Coast FIRE calculator

How this Coast FIRE calculator works

This calculator models Coast FIRE as the two phases it actually is. In the setup you give your age, invested balance, target retirement age, and either your expected annual spending (multiplied by 25, the 4%-rule inversion) or a target number of your own. Then the plan runs in sequence: Phase 1, the contribution phase — you keep saving while compounding builds a base; then Phase 2, the coast phase — zero contributions, the balance simply compounds to your retirement age. Each phase takes its own real return, so you can model de-risking to a more conservative allocation during the coast years. The retirement outcome — where the ending balance lands in today's dollars and what it supports at a 4% withdrawal — sits below the chart.

There are two modes. Solve for coast age (the default) answers the headline question: given your savings rate, return, and target retirement age, what's the earliest age you can stop contributing and still reach the target on compounding alone? It fills in the contribution-phase length and marks that coast point directly on the phase timeline. Plan the phases flips it around: you set how long you contribute, and the calculator tells you whether that plan hits the target — and how early you could have stopped. The timeline strip shows the contribute-then-coast split at a glance; the chart overlays your plan (yellow) against coasting from today with no further saving (blue), with the dashed target and the orange coast-age marker.

Because the returns are real (after inflation), every figure is already in today's purchasing power. The retirement summary below the chart shows what the balance sustainably supports at 4% a year — but for the actual drawdown question (does the money last, and does the order of returns sink it?), Coast FIRE math is deterministic and can't see sequence risk. That's a different job: run your retirement balance through the Retirement Phases calculator, which tests withdrawals against real historical market sequences.

What Coast FIRE actually means

Coast FIRE is the least extreme member of the FIRE (Financial Independence, Retire Early) family, and arguably the most useful for ordinary savers. Full FIRE demands a portfolio big enough to live on now — 25 times your annual spending, often seven figures. Coast FIRE asks a humbler question: do you have enough invested that compounding alone will finish the job by a normal retirement age? Because compounding does the heavy lifting over multi-decade horizons, the answer arrives far sooner than most people expect. A 30-year-old targeting $1.5 million at 65 needs roughly $272,000 today at a 5% real return — not $1.5 million.

Reaching Coast FIRE doesn't mean quitting work; you still need income for rent and groceries. What it changes is the pressure. Retirement saving becomes optional. That's the license people use to switch to meaningful-but-lower-paying work, start a business, go part-time while raising kids, or simply stop feeling behind. Every dollar you save after the coast point buys earlier retirement or a richer one — but it's a choice, not an obligation.

The math behind the coast number

The formula is compound interest run backwards — a present-value calculation:

Coast FIRE number = target ÷ (1 + r)ⁿ

where r is your expected annual return and n is the years until your target retirement age. At the defaults — a $1,500,000 target ($60,000 spend × 25), 35 years out, 5% real return — that's $1,500,000 ÷ 1.05³⁵ ≈ $272,000. The same target at age 40 requires about $443,000, and at 50 about $722,000: the coast number roughly doubles every 14 years you wait at 5%, which is why the early years of saving matter so disproportionately. To see this mechanic from the other direction — money compounding forward through different phases of life — the multi-phase compound interest calculator is the companion tool, and the dollar-cost averaging calculator shows what those steady contributions look like against real market volatility.

One assumption deserves emphasis: this calculator defaults to a real (after-inflation) return of 5%, so your target stays in today's dollars and the results mean what they appear to mean. If you use a nominal return like 7%, be consistent and inflate your target too — otherwise you're planning to fund today's lifestyle with dollars that have lost decades of purchasing power.

Coast FIRE vs. full FIRE vs. Barista FIRE

The three aren't competing philosophies so much as stops on one road. Coast FIRE is the first checkpoint: your future is funded, your present still needs a paycheck. Barista FIRE is the second: the portfolio starts sharing the load while you downshift. Full FIRE is the terminus. Knowing which checkpoint you've passed changes career decisions today — that's the entire point of computing the number.

The honest caveats

Coast FIRE math is deterministic; markets aren't. The projection assumes the same return every year, but a weak first decade — exactly when your balance is supposed to compound hardest — can leave a coasting portfolio behind schedule with no contributions flowing in to buy cheap shares. Your spending estimate decades from now is a guess, healthcare costs are the usual wildcard, and the 4% rule itself was built on 30-year retirements, not 40-year ones. None of this breaks the concept; it argues for margin. Verify your coast status annually, keep a buffer above the bare threshold before making irreversible career moves, and treat the number as a milestone, not a permission slip. This is education, not financial advice — a fee-only planner can pressure-test the assumptions against your actual life.

Frequently asked questions

This calculator is for education, not financial advice. Returns are assumptions, not guarantees — confirm your plan with a qualified advisor before making career or investment decisions.