Guides
The arithmetic behind the number.
The calculator gives you a figure. These guides explain where it comes from, which assumptions it is most sensitive to, and what the standard FIRE advice gets wrong when your retirement is forty years long instead of thirty. Every worked example uses the same engine the calculator runs on.
Core method · 11 min
How to Calculate Your FIRE Number, Step by Step
The 25× shortcut is a 30-year approximation dressed up as a law. Here is the arithmetic that actually sizes an early retirement — horizon, real returns, buffer and goals — worked through end to end.
Read the guide →Withdrawal · 10 min
The 4% Rule, and Where It Breaks for Early Retirees
Where the 4% rule came from, what the Trinity Study actually tested, the five assumptions baked into it, and why a 40-year retirement needs a different answer than the 30-year one it was built for.
Read the guide →Risk · 9 min
Sequence of Returns Risk: Why the First Decade Decides Everything
Two retirees, identical average returns, opposite outcomes: one dies broke at 70, the other leaves $3 million. Why the order of returns matters only after you stop earning — and the five defences that actually work.
Read the guide →Milestones · 8 min
Coast FIRE: When You Can Stop Contributing
Coast FIRE is the point where your existing portfolio, left completely alone, grows into your full retirement target on its own. Here is how to calculate it, why the real return assumption decides everything, and what it actually buys you.
Read the guide →Accumulation · 9 min
Savings Rate vs Investment Return: Which Actually Gets You There
Your savings rate sets your timeline; your return only decides how much of the work compounding does for you. The arithmetic, a full savings-rate-to-years table, and where each lever stops helping.
Read the guide →Choosing a target · 8 min
Lean, Chubby and Fat FIRE: Picking the Target You Actually Want
The five points on the FIRE spectrum are just different spending assumptions — but the double effect of spending on both savings and target means the gap between them is far larger than it looks. The full comparison, with years attached.
Read the guide →Milestones · 8 min
Barista FIRE: The Math Behind Part-Time Early Retirement
Covering half your expenses with part-time work cuts the corpus you need at 45 by nearly 40% — and defends you against the one risk that breaks early retirements. The full calculation, plus what the model leaves out.
Read the guide →Assumptions · 8 min
How Inflation Quietly Rewrites Your FIRE Number
One point of inflation moves a 40-year target by $266,000. Why real returns divide rather than subtract, why comparing a future balance to a today's-money target is the most common DIY mistake, and how to model it properly.
Read the guide →Withdrawal · 10 min
Five Withdrawal Strategies for After You Retire Early
Reaching your number is the accumulation problem. Turning it into forty years of income is a different one. Fixed real, percentage, guardrails, floor-and-upside and bucket strategies — how each behaves, and who each suits.
Read the guide →Start with the tools
The guides are most useful alongside your own figures. Thecalculator runs entirely in your browser, themethodology pagedocuments every formula it uses, and theglossarydefines the terms these guides assume.