Wealth & Wellness

A 5-minute read on why most “retire by 40” advice quietly assumes you’ll keep spending what you spend now — and the rule that fixes that.

The advice you hear most about early retirement starts with income. Earn more. Hustle. Side gig. Negotiate harder. All of which is fine. None of which is the actual math.

The math comes from a 1998 paper out of Trinity University (Cooley, Hubbard, and Walz) that asked a simple question: if you retire and pull a fixed percentage of your portfolio every year, what’s the highest rate that still leaves money in the account thirty years later? The answer that became famous is roughly 4%. Withdraw 4% of a balanced stock-and-bond portfolio every year, adjusted for inflation, and historically you almost never run out.

Mr. Money Mustache, the financial blogger who turned this into a movement, took the math one step further. If 4% is the safe withdrawal rate, then your retirement number is 25 times your annual spending. Not 25 times your income. Twenty-five times what you actually spend. And the years to get there depend on one number: your savings rate, defined as the percent of your take-home income you don’t spend.

The table that came out of his 2012 essay “The Shockingly Simple Math Behind Early Retirement” is brutal once you see it. Save 10% of your take-home and it takes about 51 years. Save 25% and it’s 32 years. Save 50% and it’s 17 years. Save 75% and it’s 7 years. The number that matters isn’t your salary. It’s the gap between what you earn and what you spend.

Most “retire by 40” advice has it backwards because it lives entirely on the income side. Earn more, but keep spending the same, and you’ve added a year to the timeline for every year of lifestyle creep. Spend less and you compound the savings twice — you save more now, and you need less to retire on later.

One small action this week. Calculate one number: your actual savings rate over the last three months. Take-home pay minus total spending, divided by take-home pay. Don’t try to fix it yet. Just look at it. Most people’s first reaction is “lower than I thought.” That’s the start.

P.S. — Next Sunday: the 24-hour rule for big purchases, and the lighter version that works on Tuesday-night Amazon orders.