Tape Test History: How the Circumference Method Estimates Body Fat
Estimate body fat with a tape measure. The method’s origins, how to measure and what its limits are.
Home / Blog
Calculator Guides & Articles
Formulas explained, where they come from and what has changed. Every article links to calculators you can use right away.
AllFinancialFitness & HealthMathOther
BMI History: Quetelet’s Index and Why It Has Limits
BMI began as a statistical tool in the 1830s. Here is how it works, where it falls short and what is changing.
Dog Years Formula: From “Multiply by 7” to the Epigenetic Clock
The old 7× rule is a myth. Here is the science-based alternative and how size changes the answer.
Flesch-Kincaid Readability: History, Formulas and Limits
The two readability formulas behind most “reading level” tools, and how to use them without being misled.
The 4% Rule and the Rule of 72: Origins, Examples and Limits
Two rules of thumb every retirement planner uses, where they came from, and what to keep in mind.
Kelly Criterion: History, Formula and Fractional Kelly
The bet-sizing formula from a 1956 Bell Labs paper: how it works, why it is aggressive and how to tame it.
Mifflin-St Jeor Equation Explained: BMR, TDEE and Calories
The equation behind most modern calorie calculators, where it came from and how to use it well.
Percentage Change Formula and the History of the Percent Sign
How to calculate percentage change correctly, why +50% then −50% is not zero, and where the percent sign came from.
RMD Rules Explained: Age 73, Age 75 and What Changed
RMD age is now 73 and rising to 75. Here is how the rules changed and how the calculation works.
Square Roots: Babylonian Clay Tablets to Your Calculator
A 3,700-year-old tablet already knew √2. Here is how square roots work and how machines compute them.
Theoretical Yield and Electronegativity: Two Chemistry Calculations Explained
Two core chemistry calculations, with the history behind them and worked examples.