The FIRE number sounds technical, but it is really just an answer to a simple question: how much do you need invested so that your investments can cover your living costs? Many guides bury that answer under spreadsheets, calculators, and assumptions, which puts people off before they start. You can get a useful, honest estimate with a pen, a few minutes, and three numbers you probably already know. This guide walks through the method step by step, with no software required.

Start with what you actually spend in a year, not what you think you should spend. Look at your bank statements for the last twelve months and add up the outgoings, or take a typical month and multiply by twelve, adjusting for annual bills like insurance. Include everything: housing, food, transport, bills, and fun. This number is the foundation of the whole calculation, so honesty matters more than precision. A rough but honest figure beats a precise fantasy every time.

The classic shortcut is the rule of twenty-five: multiply your annual spending by twenty-five to get your FIRE number. If you spend 30,000 a year, your number is 750,000. If you spend 50,000, it is 1,250,000. This comes from the idea of withdrawing 4 percent of your portfolio each year, since 4 percent of 750,000 is 30,000. For a longer retirement, some planners use twenty-eight or thirty instead, which gives a larger, more conservative target. Pick the multiplier that matches your comfort with risk.

Your FIRE number is a target, not a starting line. Subtract your current investments from it to see the gap you still need to close. If your number is 750,000 and you already have 150,000 invested, the remaining gap is 600,000. This is encouraging, because it turns an abstract goal into a concrete distance, and every month of saving visibly shrinks it. Also consider whether you expect other income in retirement, such as a pension, and whether that should reduce your target.

Before you commit to the number, test it against your life. Does the spending figure include costs that will disappear, like a mortgage that will be paid off, or costs that will appear, like healthcare? Would you be comfortable living on that spending level for decades? If the number feels impossibly large, the most powerful lever is reducing target spending, since every 1,000 cut from annual spending removes 25,000 from the target. Adjust, recalculate, and you have a plan you can actually follow.

Once you have your number, use it as a compass rather than a verdict. Divide the gap by your annual savings to get a rough timeline, then focus on the two levers that matter: saving more and spending intentionally. Revisit the calculation once a year, when your spending or investments change meaningfully. A FIRE number you understand and believe in is far more motivating than a spreadsheet you never open.

Your FIRE number is not carved in stone, so revisit it once a year, ideally at the same time you review your investments. Update your spending figure with the last twelve months of actual outgoings, adjust for any life changes such as a move, a new child, or a paid-off mortgage, and recalculate with the same simple multiplication. You may find the number falls as you spend more intentionally, or rises as your life grows, and both are fine. What matters is that the number stays honest and current, so the plan built on it stays realistic. A yearly recalculation takes ten minutes and keeps a decades-long plan grounded in real life.