Guides
The reasoning behind every number on this site: where the formulas came from, what the thresholds actually mean, and where each measurement stops being trustworthy. Every claim is sourced to primary literature, listed at the foot of each guide, and nothing here claims a medical review that has not happened.
Start here
Four guides, in this order, cover most of what people come here to understand.
Understanding BMI
What body mass index actually measures, the three recognised classification systems, and the 2025 framework that ended BMI-alone diagnosis.
BMI, explained properly
Where body mass index came from, what the categories actually mean, why the cut-offs sit where they do, and the specific situations where BMI misleads.
Run the numbers: BMI, waist-to-height
Why BMI is wrong for athletes
Muscular people routinely read overweight or obese on BMI while carrying low body fat. Here's exactly why the formula fails, and the measures that don't.
Asian BMI cut-offs, explained without the muddle
Why lower BMI thresholds exist for Asian populations, what the WHO actually decided in 2004, and how to read both systems without the usual muddle.
What changed in the 2025 obesity definition
The Lancet Commission's 2025 framework ended BMI-alone diagnosis: BMI now needs a fat-distribution measure, plus a clinical/preclinical split.
Run the numbers: BMI, waist-to-height
Measuring and body fat
How to take measurements you can trust, what every body fat method really costs in accuracy, and how to track change without buying scans.
How to measure your waist accurately
The exact landmark, tape technique and timing for a waist measurement you can trust, the half-centimetre details that move every waist-based result.
Run the numbers: waist-to-height, waist-to-hip
Body fat percentage methods, compared honestly
DEXA, BIA scales, skinfolds, tape formulas, air and water displacement: real accuracy, real cost, and which body fat method is actually worth using.
Run the numbers: body fat, lean body mass
How accurate is the Navy body fat formula?
Validation data for the US Navy tape method: the real ±3–4 point error band, exactly who it misreads, and how to get the most accuracy out of it.
Run the numbers: body fat, army body fat
How to track body composition without a DEXA scan
A tape measure, a scale and twenty minutes a month outperform expensive scans for tracking change. The exact protocol, and how to read the trend lines.
Run the numbers: body fat, lean body mass
Energy, intake and comparisons
Where your daily calories go, why every estimate carries a band, and which of the three headline numbers answers which question.
TDEE, explained from the ground up
The four components of total daily energy expenditure, why NEAT is the hidden variable, what adaptive thermogenesis does, and how to calibrate.
BMI vs body fat vs waist ratio: which number matters?
Three measurements, three different questions. What each can and cannot tell you, where they disagree, and the two-measure combination to use.
Eating: calories, protein and macros
Setting a calorie target you can actually hold, what the protein evidence supports at every tier, how calories become grams, and why a plateau is rarely your metabolism.
How many calories should I eat?
Work from basal rate to an intake target: the activity factor, the error band on the whole chain, the floors that make it safe, and how to correct it.
How much protein do you actually need?
What the 0.8 g/kg RDA really represents, the higher intakes the training and dieting evidence supports, meal distribution, ageing, and the kidney question.
How to set your macros
Set protein by body weight, floor the fat, give the remainder to carbohydrate. Why ratios matter less than total energy, and what tracking accuracy really is.
Metabolic adaptation and weight loss plateaus
How large adaptive thermogenesis really is, how much is lost tissue, why NEAT falls quietly, and why a plateau in week three is almost never adaptation.
Run the numbers: TDEE, calorie deficit
Training: zones, strength and energy cost
Why the age-based maximum heart rate is a weak estimator, how far a rep-max equation can be trusted, and what a MET value can and cannot tell you about calories burned.
Heart rate zones, explained properly
Why 220 minus age is a poor estimator, what Tanaka and Gellish fixed, how percentage of maximum differs from heart rate reserve, and what each zone is for.
Run the numbers: heart rate zones, calories burned
How to estimate your one rep max
The published 1RM equations, how far apart they get at high rep counts, why accuracy collapses past about five reps, and when testing beats estimating.
Run the numbers: one-rep max, FFMI
What a MET is, and how calories burned is estimated
The 3.5 ml per kilogram per minute convention, how the Compendium of Physical Activities was built, and why every calorie-burn figure is a population average.
Run the numbers: calories burned, TDEE
Children and pregnancy
Why a growing child gets a percentile rather than a fixed cut-off, the difference between a growth reference and a growth standard, and where the pregnancy weight ranges came from.
Child BMI percentiles, explained
Why children get percentiles not fixed cut-offs, what the CDC LMS method does, how a reference differs from a standard, and why trajectory beats one reading.
Pregnancy weight gain, explained
What the Institute of Medicine ranges say by pre-pregnancy BMI, the evidence behind them, where it is weak, the twin gap, and the firm limits of any calculator.
Run the numbers: pregnancy gain, BMI
Losing weight on purpose
How long a safe cut actually takes, how to read a weekly weigh-in, when the scale is hiding a recomp, how to exit a deficit, and what fasting, metabolic age and alcohol do to the numbers.
How long does safe weight loss take?
Safe loss is 0.25 to 1% of body weight a week. Convert kilograms to weeks with 7,700 kcal per kg, then ignore any finish date that skips that energy maths.
Run the numbers: weight-loss time, energy in fat
How to read a weekly weigh-in
A weekly average hides water and glycogen noise. Compare expected kilograms from your deficit with the trend, then hold, recheck the log, or slow down.
Run the numbers: check-in, weight-loss time
Body recomposition versus weight loss
Recomp is fat down and lean up at nearly the same weight. The scale can sit still. Protein, lifting and a small deficit decide fat loss versus shape change.
Reverse dieting after a cut
After a cut, add 50 to 200 kcal a week back toward TDEE. Reverse dieting is a controlled exit, not a metabolism repair. Watch the weekly average, not one day.
Run the numbers: reverse diet, TDEE
What metabolic age actually is
Gym-scale metabolic age matches your BMR to a BMI-22 person of the same height. It is a marketing label, not tissue age. Lean mass is what actually moves BMR.
Run the numbers: metabolic age, TDEE
Fasting windows and calories
A fasting window times meals; it does not add a fat-loss bonus. Weekly calories versus TDEE still decide the scale. 16:8 only works if intake stays below cost.
Run the numbers: fasting window, calorie deficit
Alcohol and the calorie budget
A regular beer is about 153 kcal. Two a night, four nights a week, is a surplus you will not see on Tuesday. Log the drink first, then decide what else fits.
Run the numbers: alcohol calories, TDEE