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. 36 app-revenue
ARPDAU explained: the math behind mobile app and game revenue
ARPDAU is your net daily revenue divided by daily active users. Here's the formula for both streams — ad impressions × eCPM and in-app purchases after the store cut — with a worked example.
Read → - No. 35 fire
What is Lean FIRE? The Number, the Lifestyle, and the Risks
Lean FIRE is early retirement on a deliberately small budget — usually $25–40k a year, a $625k–$1M portfolio at 4%. The math, the targets table, and the honest risks.
Read → - No. 34 depop
Depop fees explained: what Depop takes per sale
Depop dropped its US seller fee in 2024. The only cut is 3.3% + $0.45 payment processing — about $2.27 on a $55 order, or roughly 4%, as of June 2026.
Read → - No. 33 investing
Dividend reinvestment (DRIP) and yield on cost, explained
How reinvesting dividends compounds a portfolio, why yield on cost rises over time, and the math behind both — with a live DRIP calculator to run your own numbers.
Read → - No. 32 vinted
Vinted fees explained: why sellers keep 100%
Vinted charges sellers $0 — no commission, no listing fee. The buyer pays a ~5% + $0.70 protection fee instead. What you keep, and what a fee costs elsewhere.
Read → - No. 31 fire
Coast FIRE vs Barista FIRE: what's the difference?
Coast FIRE: compounding finishes your retirement fund while you pay your own bills. Barista FIRE: the portfolio pays part of them now. The math, side by side.
Read → - No. 30 etsy
Etsy fees explained: what Etsy takes per sale
Etsy takes about 11% of a typical order: $0.20 listing, 6.5% transaction, and 3% + $0.25 processing — $3.30 on a $25 sale with $5 shipping, as of June 2026.
Read → - No. 29 fire
FIRE number vs net worth: what's the difference?
A FIRE number is a target; net worth is a snapshot of what you own. Track FIRE progress with invested assets only — home equity can't fund a 4% withdrawal.
Read → - No. 28 fire
How much do you need to retire at 55?
Retiring at 55 means funding roughly 40 years, so plan on a 3.25–3.5% withdrawal rate — about 28–31× annual spending. $60,000 a year needs about $1.7M–$1.85M.
Read → - No. 27 taxes
Marginal vs Effective Tax Rate: 2026 Brackets Explained
Your marginal rate hits only the dollars above each bracket line; your effective rate is total tax divided by income. Why a raise cannot cut your take-home pay.
Read → - No. 26 mercari
Mercari fees explained: how much sellers actually keep
Mercari charges sellers a flat 10% of item price plus buyer-paid shipping, effective Jan 6, 2025 — no per-order fee. The payout math, with a live calculator.
Read → - No. 25 debt
Negative compound interest: debt, fees, and losses
Negative compound interest is compounding with the sign flipped: value × (1 − r)^n. How card debt snowballs and a 1% fee eats a quarter of a 30-year balance.
Read → - No. 24 poshmark
Poshmark fees explained: flat $2.95 vs the 20% cut
Poshmark takes a flat $2.95 on orders under $15 and 20% on orders of $15 or more, as of June 2026. The $15 cliff, offer math, and a live payout calculator.
Read → - No. 23 investing
ROI vs CAGR: How to Measure Investment Returns
ROI is total gain: end ÷ start − 1. CAGR converts it to a steady annual rate: (end ÷ start)^(1/years) − 1. Why CAGR is the honest number, with worked examples.
Read → - No. 22 fire
What is Fat FIRE? Definition, Numbers, and Who It Fits
Fat FIRE is early retirement on an above-average budget, commonly $100,000+ a year — a $2.5M+ portfolio at 4%. The math, targets, and Fat vs Standard vs Lean.
Read → - No. 21 inflation
What will your money be worth in 20 years? Inflation math
At 3% inflation, $100,000 buys in 20 years what $55,368 buys today — a 44.6% loss of purchasing power. The formula, a 10–40 year table, and a calculator.
Read → - 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 → - 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 → - 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 → - No. 17 fire
Is $2 Million Enough to Retire?
Yes, for most households: at a 4% withdrawal rate, $2 million supports about $80,000 a year. How long $2M lasts, retiring at 50 vs 60, and the math for couples.
Read → - 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 → - 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 → - 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 → - 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 → - 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 → - 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 → - No. 10 ebay
eBay Profit Margin: The Fees Most Resellers Miss (2026)
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 → - 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 → - 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 → - 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 → - 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 → - 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 → - 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 → - 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 → - 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 → - No. 01 fire
How to Calculate Your FIRE Number (2026 Guide)
Your FIRE number is annual expenses ÷ safe withdrawal rate — 25× expenses at 4%. The formula, worked examples, the four FIRE variants, and live calculators.
Read →