The math behind the headlines.
Long-form posts on FIRE math, compounding, and the back-of-the-envelope reasoning that turns money decisions into something you can verify. No motivational posters.
- No. 20 fire
What is the average FIRE number?
There's no single average FIRE number — it's set by your spending. Most US households land between $1M and $2.5M, because typical retirement spending of $40k–$100k a year is 25× that range at a 4% withdrawal rate.
Read the post → - No. 19 fire
Coast FIRE number by age (with table)
Your Coast FIRE number is your full FIRE number discounted back to today by compound growth. A table of Coast FIRE targets at ages 25–45 for a $1M goal, the formula, and a live calculator.
Read the post → - No. 18 fire
FIRE number for a $50k or $60k salary
Your FIRE number is driven by what you spend, not what you earn. On a $50k–$60k salary that usually means a $1M–$1.5M target at 4%. The math worked through, plus the $100k case, with a live calculator.
Read the post → - No. 17 fire
Is $2 million enough to retire?
At a 4% withdrawal rate, $2 million supports about $80,000 a year of inflation-adjusted spending. For a 50-year early retirement at 3.25–3.5%, closer to $65,000–$70,000. The math, the caveats, and a calculator.
Read the post → - No. 16 fire
The FIRE formula (and the 25x rule)
The FIRE formula is annual expenses ÷ safe withdrawal rate — at 4%, that's 25× expenses. The equation explained, the Coast, Lean, and Barista variants, the time-to-FIRE formula, and a worked example.
Read the post → - No. 15 fire
What is a FIRE number?
A FIRE number is the portfolio that covers your annual expenses indefinitely at a safe withdrawal rate — annual expenses ÷ 4%, or 25× expenses. What it means, a worked example, and how it differs from net worth.
Read the post → - No. 14 investing
Compound interest: the most misunderstood formula in finance
The formula everyone has heard and almost nobody runs the numbers on. What compound interest actually does, where the curve bends, the Rule of 72, and why frequency matters less than time. With a live calculator inline.
Read the post → - No. 13 fire
Sequence-of-returns risk: why the order of bad years matters more than the average
Two retirees with the same average return can have completely different outcomes. Why early bad years break the 4% rule, where the risk window lives, and how to defend against it.
Read the post → - No. 12 fire
What is the 4% rule, exactly? Bengen, Trinity, and the real math
The 4% rule isn't a guarantee — it's a 30-year survival rate. Where the number came from, what assumptions it makes, and the cases where you should use 3.5% or 4.5% instead.
Read the post → - No. 11 debt
Debt snowball vs avalanche: which actually pays off faster?
The math says avalanche; the behavioral data says snowball. Both are right — and the gap between the two methods is much smaller than either camp claims. With a live side-by-side calculator inline.
Read the post → - No. 10 ebay
eBay profit margin: the fees most resellers miss
13.6% Final Value Fee + a per-order fee is the headline. Add shipping, packaging, and the per-item time cost, and a $50 sale nets less than half what most sellers think. The math, with a live calculator inline.
Read the post → - No. 09 freelance
How to Calculate a Freelance Hourly Rate
Salary ÷ 2,080 is the wrong start for a freelance rate. The honest math works backward through tax, business expenses, and unbookable hours — usually 2× the naive number. With a live calculator inline.
Read the post → - No. 08 lifetime-cost
Lifetime cost of a kid or pet: the headline numbers and what they hide
USDA says ~$310k to raise a child. ASPCA says ~$20k for a dog. Both numbers are true; both miss the bigger picture. The math, what it leaves out, and a live calculator with presets.
Read the post → - No. 07 mortgage
Mortgage payments explained: PITI, PMI, and the math nobody teaches
A mortgage payment is PITI — Principal, Interest, Taxes, Insurance — plus PMI when your down payment is under 20%. The amortization formula, when PMI ends, and the costs the math leaves out.
Read the post → - No. 06 net-worth
Net worth: the only financial number that really matters
Salary measures flow. Net worth measures the only thing that actually compounds. The formula, what counts as an asset, what to exclude, and how to track it. With a live calculator inline.
Read the post → - No. 05 fire
Savings rate: the one number that decides your time to FIRE
Years to financial independence depend almost entirely on savings rate — not income, not stock-picking, not asset allocation. The math, the assumptions it hides, and what to do with it. With a live calculator inline.
Read the post → - No. 04 student-loans
Student loans: pay them off, or wait for IDR forgiveness?
Standard, Extended, or IDR with 20-year forgiveness — the same balance can cost $20k or $130k depending on plan. The math, the political risk, the breakeven. With a live three-plan comparison inline.
Read the post → - No. 03 car
The true cost of owning a car: what your $30,000 vehicle actually costs over 10 years
Sticker price is the small half of the answer. Depreciation, fuel, insurance, maintenance, and the foregone investment returns — combined, they typically double the headline. The math, with a live calculator inline.
Read the post → - No. 02 wages
True hourly wage: what your job actually pays
Your salary ÷ 2,080 is a fiction. The honest hourly wage nets out taxes, commute, and job expenses — and is usually 30-50% below the headline number. With a live calculator inline.
Read the post → - No. 01 fire
How to Calculate Your FIRE Number (2026 Guide)
Your FIRE number is annual expenses ÷ safe withdrawal rate. At 4%, that's 25× expenses. The full math, the assumptions, the four FIRE flavors, and worked examples — with live calculators inline.
Read the post →