Methodology

Every calculator on this site runs the same open, deterministic simulation in your browser. Nothing is sent to a server.

The monthly loop

1. Convert the annual return r to a monthly rate: (1 + r)^1/12 − 1. Apply it to the balance.
2. Subtract the withdrawal. If a tax rate t is set, the amount removed is withdrawal ÷ (1 − t) so the after-tax amount equals what you entered.
3. Every month, increase the withdrawal by the monthly-equivalent inflation rate so that it rises by exactly the annual inflation figure each year.
4. Stop when the balance reaches zero (report the month) or after 1,200 months (report "indefinitely").

Safe withdrawal for N years

We search for the largest starting withdrawal whose simulation survives N×12 months, using 40 rounds of bisection between $0 and the full balance. Accuracy is well under $1.

What the model does not do

• It does not model variable returns or sequence-of-returns risk. A constant return is optimistic for retirees who hit a bear market early.
• It does not model fees separately — subtract them from the return.
• It does not model progressive tax brackets, Social Security taxation, RMDs, or state-specific rules. The tax field is a flat rate.
• Amount-page tables use 2.5% inflation and no tax.

Sources for the rules of thumb quoted across the site: the original 4% rule (Bengen, 1994) and the Trinity Study (1998); IRS Publication 590-B for IRA distribution rules; SSA data for average retired-worker benefits; TSP.gov and IRS guidance for plan-specific rules. Figures are reviewed annually.