Your FIRE number is the amount of invested assets you need for work to become optional, and calculating it takes about ten minutes once you know your annual spending. The standard formula multiplies your yearly expenses by 25, which comes from the 4 percent guideline: withdrawing 4 percent of a portfolio each year, adjusted for inflation, has historically lasted through long retirements in the studies behind the rule. The formula is simple, but the inputs deserve care, because a wrong spending estimate produces a wrong target.

Start by adding up your actual annual spending, not your income and not a guess. Pull twelve months of bank and card statements, group spending into categories like housing, food, transport, insurance, and discretionary costs, and total them. If your spending varies a lot, use an average of the last two years rather than a single unusual year. This is the most important step, because every dollar of annual spending adds 25 dollars to your FIRE number.

Next, adjust for retirement differences. Some costs will fall, such as commuting and work clothing, while others may rise, such as healthcare and leisure travel. Add any costs you expect to keep paying, like a mortgage that will still be running, and remove costs that will genuinely end. Be honest here rather than optimistic: underestimating retirement spending is the most common reason FIRE plans fall short.

Now apply the formula. If your adjusted annual spending is 36,000 dollars, your FIRE number is 900,000 dollars. If it is 50,000 dollars, the number is 1,250,000 dollars. This is why spending matters so much: cutting 5,000 dollars a year from your lifestyle reduces your target by 125,000 dollars, which can be worth years of additional saving. Small spending changes have an outsized effect on the goal.

Consider whether the standard 25-times rule fits your situation. It was built around 30-year retirements, so someone retiring at 40 with a potentially 50-year retirement may prefer a more conservative multiple, such as 30 or 33 times spending, which corresponds to a withdrawal rate closer to 3 percent. A larger multiple means a bigger target but also a larger safety margin, and the right choice depends on how much risk you are comfortable carrying.

Once you have your number, compare it with your current invested assets to find the gap, and estimate how long closing it will take at your current savings rate. Online compound growth calculators make this quick: enter your current portfolio, monthly contribution, and a cautious return assumption, and see when the lines cross. If the date feels impossibly far away, revisit spending first, since that moves the target faster than any realistic change in returns.

Recalculate your FIRE number once a year, because spending, income, and life circumstances all change. Treat it as a living target, not a one-time calculation. The ten minutes you spend each year keeping it current will keep your saving focused, your expectations realistic, and your motivation grounded in a number you actually trust.

Once you have your FIRE number, a useful follow-up is to calculate your savings rate and your estimated time to reach the target under a few different scenarios, such as your current rate, a rate five points higher, and a rate five points lower. Seeing how much faster a modest increase in savings moves the date is often the most motivating part of the whole exercise, because it turns an abstract goal into a concrete trade-off you can act on this month. Also consider calculating a lean version of your number based on essential spending only, which gives you a minimum safety target, and a comfortable version based on your ideal lifestyle, which gives you the full goal. Having both numbers lets you celebrate reaching the lean target as real progress while still working toward the comfortable one, rather than feeling like nothing counts until the single big number is hit.