Invest every surplus in low-cost index funds immediately

FI is built in the gap between income and spending, compounded by market returns over time.

Why it works

Compounding produces exponential rather than linear growth — each dollar invested today grows to produce returns on returns for decades. The magnitude of compounding means that early investment is disproportionately valuable: a dollar invested at 25 has 40 years to compound before 65, while the same dollar invested at 45 has only 20. Every month of delay has a mathematically calculable cost.

How to do it

1. Calculate the monthly surplus remaining after fixed costs and intentional spending.
2. Invest the entire surplus the same week it appears — do not allow it to accumulate uninvested in checking.
3. Use tax-advantaged accounts first (401k, IRA, HSA), then taxable brokerage for overflow.

Evidence

Compound growth is mathematical, not contested. Long-term US equity market returns have averaged roughly 7% annually in real terms over the past century, though with significant year-to-year variance and no guarantee of future performance. (mechanistic)

Historical US equity returns are high by global standards; international equity and future US returns may be lower, which extends FI timelines and may require higher savings rates.

Sources

• Siegel (2014), Stocks for the Long Run — long-run equity return data

Common mistake

Waiting to invest until you have a "meaningful amount" — the cost of a 6-month delay in starting compounds across the entire investment horizon and is typically larger than expected.