FIRE calculator: how much capital you need and how long it lasts

It comes from William Bengen's work (1994) and the Trinity study (1998): with US stock and bond portfolios, withdrawing 4% of capital in the first year and then raising it with inflation lasted 30 years in the vast majority of historical periods. It equals building up 25 times your annual spending. For longer horizons, such as an early retirement of 40 or 50 years, or outside the US, many later analyses suggest more prudent rates of around 3 to 3.5%.

With a constant return the order of the years does not matter, but in reality it does. A sharp fall in the first years of withdrawals forces you to sell low and leaves less capital for the recovery; the same fall at the end barely matters. That is why people keep a cash buffer or cut spending in bad years. This calculator uses a constant return: treat it as a starting point, not a guarantee.

The target capital is your annual spending divided by the withdrawal rate. In the second part, the withdrawal is taken at the start of each month, rises every year with inflation, and the remaining capital earns the annual return you choose. The sustainable withdrawal is the one that runs the capital down exactly at the end of the horizon. Taxes on what you sell, fees and pensions are not included.

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