How to Calculate Your LeanFIRE Number (And Why Most People Overshoot It)

What LeanFIRE Actually Means

LeanFIRE is early retirement on an annual budget under $40,000. Most practitioners target $25,000 to $35,000 per year. That number covers shelter, food, healthcare, and modest discretionary spending, nothing more.

This is not a lifestyle of deprivation for everyone. Geographic arbitrage, paid-off housing, and low baseline spending make $30,000 a genuinely livable annual income in many U.S. markets and most international ones.

The financial math, however, is unforgiving. Retiring at 35 or 40 means your portfolio must last 50 to 55 years. That time horizon changes the entire withdrawal rate calculus.

The Withdrawal Rate Problem Most Calculators Ignore

The original 4% rule came from the 1994 Trinity Study. It was designed for 30-year retirements. A 65-year-old retiring in 1994 was 95 by 1994's standard mortality tables. The math fit.

A 35-year-old retiring today needs money until age 85 to 90 at minimum. More likely until 95 or beyond. That is a 50 to 60-year horizon.

Historical backtests using Vanguard and Morningstar return data show the following portfolio survival rates for a 60/40 portfolio:

• 4.0% withdrawal rate over 50 years: approximately 72% success rate
• 3.5% withdrawal rate over 50 years: approximately 87% success rate
• 3.0% withdrawal rate over 50 years: approximately 95% success rate

The difference between 4% and 3.5% is not academic. It represents a real-dollar gap in your target number and a meaningfully different probability of running out of money in your 70s.

How to Calculate Your LeanFIRE Number

The formula is simple. The inputs require precision.

LeanFIRE Number = Annual Spending / Withdrawal Rate

At 3.5%: divide by 0.035, or equivalently multiply by 28.6. At 3.0%: divide by 0.030, or multiply by 33.3.

The variable that does the most damage is annual spending. Most people undercount it by 15 to 25% when they first attempt this calculation.

Step 1: Build an Honest Annual Spending Figure

Do not estimate. Pull 12 to 24 months of actual transaction data. Categorize every dollar. Include:

• Housing (rent or ownership costs, property tax, insurance, maintenance)
• Food, both groceries and restaurants
• Transportation, including registration, insurance, fuel or public transit
• Healthcare premiums, out-of-pocket costs, prescriptions
• Insurance outside of healthcare
• Annual subscriptions and recurring services
• Travel and discretionary spending
• Irregular but predictable expenses, car replacement, appliances, home repairs

A common honest figure for a single LeanFIRE practitioner in a low cost-of-living area lands between $24,000 and $32,000 per year. For a couple in a moderate cost-of-living area, $36,000 to $46,000 is realistic.

Step 2: Add a Healthcare Buffer

Pre-Medicare healthcare is the largest variable cost in early retirement. The average ACA marketplace premium for a 40-year-old in 2025 was $541 per month before subsidies, or $6,492 per year. Out-of-pocket maximums on silver plans averaged $9,100 in 2025.

In a bad health year, you spend $15,000 or more on healthcare alone. Build at least $8,000 to $12,000 annually into your LeanFIRE spending figure if you are retiring before Medicare eligibility at 65.

Step 3: Apply the Correct Multiplier

Once you have an honest annual spending number, apply the multiplier appropriate to your expected retirement duration.

Retiring at 40: use 28.6 (3.5% rate) at minimum. Conservative practitioners use 33.3 (3.0% rate). Retiring at 35: use 33.3 to 40 (3.0% to 2.5% rate). Retiring at 45: 28.6 remains defensible. Some argue 25 (4%) is acceptable, though the sequence-of-returns risk remains elevated.

Worked Example 1: Single Person, Age 37, Low Cost-of-Living Area

Rachel earns $78,000 and lives in Knoxville, Tennessee. Her tracked annual spending is $26,400. She adds $9,600 for healthcare, bringing her honest annual figure to $36,000.

She plans to retire at 37 and conservatively assumes a 50-year retirement window.

LeanFIRE Number = $36,000 / 0.035 = $1,028,571

She rounds to $1,030,000 as her target.

