How is this different from the Compound Interest Calculator?
Compound Interest assumes a fixed, contractual rate (savings account APY, CD rate, bond coupon) and gives you precise dollar outcomes. Compound Investment models a variable-return investment portfolio (stocks, ETFs, mutual funds) where the rate you enter is a long-run average, not a guarantee. The math is the same — the difference is interpretation. Use this calculator for retirement and brokerage projections; use Compound Interest for savings accounts and CDs.
What expected return should I use?
Match the assumption to your portfolio. Historical US data (Damodaran NYU 1928–2024) suggests roughly 10% nominal / 7% real for US large-cap stocks, 5% nominal / 2% real for 10-year Treasury bonds, and 3% nominal / 0% real for T-bills. For a 60/40 stocks-bonds portfolio, 7–8% nominal is reasonable. Vanguard and BlackRock's forward-looking 10-year capital market expectations are typically 1–2 points lower than the historical average.
Should I use real or nominal returns?
Use real (inflation-adjusted) returns if you want the output to represent today's purchasing power — most useful for retirement planning. Use nominal returns if you're comparing against a specific dollar goal (e.g., a $500,000 down payment in 2046). The relationship is approximate: real return ≈ nominal return − inflation. The historical US inflation average is about 3% per year, so a 10% nominal stock return is roughly a 7% real return.
How often should I rebalance my portfolio?
Most evidence-based advice points to annual rebalancing or threshold-based rebalancing (when an asset class drifts more than 5 percentage points from target). Vanguard's research finds that more frequent rebalancing adds transaction costs without improving risk-adjusted returns. This calculator assumes you maintain your target allocation, so its return assumption already implicitly bakes in periodic rebalancing.
What about investment fees and expense ratios?
Subtract them from your assumed return before running the calculator. If you expect a 7% gross return and your fund's expense ratio is 0.5%, model 6.5%. Over 30+ years, even small fee differences compound dramatically: a 1% vs 0.05% expense ratio on a $500,000 portfolio costs about $4,750 per year in foregone returns, which itself compounds. The SEC and FINRA both flag fees as one of the largest controllable variables in long-term investing outcomes.
What is dollar-cost averaging (DCA)?
DCA is the practice of investing a fixed dollar amount on a regular schedule (e.g., $500 on the 1st of every month) regardless of market conditions. The Monthly Contribution field in this calculator implicitly models DCA. The benefit is behavioral — it removes market-timing decisions — and you buy more shares when prices are low. The drawback is that DCA-ing a lump sum often underperforms investing it all at once, because markets rise more often than they fall.
Does this account for sequence-of-returns risk?
No. The calculator assumes a smooth, constant return every year. In reality, the order of returns matters enormously in retirement (a -30% year early in withdrawal can permanently impair a portfolio) but matters far less during the accumulation phase, when you're buying through volatility. For a 30-year-out projection, the constant-return model is a reasonable simplification; for the 5 years before and after retirement, use a more sophisticated Monte Carlo simulator.
What happens if I stop contributing after some years?
Run the calculator twice. First, calculate the future value at the year you stop contributing. Second, run it again with that ending balance as the new principal, monthly contribution set to $0, and the remaining years. For example: $500/month for 10 years at 7% → about $86,000. That $86,000 then compounds alone for another 20 years at 7% → about $333,000. The compounding power of the early decade dwarfs the missed later contributions.
Is a 7% return realistic going forward?
It's plausible but not guaranteed. The Vanguard Investor Resources 2026 capital market assumptions project 4.5–6.5% nominal for US equities over the next decade, lower than the historical 10%, due to elevated current valuations. A 7% long-run average across a 30+ year horizon remains within the range of historical experience, but using a more conservative 5–6% gives a margin of safety in case the next decade underperforms history.
What about taxes on investment growth?
The calculator returns pre-tax future values. In tax-deferred accounts (Traditional 401(k)/IRA), the entire balance is taxed at ordinary income rates on withdrawal. In Roth accounts, qualified withdrawals are entirely tax-free. In taxable brokerage accounts, you owe taxes on dividends and realized gains each year, plus capital gains tax on the appreciation when you sell. For taxable-account projections, reduce the expected return by your effective tax drag (often 0.5–1.5 percentage points).
Why does my Effective Return look so high?
Effective Return is shown as the cumulative multiplier on contributed dollars (FV ÷ Total Contributions − 1), not an annualized rate. Over 30–40 years, a 7% annualized return naturally produces a 4–10× lifetime multiple, which displays as 300%–900% cumulative. To get the annualized equivalent, take (FV ÷ Total Contributions)^(1/t) − 1, where t is the number of years — but note this formula is only exact for lump-sum investments, not for contribution streams where dollars compound for different lengths of time.
How does the calculator handle the timing of monthly contributions?
The widget treats contributions as end-of-month (ordinary annuity), which is the convention used in most retirement-account formulas. If your real contributions hit at the start of each month (annuity-due), the future value is slightly higher — multiply the contribution portion by (1 + i), where i = r/12. The difference is usually less than 1% of the final balance, so the end-of-month assumption is fine for planning purposes.
