Savings rate is the whole game
The reason FIRE arithmetic feels surprising is that a savings rate does two things at once. Saving half your take-home pay builds the portfolio twice as fast as saving a quarter, and it also means you live on half as much — so the portfolio you need is smaller. Both effects push in the same direction, which is why the time to independence collapses so quickly as the rate rises.
It also explains why absolute income matters less than people expect. Someone earning $80,000 and saving 50% reaches independence sooner than someone earning $200,000 and saving 15%, because the second person has built a much more expensive life to fund.
Working out your number
Annual spending divided by your chosen withdrawal rate. At 4% that is 25 times spending; at 3.5%, roughly 29 times; at 3%, about 33 times. The choice of rate matters more here than anywhere else in the plan, because it is being applied to a much longer horizon than the research it came from.
Two adjustments worth making. First, use spending rather than income, and use the spending you expect in retirement rather than today's — for many people it is genuinely lower once commuting, saving and mortgage payments come out. Second, remember the number is pre-tax. Killion's projections are pre-tax and say so, but your withdrawals will not be.
The safe withdrawal rate calculator converts between the two directions, and the FIRE glossary entry covers the lean, fat, barista and coast variants.
Coast FIRE and the other milestones
Coast FIRE is the point at which what you already have invested would grow to your target on its own, with no further contributions, by your retirement age. It is the most practically useful milestone on the way, because it converts retirement saving from an open-ended obligation into a solved problem.
That matters even if you never intend to slow down. Reaching Coast FIRE makes a lower-paid job, a sabbatical or a career change financially survivable rather than reckless, which is a bigger change to your life than the eventual retirement date. The Coast FIRE calculator runs the number, and the result is highly sensitive to the real return you assume — which is the honest argument for simulating a range rather than trusting one figure.
Why a long horizon changes the rules
Almost every rule of thumb in retirement planning was derived from 30-year windows, because that is roughly what a retirement at 65 looks like. Retiring at 45 can mean fifty years, and three things get harder in that stretch.
- The sustainable withdrawal rate falls. More years means more chances to meet a bad sequence, and less time for a portfolio to recover from one.
- Inflation compounds further. At 3%, prices roughly double over 24 years and quadruple over 48. Any fixed nominal income is worth a fraction of itself by the end.
- Flexibility becomes more valuable than precision. Over fifty years you will almost certainly earn something, spend differently than planned, or adjust. A model that assumes rigid spending for five decades is modelling someone who does not exist.
The practical conclusion is not to pick a lower rate and stop thinking. It is to test the horizon you actually have, with the spending flexibility you actually have.
The risks during accumulation
Most FIRE content is written about the withdrawal phase, and most people reading it are accumulating. The risks are genuinely different in that phase.
A crash while you are saving is not a threat — it is a discount, because contributions keep buying at lower prices. The threat during accumulation is behavioural: stopping. Forty years of S&P 500 data shows what missing the best months costs, and the best months cluster inside the worst periods, which is exactly when the urge to step aside is strongest. The full analysis is in timing the market vs staying invested.
The other accumulation-phase risk is fees, which do their worst damage over long horizons — precisely the horizons FIRE plans have. See what a 1% fee costs over 40 years.
How to actually test the plan
A spreadsheet compounding one assumed return will tell you a FIRE date. It will not tell you how confident to be in it, and the confidence is the part that matters when you are deciding whether to leave a career.
- Run the plan over your real horizon, not a 30-year default.
- Read the worst decile before the median. That is the world you would have to live in.
- Test a market shock in the first years of retirement. That is where sequence risk concentrates.
- Compare a rigid withdrawal rule against a flexible one and see what the flexibility buys.
- Backtest it against every start year since 1928, including 1929 and 1966.
The live demo does all of this on a fictional household with no account, which is the fastest way to see whether the approach is worth applying to your own numbers.
Is the 4% Rule Still Safe? We Ran 5,000 Simulated Retirements
Is the 4% rule still safe? Flexible 4% lasted in 100% of 5,000 simulations; rigid 4% failed 1.1%. Full safe withdrawal rate trade-off table.
Sequence of Returns Risk: What If the Market Crashes the Year You Retire?
Sequence of returns risk explained: the same 40% crash dropped success from 86% to 71% at retirement age 65, but only to 81% at age 48. 5,000 simulations.
Timing the Market vs Staying Invested: 40 Years of S&P 500 Evidence
Timing the market vs staying invested: miss the 10 best months since 1985 and ~$744K becomes ~$264K. Forty years of S&P 500 data on why time in the market wins.