How the numbers are made

Calculators

Formulas live in code, separate from the page. FIRE number is spending divided by a withdrawal rate. Compound growth adds the contribution at the end of the year. Years to FI solves that formula for time. Coast FIRE discounts the FIRE number by a real return. The 401(k) match applies the match rate only up to the percent of salary the formula names, then caps the employee deferral at a labeled IRS limit.

Returns and inflation

Public growth tools default to a 5% real return. Spending stays in today’s dollars. That avoids counting inflation twice. 5% is a round planning assumption so examples stay readable. It is not a forecast, and it is not a historical average Arc claims as its own. Lower it if you want a more cautious date.

Withdrawal rates

4% is the familiar starting rate from U.S. historical research associated with a 30-year retirement. The calculators do not rerun those studies. They multiply a portfolio by the rate you choose and label it as year-one spending. Sequence-of-returns risk is explained in writing. It is not simulated on the public tools.

Benchmarks

Age tables use the Federal Reserve 2022 Survey of Consumer Finances percentile extract already in the product. Federal Reserve SCF 2022 (inflated to July 2026$ via CPI-U). Household net worth uses the survey definition. Retirement balances are among households that hold a retirement account, and they combine IRAs with employer plans. This extract has no mean, and no 401(k)-only series. Those pages say so instead of filling the gap.

Tax-year limits

The match calculator’s default deferral limit is tax year 2026. IRS Notice 2025-67. Employee elective deferral for 401(k), 403(b), and most 457 plans. The number is visible, and the age-50 catch-up is a checkbox, not a hidden change.

What an output is

An output is an estimate under the assumptions on the page. It is not a guaranteed balance, a tax determination, or a recommendation to buy or sell anything.