What if?

  1. The question

    What if I invested $100 a month in the S&P 500?

    $100 every month into the S&P 500 since 2015 — contributions and ending value on real prices. A small, honest monthly what-if.

    Quick answer

    If you invested $500/month into S&P 500 from 2020-01-03 through 2026-10-16, total contributions would have been about $40,960 and a blind DCA portfolio would be worth about $72,893 (+78%) on this historical path — before fees and taxes.

  2. Assumptions

    What this page actually assumes

    • $100 per month in US dollars into the S&P 500.
    • Buys follow the monthly cadence in our backtest (not twelve identical weeks).
    • Held to the last date. No withdrawals, no timing the dips.
    • Blind DCA is a flat $100. SteadyGrow uses the same long-run budget with week-to-week sizing.

  3. Historical data

    2020-01-03 → 2026-10-16

    S&P 500 from 2015. This is the common “what if I invested $100 a month” question people ask on Reddit and forums — answered with prices, not a fixed 8% toy.

    Series: S&P 500. Same contribution calendar. No data before 2015.

  4. Interactive simulation

    Change the habit, stay on the real curve

    Amount
    Start year

    $500/month · S&P 500

    2020-01-03 → 2026-10-16 · $500/month · S&P 500

    Contributed$40,960
    Held to the last date$72,893
    Return+78%
    S&P 5002020-01-03 → 2026-10-16 · $500/month
     +$31,933 vs cash inEnded $72,893 on $40,960 contributed
    Portfolio (DCA)Cash contributed
    $0$19k$37.9k$56.9k$75.8k2020-012021-052022-092024-012025-062026-10

  5. Results

    What the numbers say

    Contributions ≈ months × $100. Ending value is that cash marked to the index. Switch the year chips for a shorter window; we will not invent a 1980s series we do not have.

    If $100/month feels small, open the $500/month and $1,000/month stories — same engine, bigger habit. Or jump to the monthly investment calculator to browse amounts.

    The chart marks portfolio value over time for fixed-amount DCA and (when shown) a same-capital lump sum. Contributed cash rises in steps; ending value is mark-to-market on the last date (2026-10-16).

  6. What changes if…

    How did we calculate this?

    • Window: real market history from our published backtest bundle, typically from 2015 through the latest Friday in the series — we do not invent older decades.
    • Contribution calendar: the engine runs on a weekly Friday grid. A “monthly” habit is converted to an equivalent weekly cash rate ((amount × 12) / 52) so the same engine can compare habits.
    • Purchase timing: each period’s contribution is applied on that week’s bar in the series (close-based weekly path), not an intraday open fill.
    • Assets: S&P 500, Bitcoin, and other series we publish — USD only. No FX conversion.
    • Shares: fractional units are assumed. No brokerage commissions, bid–ask, or slippage are modeled.
    • Dividends / income: returns follow the wealth path in the backtest bundle for that asset (not a fixed 7–10% toy rate).
    • Lump sum (when shown): the same total cash as the DCA habit, invested on day one of the window, then marked to the same path.

    Frequently asked questions

    What if I invested $100 a month in the S&P 500?
    From 2020-01-03 to 2026-10-16, $500/month into S&P 500 contributed about $40,960 and a blind DCA habit ended near $72,893 (+78%).
    Are these numbers a forecast?
    No. They replay published market history from 2015 onward. Future returns can look nothing like this window.
    What assumptions matter most?
    Start date, contribution size, weekly vs monthly cadence, and whether you held through drawdowns. Fees and taxes are not modeled.
    Can I change the amount or start year?
    Yes — use the chips on this page when available, or open a related what-if / calculator with a different habit.
    How is this different from a 7% compound calculator?
    Fixed-rate toys assume a smooth return. We mark contributions to a real index/crypto path, including crashes in the window.
    What if I had traded instead?
    See The Boring Alternative for story-shaped counterfactuals (options losses, round-trips), then run the same dollars through an index DCA calculator.

    Run a related reality check

    Same contribution schedule, different assets — or a fixed-rate planning calculator when you want an illustration instead of history.

    S&P 500 vs BitcoinCompound interest →Boring Alternative →