How your QuitNumber is calculated
Three straightforward assumptions, combined into one formula. Figures below use United States and a worked example at age 25.
Monthly cost of a good-enough life
An estimated cost of living for one adult, refreshed in the background and converted to local currency.
Years you'll need to fund
Your country's average life expectancy, refreshed every few months from the World Bank, minus your current age.
The lump sum that covers it
Invested at an assumed real annual return, the amount that pays your monthly cost every month for exactly that many years.
Your funding window
The formula
This is the standard present value of a finite annuity. As n grows large (a young
age relative to life expectancy), it converges toward Monthly cost × 12 ÷ r
— the familiar "save 25× your annual expenses" rule. A shorter horizon correctly asks for
less, because there are fewer years left to fund.
Lump sum needed vs. years to fund
Holding the monthly cost fixed, here's how the required lump sum changes with the funding horizon.
Dashed line: the infinite-horizon "25×" value the curve approaches. Marker: your worked example above.
For someone in United States at age 25, funding $3,200/month for 53.9 years at a 4.0% real return:
QuitNumber = $844,031
(vs. $960,000 under the classic infinite-horizon 25× rule)