FORGING SMART FUTURES · Free tool

Index Investing Scenario Explorer

See how contributions, costs and inflation interact. Choose your own assumptions; these are illustrative inputs, not a recommended portfolio or expected returns.

Paul Foster and FSF are not financial advisers. General education and opinion, not personalized investment, tax or legal advice. Consult a qualified financial professional and tax adviser about your situation. Investments can lose value. Past performance does not guarantee future results.

1. Choose an illustrative allocation

Weights affect the expense estimate and hypothetical shock only. The growth model uses your single portfolio-return assumption below; it does not forecast each asset class. Starting weights are an example, not advice.

U.S. stock exampleU.S. stocks (%) International stocks — FTIHX (%)U.S. bonds — FXNAX (%)International bonds — FBIIX (%)

Must total 100%. S&P 500 and total U.S. market substantially overlap, so choose one here. Example expenses, June 2026 Fidelity lineup: 0.015%, 0.060%, 0.025%, 0.060% respectively. Verify current prospectuses. These mutual funds are examples, not endorsements.

2. Compare growth and buying power

Starting amount ($)Deposit at each month end ($)Whole yearsAssumed gross annual portfolio return (%)Constant illustrative rate before fund expenses. Not historical performance or a forecast.Assumed annual inflation (%)

3. Test one hypothetical downturn

Stock decline (%)Bond decline (%)

Applied once to the ending balance using your allocation. Not included in the constant-return chart. Actual losses can differ or be greater.

4. Separate contribution tax example

Eligible contribution ($)Contribution typeFederal marginal income-tax rate (%)Applicable employee payroll tax for HSA payroll (%)Used only for eligible HSA payroll mode. Verify your applicable rate and wage-base effects.

This is multiplication, not an eligibility or contribution-limit check. The entire contribution is assumed deductible/excludable at the entered marginal rate. Traditional 401(k) deferrals do not avoid employee payroll taxes. No second HSA deduction for excluded payroll amounts. Future withdrawal taxes and state rules are excluded.

Your calculated illustration

Teal: nominal balance · Gray: contributions · Gold: purchasing power. Horizontal axis: years from start; common vertical scale: dollars. Exact values below.

Contribution tax illustration

Year	Contributions	Nominal balance	Starting-year dollars

The workbook downloads as a JSON file: it is saved input data, not a readable report. Reopen it using the control below. Use CSV for spreadsheet results. On iPhone, use Safari for printing and downloads.

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How this model works

Net annual growth = (1 + gross return) × (1 − weighted annual fund expense) − 1. An equivalent monthly rate is applied before each month-end deposit. Allocation and fees remain fixed. Inflation-adjusted value = nominal balance ÷ (1 + inflation)^years. Exact lump-sum doubling = ln(2) ÷ ln(1 + net return); Rule of 72 = 72 ÷ net return expressed as a percent. Deposit timing, variable returns, taxes, withdrawals, rebalancing trades, advice/platform charges and changing inflation are not modeled. No historical return is used as a forecast.

Inputs stay in this calculator in your browser unless you download a file. No inputs or workbook contents are sent to analytics. Refreshing clears unsaved changes. Read the investing guide and source notes.