The 4% rule turns annual spending into the portfolio that would cover it by withdrawing 4% a year. Spending of 2,000 a month is 24,000 a year. At 4%, the target is 24,000 ÷ 0.04 = 600,000.
Target = annual spending ÷ withdrawal rate. At a zero rate, months = (target − portfolio) ÷ contribution.
With nothing saved, a contribution of 2,000 and a zero rate, 600,000 ÷ 2,000 = 300 months, or 25 years. With a positive rate, the calculator finds the first month when the compound balance reaches the target. This page stops at 600 months.
What the calculation does not promise
A fixed withdrawal ignores inflation, taxes and a run of bad years. If the balance after 50 years is still short, the page says so instead of inventing a date.