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US · 2025

Index Fund Calculator

Project index fund investment growth with lump sum and regular contributions. See long-term compound returns.

Last reviewed: 10 February 2026Source: FCA — Investment basicsUpdated every: tax year
Index Fund Calculator · USInvestments & Savings

Rates & sources

Compound growth assumes reinvested returns and no platform fees. Past performance is not a guide to future returns.

Source: FCA — Investment basics — figures refreshed at the start of each tax year.

When to use this calculator

  • Before choosing between saving, investing, or increasing your monthly contribution.
  • When you want to compare best-case, base-case, and cautious return assumptions.
  • When you need a quick projection before making a longer-term portfolio decision.

A realistic US planning example

Use these sample inputs as a quick scenario test, then change one variable at a time to compare outcomes.

Initial Investment ($)

$15,000

Monthly Contribution ($)

$250 per month

Expected Annual Return (%)

8

Years

10 years

After entering these figures, compare portfolio value, total invested and growth before deciding which scenario looks strongest.

How to read your results

Portfolio Value

Use this metric to compare scenarios side by side and understand how the key drivers affect the final outcome.

Total Invested

This is the headline outcome, but it is most useful when viewed alongside the supporting metrics below it.

Growth

Use this metric to compare scenarios side by side and understand how the key drivers affect the final outcome.

Method & assumptionsAuthoritative sources

Enter an initial lump sum, regular monthly contribution, expected annual return, and investment horizon to project your index fund portfolio value. Global index funds tracking the MSCI World or S&P 500 have historically returned around 7–10% per year after inflation over long periods. This calculator does not account for fund charges — a 0.1–0.2% annual charge on a low-cost index fund has a negligible impact over most horizons.

Common mistakes

  • !Assuming a constant return without checking a more conservative growth rate.
  • !Forgetting to include ongoing contributions, fees, or tax wrappers where relevant.
  • !Focusing only on the final balance instead of the path required to reach it.

What to do next

  • Test a cautious, expected, and optimistic growth rate instead of relying on a single projection.
  • Compare this result with related savings or retirement tools before committing more money.
  • Use the linked guides to understand which assumptions matter most over longer periods.

Frequently asked

Global equity index funds have historically returned around 7–10% per year over long periods (20+ years), though past performance does not guarantee future results.

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