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The FIRE Calculator explained

The FIRE Calculator: Lean, Regular, Fat, and Coast variants in one screen, with a projection chart and every assumption you can tweak.
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Quick start

The FIRE screen is where your entire financial picture turns into a concrete target: how much you need, how close you are, and how long it'll take.

  • Four tabs at the top switch between the four FIRE variants: Lean, Regular, Fat, and Coast. Each represents a different definition of "enough".
  • The big number on each tab is your FIRE number for that variant — the portfolio size you need to retire.
  • The progress bar below shows how far you've come; the years to FI figure projects forward based on your current savings rate.
  • The projection chart plots your portfolio's growth from today until it hits your target, assuming the return + contribution figures you've entered.
  • Change the assumptions via the icon at the top of the screen — return rate, inflation, tax, withdrawal rate.

The rest of this guide is for users who want to tune the screen. If you're happy with the defaults, you can stop here — everything below is optional customisation.

Going deeper

The 25× rule

All four FIRE variants derive from the same core idea: the portfolio size that can sustainably fund your annual spending forever is roughly 25× your annual expenses (the inverse of the 4% safe-withdrawal rate from the Trinity Study). FirePath's default withdrawal rate is 4% — adjust in Settings if you prefer a more conservative 3.5% (which corresponds to roughly 28× spending).

Expenses come from your Income & Expenses screen, normalised to annual. The calculator multiplies by 25 (or your chosen multiplier) to land on your FIRE number.

Lean vs Regular vs Fat

Regular FIRE uses all your expenses: the spending level you currently enjoy, sustained indefinitely.

Lean FIRE uses only expenses you've flagged Include in Lean FIRE. The idea: you could cut back to just essentials (rent, food, insurance, basic transport) and still be free. Lean's FIRE number is usually 40-60% of Regular's.

Fat FIRE is Regular times a multiplier — default 1.5×, configurable in Settings. The idea: retire with significant headroom for travel, upgrades, generosity. Fat is for people who want a safety margin beyond the 25× baseline.

Coast FIRE: the most useful milestone

Coast FIRE tab with retirement-age and projection
① Coast target. ② "You can stop saving at age" callout.

Coast FIRE is the point at which your existing investments will grow to a full FIRE target by your retirement age with zero further contributions. In other words: you've saved enough that you can stop contributing and just let compounding carry you home.

Mathematically it's the present value of your target at your retirement age, discounted at your expected real return. Example: Regular FIRE number $1.5M, retirement age 60, you're 35, real return 5%. Coast FIRE = $1.5M / (1.05 ^ 25) = ~$443k. Hit $443k invested and you can "coast" to $1.5M by 60.

Psychologically it's the biggest milestone most FIRE-chasers hit. Many people reach Coast FIRE in their 30s, then optionally reduce hours, switch to meaningful work, or keep saving to reach full FIRE earlier.

The Coast tab shows your progress to the Coast number. When you cross it, the tab reveals the "you can stop contributing" message — a celebratory moment many users screenshot.

Years to FI

The "years to FI" figure under each variant is a projection: how long until your current Investment NW plus your current savings rate compound up to the target. Uses your real-return assumption and subtracts projected inflation so the number is in today's dollars.

It's a single-point estimate, not a probability distribution — markets don't deliver the average every year. But it's a useful directional signal: cutting 4 years off your FI date by increasing savings by $200/month is a concrete motivator.

The projection chart

FIRE projection chart with years-to-FI
① Portfolio growth curve. ② FIRE target line. ③ Years-to-FI at crossover point.

Plots two series: your portfolio's projected growth from today to your FIRE target (solid line), and your FIRE target (horizontal dashed line). The point where the lines cross is your projected FI date.

The chart respects the FIRE uses Total NW toggle — flip it on if you want the projection to include your entire net worth rather than just investment-flagged assets.

FIRE uses Total NW: when to flip it on

Default is off (Investment NW only) because for most users, the primary residence and car aren't funding retirement. But some users plan to downsize, sell the house, or liquidate other personal-use assets as part of their retirement plan. If that's you, flip the setting on and the projection runs against your full net worth.

You can flip it back and forth to see both projections — the only thing that changes is which column from your snapshots is fed into the maths.

Make it yours — Settings that affect this screen

  • Default FIRE Type — Settings → Display. Which variant loads when you open the screen.
  • Show Assumptions on FIRE — Settings → Display. Keeps a summary of your return/inflation/tax assumptions visible at the top of the screen for reality-check at-a-glance.
  • Show FIRE Numbers on Coast — Settings → Display. Toggles the numeric reveal on the Coast tab (some users find it motivating, others find it distracting).
  • FIRE uses Total Net Worth — Settings → Display. Flip to run the projection against Total NW instead of Investment NW. Off by default because Investment NW is what actually funds retirement.
  • FIRE Assumptions — Settings → FIRE. The full set of knobs: expected real return, inflation, effective tax rate, safe-withdrawal rate, retirement age for Coast FIRE.
  • Lean FIRE Included Expenses — per-expense toggle on the Add/Edit Expense screen. Controls which expenses count toward the Lean variant's target.

Questions or feedback? Email us at woohoosoftware@gmail.com