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· 12 min read
Frame size exists because height alone does not fix skeletal mass, and the published methods are crude proxies with real cut-offs. The bigger story is the ideal-weight formulas: four of them, all built for drug dosing rather than health, disagreeing by up to 27 lb at the same height.
· 10 min read
The correction factor says how far one unit of insulin moves glucose, and the classical estimate is 1800 divided by the total daily dose in mg/dL — or 100 divided by it in mmol/L. Those are the same rule, and here is the arithmetic that proves it, plus why stacking is the error that actually hurts.
· 10 min read
High blood glucose pulls water out of cells and dilutes the sodium in it, so a measured sodium in a hyperglycaemic patient reads lower than the real one. Here is the correction, both of the published factors, a case where they disagree by 4 mmol/L, and the different problem that lipids cause.
· 10 min read
The ratio says how many grams of carbohydrate one unit of insulin covers, and the usual starting estimate is 500 divided by the total daily dose. Here is where that number comes from, how sharply it moves across daily doses, why a 450 version exists, and the three things it quietly assumes.
· 13 min read
Milligrams per kilogram is one multiplication, and that is exactly why it fails: the documented errors are unit slips, misplaced decimal points and the wrong body weight, not hard maths. Here is the arithmetic done properly, the three body weights that give three different answers, and what body surface area changes.
· 2 min read
A pack-year sums up a smoking history in one number. Here is how it is calculated and why doctors use it to judge risk and screening.
· 2 min read
Body surface area estimates your body's external area in square metres. Here is how it is calculated and why clinicians use it to dose medicines.
· 3 min read
The healthy BMI band of 18.5–24.9 turns into a weight range once you know your height. Here is how to work out yours step by step.
· 2 min read
Weight depends on gravity, but mass does not — here is why you would weigh differently on the Moon, Mars and Jupiter, and how to work it out.
· 14 min read
Plasma is back in a day, red cells in weeks, iron in months — and it is the iron that sets the interval. The arithmetic of a unit, and the minimum intervals published by seven national services.
· 15 min read
The band and the cup are computed differently in the UK, the US, the metric EU, the French and Spanish markets and Italy. One body, its four labels, and the arithmetic behind each.
· 14 min read
A nap's effect depends on which sleep stage the alarm interrupts, and length is the only part of that you control. What 10, 20, 30 and 90 minutes actually do, with the arithmetic for the alarm.
· 11 min read
Doubling time is ln(2) times the interval divided by the natural log of the ratio. Here it is derived and worked on real pairs, with the two things most often got wrong online: the expected rate slows as the level rises, and a single value is meaningless because the reference range at one gestational week can span more than two hundredfold.
· 12 min read
How early you can test is the sum of three delays: when implantation happens, how fast hCG climbs from that moment, and what the test can detect. Each one is measured in the literature, so the chain can be computed end to end — and it explains exactly why an early negative is so often wrong.
· 12 min read
Ovulation is estimated far better by counting backwards from the next period than forwards from the last one, because the phase after ovulation is the stable one. Here is the inversion worked for every cycle length, the fertile window from the study that defined it, and the failure rates that make this unusable as contraception.
· 13 min read
Gestational age is counted from your last period, about two weeks before conception, so a pregnancy of 6 weeks contains an embryo of about 4 weeks. Here is Naegele's rule derived, the correction for a cycle that is not 28 days, and when ACOG says an early scan should replace your dates.
· 2 min read
How to split your daily calories across meals and snacks, with typical per-meal ranges and how to adjust them to your goals.
· 3 min read
Keeping your waist under half your height is a simple health marker. Learn what waist-to-height ratio means, how to measure, and why under 0.5 is the goal.
· 3 min read
A drink's alcohol content depends on its strength (ABV) and volume. Learn how standard drinks and units are calculated and how to count yours.
· 3 min read
Lean body mass is everything in your body that isn't fat. Learn how the Boer formula estimates it and why it matters for health and training.
· 3 min read
Health bodies suggest losing about 1-2 lb (0.5-1 kg) per week. Learn the ~500-1000 kcal/day deficit behind that and why slow-and-steady lasts.
· 3 min read
Your basal metabolic rate is the energy your body burns at complete rest. Learn how the Mifflin-St Jeor equation estimates it and what raises or lowers it.
· 14 min read
The Rockport test turns a brisk one-mile walk into a VO2max number using five inputs and one published regression. Here is the equation, one person worked through it, and the two assumptions — a stated standard error and a genuinely brisk pace — that decide whether the answer means anything.
· 12 min read
A headline saying 50% higher risk can describe a change of two-tenths of a percentage point. Both numbers come from the same 2x2 table, and computing both takes one line each — here is how, with a worked example, number needed to treat, and where the odds ratio stops approximating relative risk.