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Guides

The why behind the numbers.

Comparisons and step-by-step explainers to read alongside the calculators: which formula to use, when, and what can go wrong.

ComparisonActual, ideal, adjusted or lean body weight: which weight is whichActual body weight is what the scales show, ideal body weight comes from height and sex, and adjusted body weight sits between the two. In an average-sized adult they are close. In obesity they can differ by 50 kg or more, and any weight-based dose moves with them.Read the guide →ExplainerCHA₂DS₂-VASc or CHA₂DS₂-VA? What changed with the 2024 ESC guidelineCHA₂DS₂-VA is CHA₂DS₂-VASc without the point for female sex, and the 2024 ESC atrial fibrillation guideline adopted it in place of the older score. Treatment decisions are almost unchanged. What changes is how you read the number.Read the guide →Step by stepDosage calculation methods, step by stepDosage calculation methods come in three classic forms: the formula (D/H × V), ratio and proportion, and dimensional analysis. Done correctly, all three give the same answer. Below they are applied to the same problems, so you can compare them and pick the one that suits you.Read the guide →ComparisoneGFR vs creatinine clearance: what each one tells youeGFR and creatinine clearance both start from serum creatinine, but they are not interchangeable. eGFR is scaled to 1.73 m² and stages kidney disease; Cockcroft-Gault CrCl is an absolute figure in mL/min that many drug labels still dose on. Two worked cases show how far apart they can land.Read the guide →Step by stepHow to read an arterial blood gas (ABG) in six stepsTo read an arterial blood gas, it helps to take it in the same order every time: the patient and sample, pH, primary disorder, compensation, anion gap, then oxygenation. Done that way, a wall of numbers becomes a short, repeatable piece of reasoning.Read the guide →ExplainerMedication unit conversions explainedMedication unit conversions follow a few fixed rules: g, mg, mcg and ng step by 1000, a % strength × 10 gives mg/mL, and units never convert to mg. Unit and decimal slips cause many of the most dangerous calculation errors, the tenfold and thousand-fold ones, so each conversion below comes with the trap hidden in it.Read the guide →ExplainerNormal vital signs by ageNormal vital signs change with age: a newborn’s heart rate runs at 100–205/min awake, an adult’s at 60–100/min at rest. The tables below give heart rate, respiratory rate, blood pressure, temperature and SpO₂ from birth to old age, with their sources and why those sources disagree.Read the guide →ComparisonQTc formulas compared: Bazett, Fridericia, Framingham and HodgesFridericia and Framingham correct the QT for heart rate better than Bazett, which overcorrects at fast rates and undercorrects at slow ones. On the same ECG at 110/min, the four common QTc formulas can differ by more than 50 ms.Read the guide →ComparisonWhich body surface area formula? Mosteller, Du Bois, Haycock and Gehan & GeorgeFor most adults it hardly matters which BSA formula you use: Mosteller, Du Bois, Haycock and Gehan & George agree within about 2%. In obesity and in newborns they drift apart by 10% or more, and that matters when a dose is prescribed per m².Read the guide →