How this simplified Coast FIRE model works
Let A be annual retirement spending in today’s money, w your target-conversion withdrawal percentage as a decimal, y the years until your chosen retirement age, g the fixed effective annual return and f fixed inflation. Today’s target = A / w. Future nominal target = (A / w) × (1 + f)^y. Required investments now = future nominal target / (1 + g)^y.
This is a zero-additional-contribution accumulation threshold. The spending percentage only converts your spending assumption into a target; it does not run withdrawals after retirement. The current-money difference is current investments minus the required current amount. If the ages are equal, the required amount is simply A / w.
Illustrative example: age 30 to 40, annual spending 40,000 in today’s money, a 4% conversion percentage, 0% return, 0% inflation and current investments 100,000. The target is 1,000,000 in both today’s and future money; required investments now are 1,000,000 and the current-money difference is −900,000. The 4% value is an editable illustration, not a safe withdrawal rate.
Hypothetical educational calculations, not financial advice or predictions. Fixed returns and inflation cannot model market variability or actual future living costs. This tool does not estimate the earliest retirement age, failure probability, asset lifespan, spendable income or a withdrawal path. It excludes Social Security, pensions, account rules, fees and taxes; it does not recommend an age, return, spending level or withdrawal percentage.