Your FIRE date is a set of assumptions
A FIRE date is not one fixed truth. It is the output of assumptions about income, savings, spending, growth, and withdrawal rate. Change one variable and the date moves — sometimes by years.
Savings rate moves the date fastest
Savings rate usually moves the date fastest because it improves two things at once: how much you invest and how much you may need later. Going from a 20% to a 30% savings rate might move your FIRE date forward by 4–6 years, depending on starting portfolio and income.
Spending assumptions
Spending assumptions matter just as much. A lower recurring spending target reduces the portfolio required to retire, while a higher target pushes the goal further away. Many planners find that their retirement spending estimate shrinks over time as they build a clearer picture of what actually makes them happy.
Growth rate and withdrawal rate
Withdrawal rate, market return, and location costs are best treated as planning levers rather than promises. A 7% nominal return assumption is historically reasonable for a diversified US equity portfolio, but not guaranteed. A 3% real return is more conservative and appropriate for longer planning horizons.
The point of the calculator is to help you test tradeoffs and decide what to change next. Run the numbers at several different growth rates (6%, 7%, 8%) to see how sensitive your timeline is to market performance. If your FIRE date only changes by 2 years across that range, your plan is relatively robust. If it changes by 10 years, you are very dependent on market returns and should consider raising your savings rate.