How the FIRE calculator works
FIRE stands for financial independence, retire early. You're financially independent when your investments can pay for your spending indefinitely. Your FI number is your yearly spending divided by a safe withdrawal rate: at the classic 4%, that's 25 times what you spend in a year.
Your savings rate is the share of your gross income you invest. It's added each month and grows at your expected return. Using a return after inflation keeps every figure in today's dollars. The two biggest levers are how much you save and how much you plan to spend: spending less in retirement lowers the amount you need to reach.