At her current savings rate of $24,000 per year with an existing portfolio of $340,000, and assuming a 7% real return on a 70/30 equity/bond portfolio, she reaches her target in approximately 14.3 years. That puts her retirement at age 51.3.

If Rachel increases her savings rate to $36,000 per year by cutting $12,000 in annual spending now, she reaches $1,030,000 in approximately 10.6 years. She retires at 47.6. The decision to cut $12,000 in annual spending today buys her 3.7 additional years of retirement.

Worked Example 2: Couple, Age 42, Medium Cost-of-Living Area

David and Maria live in Raleigh, North Carolina. Combined annual spending, tracked honestly, is $51,200. They add $14,400 for two people's healthcare buffer. Total annual figure: $65,600.

They plan to retire together at 42 and model a 48-year horizon.

LeanFIRE Number = $65,600 / 0.035 = $1,874,286

They round to $1,875,000.

Their current combined portfolio is $890,000. They save $58,000 per year. At 7% real return, they reach their target in approximately 11.4 years. Both retire at 53.4.

David argues for a 4% withdrawal rate, which would set their target at $1,640,000. That saves them roughly 2.8 years of working. Maria notes the 50-year failure rate difference and holds the 3.5% line. The disagreement is worth $234,000 in portfolio target and 2.8 years of working life. That is a conversation worth having with a precise calculator, not a rough estimate.

The Sequence-of-Returns Risk Is Not Theoretical

Retiring in 2000 on a 4% withdrawal rate from a 60/40 portfolio produced portfolio depletion within 28 years in historical simulations. A 35-year-old who retired in 2000 would face insolvency at 63, two years before Medicare eligibility.

The first 5 to 10 years of retirement determine most of the outcome. A 20% portfolio drawdown in year one forces either a withdrawal rate reduction or a return to income. LeanFIRE budgets leave little room for either.

Mitigation Strategies That Change the Math

Three approaches meaningfully improve survival rates without requiring a larger portfolio.

Variable withdrawal rate. In down market years, reduce spending to 80 to 90% of the planned figure. This extends portfolio life by 3 to 7 years in historical simulations.

Part-time income buffer. $8,000 to $12,000 per year in supplemental income during the first decade of retirement reduces initial withdrawal pressure significantly. At $10,000 per year supplemental income on a $36,000 spending plan, you are withdrawing only $26,000, a 2.5% rate on a $1,030,000 portfolio. That is historically near-indestructible over any 50-year window.

Geographic arbitrage. Spending $36,000 per year in the United States translates to $22,000 to $26,000 per year in Portugal, Mexico, or Thailand. Some LeanFIRE practitioners drop their effective withdrawal rate below 3% through location flexibility alone.

What Your LeanFIRE Number Does Not Include

Four items derail even well-constructed LeanFIRE plans.

Inflation variability. The standard model uses 3% annual inflation. Healthcare inflation has averaged 5.2% over the past decade. A 35-year retirement compounds that gap significantly.

Tax drag on withdrawals. Traditional 401(k) and IRA withdrawals are ordinary income. A $36,000 withdrawal in retirement is not the same as $36,000 of spending after accounting for federal and potentially state income tax.

Social Security. Most LeanFIRE models exclude it as a conservative assumption. If you work 10 or more years before retiring, your eventual Social Security benefit is real money. A 35-year-old with 10 years of work history may receive $800 to $1,200 per month at age 70 in today's dollars. That materially changes the math after age 70.

One-time large expenses. A $22,000 car replacement, a $14,000 medical event, or a $30,000 roof over a 50-year retirement is not extraordinary. Budget for irregular large expenses or hold a separate cash reserve.

Run Your Actual Numbers

The examples above use specific inputs. Your numbers are different. Your spending figure is different. Your existing portfolio balance is different. Your planned retirement age is different.

The CalcMoney Retirement Calculator accepts your exact figures: current portfolio, annual contributions, expected return, planned retirement age, and target withdrawal rate. It outputs your LeanFIRE number, your projected timeline, and Monte Carlo survival probabilities across market conditions.

Rough math gets you in the ballpark. Precise inputs get you to a defensible retirement date.

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