account for whether the ETF fits your circumstances. The estimate is a transparent arithmetic tool—not a substitute for evaluating the investment or the uncertainty around it.
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