Reference formulas and worked examples below keep their stated units. Monetary examples use USD; dimensional examples use US customary units unless labeled otherwise. Metric calculator inputs are converted to these reference units before calculation. Choosing another currency does not convert example amounts or exchange money.
How to use this fire calculator
Enter assets, regular saving and the annual spending your portfolio must cover. The projection keeps amounts in today’s purchasing power.
How the calculation works
Annual portfolio-funded spending = max(0, spending − income available from retirement). Target = that annual gap ÷ the assumed withdrawal rate.
Real annual growth factor = (1 + nominal return)/(1 + inflation). Convert this factor to a monthly growth rate, apply monthly growth and then add the constant real monthly contribution.
Stop at the first month assets meet the target, or at the selected horizon. An already-funded target is identified at month zero. Not reaching it within the horizon is reported without inventing a retirement age.
Worked example
Annual spending of $40,000 and no other retirement income gives a $1,000,000 target at a 4% assumed withdrawal rate. With $400,000 already saved, $1,000 per month and 0% real growth, reaching the target takes 600 months (50 years).
Limits and assumptions
A constant real-growth savings scenario. The withdrawal-rate target is a heuristic, not proof a portfolio will last. No market-path risk, taxes, account restrictions, benefit entitlement, changing contributions or delayed retirement-income streams. Use the retirement cash-flow tools for a spending-horizon scenario.
Frequently asked questions
How is this different from Coast FIRE?
Coast assumes no further contributions before retirement. This tool includes continuing monthly saving and solves for the first modeled month you reach the target.
Are these future dollars?
No. Assets, contributions, spending and the target are all in today’s purchasing power. Nominal return is adjusted for inflation before the projection.
Can I include a pension that begins much later?
Not as income available from the FIRE date. This simplified target assumes entered retirement income is available immediately on retirement; delayed streams need a separate scenario.
What if I do not reach the target?
The tool reports the remaining gap at the chosen horizon instead of promising a date beyond that horizon.