About
Lean FIRE: aggressive frugality + early retirement. Annual spending typically under $40K. Common $1M target ($40K × 25). Requires careful planning, often LCOL area, may include gardening, side hobbies for income.
The 4% safe withdrawal rate underpinning the "25× annual expenses" Lean FIRE target comes from the Trinity Study (Cooley, Hubbard, Walz, 1998), which back-tested 30-year retirements 1926-1995 and found a 95% success rate at 4% with a 50/50 stock-bond portfolio. Bengen's original 1994 paper actually concluded with 4.5% using slightly different assumptions. Lean FIRE practitioners who retire in their 30s or 40s face a 50-60 year horizon, not 30, which historically drops the safe rate to ~3.3-3.5% (Kitces' rolling-30-year and Pfau's 60-year extensions both confirm). The standard 2026 Lean FIRE rebuttal is that 4% is still defensible because (a) most early retirees re-employ at least part-time, and (b) flexible withdrawal rules (Guyton-Klinger guardrails, dynamic CAPE-based variable spending) raise sustainable rates back to ~4% even for 50-year retirements.
Formula
Use AFTER-tax expenses, not pre-tax. A Lean FIRE retiree spending $35K/year typically falls within the 2026 federal 0% long-term capital-gains bracket (single MAGI under $48,475 / married $96,950), meaning brokerage withdrawals carry near-zero federal tax. Roth conversion ladders done before official FIRE date can move 401(k)/traditional-IRA funds to Roth at low marginal rates for tax-free later access. Healthcare is the gotcha: pre-Medicare ACA marketplace premiums for a $40K MAGI couple in 2026 net about $80-200/month after subsidies, but a 2025 enhanced-subsidy expiration could double that. Build a 15-25% healthcare buffer above your nominal expense number.
Lean FIRE pitfalls
- Sequence-of-returns risk in early years: A 30% drawdown in years 1-3 of retirement has 4-5x the failure impact of the same drawdown in years 25-27. Many planners now recommend keeping 2-3 years of expenses in cash/short-term bonds at the start (a "cash floor" or bond tent that converts to equities over the first 10 years).
- Geographic lock-in: Lean FIRE often relies on LCOL areas (rural Tennessee, Portugal, Mexico). Moving back to HCOL for elderly parents or grandchildren can collapse the math overnight. Budget for "lifestyle creep insurance" via Fat FIRE buffer ($300-500K above lean target).
Frequently asked questions
Should I use 4% or 3.5% as the safe withdrawal rate?
For a traditional 30-year retirement starting at 65, 4% remains defensible per the Trinity Study. For Lean FIRE with a 50-60 year horizon, 3.25% to 3.5% is the more conservative figure (Pfau, 2014 and Kitces ongoing). The practical compromise most lean FIRE practitioners use: model with both numbers and check the gap. A $35K/year spender targets $875K at 4%, $1.075M at 3.25%; that $200K is your "sequence-of-returns insurance" buffer. Many also adopt a "guardrails" rule: spend 4% in good years, drop to 3.25% (or accept temp part-time work) in years where portfolio falls 25%+ below starting value.
How do I handle healthcare before Medicare in Lean FIRE?
Three main paths in 2026. (1) ACA marketplace with controlled MAGI: keep dividends and Roth conversions tuned so MAGI stays in the 200-400% FPL band (about $30K-$60K single, $40K-$80K MFJ) for maximum premium tax credit. (2) Direct primary care + catastrophic-only plan: DPC memberships ($60-150/month) cover routine visits, paired with a high-deductible bronze plan. (3) Health-sharing ministries: not insurance, no ACA protections, but $200-500/month all-in for healthy applicants under 50. Whatever the route, budget $7K-$15K/year per adult for healthcare in your Lean FIRE expense number.
How accurate is the Lean FIRE Calculator?
It applies the standard formula. Accuracy is limited only by your input precision. For decisions with material consequences (taxes, medical, legal, structural), use the result as a starting point and verify with a qualified professional in the relevant field.
Is the Lean FIRE Calculator free to use?
Yes. 100% free, no signup, no payment, no API key. The site is funded by display ads around the tool but not inside the calculation flow.
Are my inputs saved anywhere?
No. All inputs stay in your browser tab. Closing the tab discards them. The site uses Google Analytics for traffic measurement (anonymized) but the analytics never see what you type into the form.
Can I use the Lean FIRE Calculator on my phone?
Yes. The tool is responsive and tested on iOS Safari, Android Chrome, and major desktop browsers. Touch targets meet Apple's 44pt and Google's 48dp minimum.
Does the Lean FIRE Calculator work offline?
Yes. Once the page has loaded, it works without internet. The calculation runs in JavaScript on your device.
How do I report a bug or suggest improvement to the Lean FIRE Calculator?
Email hi@3tej.com with the URL of this page and a description of what you saw vs expected. We typically respond within 72 hours.
Can I share results from the Lean FIRE Calculator?
Take a screenshot or copy the output. The page doesn't generate shareable URLs for specific calculations - inputs stay in your browser only.
Why are the results different from another lean fire tool?
Most likely: different formula assumptions, different default values, different rounding rules, or different applicable rates. Check the methodology if both tools document it. Both can be valid for different scenarios.
Is the Lean FIRE Calculator accurate?
The Lean FIRE Calculator applies the standard formula for lean fire. Accuracy is limited only by your input precision. For decisions with material consequences, use the result as a starting point and verify with a qualified professional or the relevant official source.
Is the Lean FIRE Calculator free?
Yes. 100% free, no signup, no payment, no API key. The site is funded by display ads that appear around the tool but not inside the calculation flow.
Are my inputs saved?
No. Inputs stay in your browser tab. Closing the tab discards them. The site uses Google Analytics for traffic measurement (anonymized) but does not see what you type into the form.
How to use the Lean FIRE Calculator
The Lean FIRE Calculator is a browser-based tool that runs entirely on your device. Inputs you enter never reach a server - all calculations happen client-side in JavaScript. This means:
- Privacy: nothing is logged, sent, or stored by 3Tej. Inputs disappear when you close the tab.
- Speed: results update as you type. No network round trip.
- Offline use: once the page is cached, it works without internet.
- No signup: no account, no email, no rate limits.
Step by step
- Enter your inputs in the form above. Each field is labeled with its unit (currency, percent, kg, etc.) and the expected range.
- Read the result as it updates. The number reflects the formula commonly accepted in Lean FIRE-related calculations.
- Adjust to see sensitivity: change one input at a time and watch how the output moves. This is the fastest way to understand which variable matters most.
- Copy or screenshot the result for later reference. The page state persists for the session if your browser allows it.
When you would use this
- Quick estimates: when you need a number now and don't want to open a spreadsheet.
- Sensitivity analysis: testing how a result changes as inputs vary, before committing to a real-world decision.
- Comparison: running the same calculation with different inputs to compare options side by side.
- Learning: building intuition for how the underlying math behaves.
- Documentation: capturing a snapshot of inputs and outputs at a point in time.