The CalcNest Journal
Long-form guides to the math behind major financial, health, and tech decisions. No fluff, no filler — just the formulas and frameworks that matter.
Mortgage Refinance Math: When Refinancing Actually Saves You Money
The RECAP framework: recovery period, equity position, the cost of the term reset, adjusted horizon, and payment vs payoff goal. Why the "1% rule" of refinancing is a trap, with a worked example where the rate cut actually loses.
Read article →What BMI Gets Wrong: A Smarter Way to Read Your Body's Numbers
The BODY-5 framework: baseline BMI, waist distribution, body-fat density, year-over-year trend, and five vital blood markers. Why BMI overstates risk for the muscular and understates it for the "skinny-fat" — and how to read your body honestly.
Read article →How Much Should You Actually Feed Your Dog? The Science Behind Portions
The PORTION framework: built on veterinary RER and MER energy science instead of the bag's weight brackets. Why two same-weight dogs need different amounts, with a full worked calorie example and the body-condition reality check.
Read article →Standard Deviation, Finally Explained: Why "Average" Hides So Much
The SPREAD framework: shape, position of the mean, range in deviations, extremes, apples-to-apples comparison, and decision. Why a lone average is untrustworthy, explained through a two-town income riddle anyone can follow.
Read article →The True Lifetime Cost of Smoking: $1.7M When You Count Everything
An original compound-cost model integrating cigarette cash, lost investment returns, insurance premium differentials, healthcare costs, and reduced earning years. The standard $200K figure undercounts by 9×.
Read analysis →AI API Cost Per Million Tokens: The Definitive 2026 Comparison
23 frontier and mid-tier models from 7 providers, with current pricing, cache discounts, batch pricing, and 5 real production cost scenarios. The cheapest-to-most-expensive frontier model spread is 167×.
Read comparison →The 4% Rule Is Outdated: What FIRE Math Actually Says
Bengen's 4% rule was built for 30-year retirement under 1990s market conditions. Research synthesis (Pfau, Kitces, Trinity) supports 2.8-3.3% for early retirees — meaning 30-40× annual spending, not 25×.
Read analysis →How Much Should I Save for Retirement? The Complete Math-Driven Guide
The Five-Layer Retirement Math (5LRM) framework: annual income need, withdrawal rate, three income sources, monthly savings calculation, and the five risk buffers most plans skip. A defensible number, not a generic benchmark.
Read article →Real Estate Investment Math: The Five-Formula Screening Framework
The Five-Formula Real Estate Screening Framework: 1% rule, 50% rule, cap rate, cash-on-cash, and the 70% rule applied through one worked example. From "looks good on Zillow" to "produces 12% cash-on-cash with margin of safety."
Read article →AI API Costs Explained: Token Pricing, Patterns, and Cost Engineering
The Cost-Per-Useful-Output (CPUO) framework: tokens, input/output asymmetry, four usage patterns, six optimization levers, and the self-host crossover. Foundation that should have been laid before the first production deployment.
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