Lean FIRE Calculator Formula

Understand the math behind the lean fire calculator. Each variable explained with a worked example.

Use the Lean FIRE Calculator

Formulas Used

Lean FIRE Number

lean_fire_number = lean_budget / (withdrawal_rate / 100)

Progress

progress = current_savings / (lean_budget / (withdrawal_rate / 100)) * 100

Remaining to Save

remaining = max(lean_budget / (withdrawal_rate / 100) - current_savings, 0)

Variables

VariableDescriptionDefault
lean_budgetLean Annual Budget(USD)30000
withdrawal_rateWithdrawal Rate(%)4
current_savingsCurrent Savings(USD)150000

How It Works

Lean FIRE

Lean FIRE Number = Lean Budget / Withdrawal Rate

Lean FIRE is achieving financial independence on a below-average budget (typically under $40,000/year for an individual).

Lean FIRE Examples

BudgetFIRE Number (4%) $20,000$500,000 $25,000$625,000 $30,000$750,000 $35,000$875,000

Worked Example

$30,000 lean budget, 4% withdrawal, $150,000 saved.

lean_budget = 30000withdrawal_rate = 4current_savings = 150000
  1. 01Lean FIRE number = $30,000 / 0.04 = $750,000
  2. 02Progress = $150,000 / $750,000 = 20%
  3. 03Remaining = $600,000

Frequently Asked Questions

What is Lean FIRE?

Lean FIRE means achieving financial independence with a frugal lifestyle budget, typically under $40,000/year. It requires less savings but more intentional spending.

Is Lean FIRE sustainable?

It can be, but leaves less room for unexpected expenses. Having a buffer or the ability to earn some income provides important safety margin.

How can I reduce my budget for Lean FIRE?

Housing is usually the biggest lever. Geographic arbitrage, house hacking, or living in a low-cost area can dramatically reduce expenses.

Ready to run the numbers?

Open Lean FIRE Calculator