TLDR
- Lean FIRE means reaching financial independence on a small budget, often around $40,000 a year or less.
- At a 4% withdrawal rate, $36,000 a year needs $900,000 invested. $48,000 a year needs $1.2 million.
- Cutting spending works twice: it lowers the number you need and raises what you save. In one example, cutting $1,000 a month takes the date from about 30 years to about 19.
- Cutting has a floor and leaves little margin. Earning more has no floor, and the two together move the date furthest.
Lean FIRE gets less respect than it deserves. The maths behind it is the strongest single lever in personal finance, and I’ll show you why.
Then I’ll show you where it runs out.
The Lean FIRE number
Your number is your yearly spending divided by your withdrawal rate. At 4%, that’s spending times 25.
| Monthly spending | Yearly | Number at 4% | Number at 3.5% |
|---|---|---|---|
| $3,000 | $36,000 | $900,000 | About $1.03 million |
| $4,000 | $48,000 | $1.2 million | About $1.37 million |
Why cutting is so powerful
Take someone spending $4,000 a month and saving $1,500, at 5% a year after inflation, starting from zero. Their number is $1.2 million, and it takes about 30 years.
| Change | Number | Saved per month | Years |
|---|---|---|---|
| Nothing | $1.2 million | $1,500 | About 30 |
| Earn $1,000 more, save it | $1.2 million | $2,500 | About 22 |
| Spend $1,000 less | $900,000 | $2,500 | About 19 |
| Earn $3,000 more, save it | $1.2 million | $4,500 | About 15 |
| Spend $1,000 less and earn $1,000 more | $900,000 | $3,500 | About 15 |
Dollar for dollar, cutting beats earning, because every dollar you stop spending lowers the number and adds to the pot at the same time. That’s the honest case for Lean FIRE, and it’s a good one.
Where it runs out
Cutting has a floor. Rent, food, a child, a parent who needs help: there’s a number below which your life stops working. Most people find the easy cuts in the first year. After that, each extra dollar costs more.
A lean budget also leaves no margin. A medical bill or a bad market year has nowhere to land except your pot.
Earning has no floor in the same way. The fourth row, earning $3,000 more, gets as far as cutting and earning together, without living any smaller. And unlike spending, earnings can keep rising.
Cut hard on what doesn’t matter or doesn’t move you closer to your goal. When there’s nothing left to cut, earn more.
What I did
Our savings rate went past 50%. But the move that changed our date most wasn’t a cut. My first salary was $1,700 a month, and selling the same skill directly took it to $4,800. No budget gets you that far.
If you want the earning side in more detail, here’s what an extra $1,000 a month actually does to your date, and here’s the version where part-time income covers part of your spending.
Run your own
Open the calculator and put in your real spending. Then lower it, and watch the date. Then put it back and pick “I don’t have a business yet” to see what a second income does instead. Every assumption is on the page.
Illustrative numbers, not advice. I’m not a licensed adviser; I show the maths and what I did, and you run yours.