FIRE — financial independence, retire early — comes down to one question: how large does a portfolio have to be before the income it throws off covers what you spend? Everything else is arithmetic around that number.
The usual starting point is the 4% rule, which came out of the Trinity Study: a portfolio of roughly 25 times your annual spending has historically survived a 30-year retirement in the great majority of cases. It is a rule of thumb, not a guarantee, and the calculators here let you move the withdrawal rate rather than treating 4% as settled fact.
Three numbers drive every result on these pages: what you spend in a year, what you save in a year, and what you assume markets will return after inflation. Savings rate matters most. Someone saving 10% of their income needs decades; someone saving 50% needs well under twenty years, and the reason is that a higher savings rate shrinks the target and grows the pot at the same time.