Guides
Every section we cover, with the explainers that go with it.
BMI and health risk
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BMI and cardiovascular risk
BMI is associated with cardiovascular disease, but most of the risk is mediated through blood pressure, lipids, and glucose. Central adiposity markers add information BMI misses.
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BMI and mortality: the curve
Observational studies show a J-shaped relationship between BMI and mortality. Reverse causation and smoking confound the low end; Mendelian randomisation suggests a more linear causal effect.
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BMI and type 2 diabetes risk
Type 2 diabetes risk rises with BMI in all populations, but the gradient begins at a lower BMI in Asian groups due to greater visceral fat and insulin resistance at the same weight.
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Metabolically healthy obesity
Metabolically healthy obesity describes people with BMI ≥30 and normal metabolic markers, but the phenotype is often transient and still carries elevated long-term cardiovascular risk.
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The risks at the low end
A BMI below 18.5 is associated with higher mortality, osteoporosis, and impaired immunity in observational studies, but reverse causation and smoking confound the relationship.
Body composition
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Body fat percentage and healthy ranges
Body fat percentage ranges differ by sex, age, and measurement method. Learn the published ranges, why a BIA number is not a DXA number, and the error margins that come with every method.
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How body fat is measured
What How body fat is measured claims, what owners report, and the one trade-off that decides whether it is the right machine.
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Smart scales and bioimpedance
Smart scales use bioelectrical impedance analysis to estimate body fat, but readings shift with hydration and carry ±3–8 percentage points of error against DXA. Learn what the number is actually worth.
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Waist circumference and what it means
Waist circumference thresholds differ by issuing body and population. Learn the NIH, WHO, and IDF cut-offs, where the tape goes according to each protocol, and the measurement error you should expect.
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Waist-to-height ratio
The NICE CG189 (2022) guideline advises keeping waist-to-height ratio below 0.5. Learn what this ratio predicts, how it differs from BMI, and the correct waist measurement protocol with its error margins.
Children and teenagers
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BMI through puberty
Why a teenager's BMI changes during puberty, the normal trajectory of adiposity rebound and growth spurt, and why adult cut-offs do not apply.
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Childhood obesity: what the data shows
Childhood obesity prevalence figures from WHO, CDC, and national surveys, with the year, source, and percentile cut-off stated for every statistic.
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How child BMI percentiles work
Why a child's BMI is compared with other children of the same age and sex, what a percentile actually means, and why it is not a body fat measurement.
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Reading a growth chart
How to read CDC and WHO growth charts, what centile lines represent, and what crossing percentiles means for a child's growth trajectory.
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Talking to a child about weight
What paediatric guidance from AAP, NICE, and the Royal College of Paediatrics says about talking to a child about weight, and what the evidence says not to do.
Measuring it properly
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How accurate bathroom scales are
How accurate consumer bathroom scales are, the factors that cause drift and variation, and a procedure for checking your scale against a known reference weight.
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How to weigh yourself so the number means something
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Measuring height accurately
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Metric, imperial and stones
Exact conversion factors for BMI units across metric, imperial, and stones, with worked examples showing how rounding during conversion can change a WHO category.
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Where the tape goes
Step-by-step protocol for measuring waist circumference using WHO and NIH landmarks, with the error each method removes and the difference the landmark choice makes.
The calculators
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Ideal body weight formulas compared
Devine, Robinson, Miller, and Hamwi ideal body weight formulas originated in clinical dosing or insurance tables, not health standards. They disagree because they were derived from different populations and purposes.
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The BMI calculator: what it does
The formula a BMI calculator runs, where the WHO 1995 classification categories came from, and how rounding during calculation can shift a person across a category boundary.
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The body fat estimate: the Navy method
The body fat calculator uses the Hodgdon and Beckett 1984 US Navy circumference equation. Its standard error against DXA is typically several percentage points, varying by population.
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The TDEE calculator: which equation and why
The TDEE calculator uses the Mifflin-St Jeor equation with FAO/WHO/UNU 2001 activity multipliers. Individual error against doubly labelled water is typically ±15–25%.
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The waist-to-height calculator
The waist-to-height ratio calculator uses the NICE CG189 0.5 boundary. This is a population heuristic, not a personal risk score, and the result depends on the measurement protocol used.
Understanding BMI
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BMI compared with body fat percentage
BMI is a weight-for-height ratio that measures no fat at all. Body fat percentage measures adipose tissue directly. They correlate in populations but diverge in individuals.
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BMI prime, the ponderal index and other variants
BMI prime divides a person's BMI by the WHO upper limit of 25. The ponderal index uses height cubed. Neither has replaced BMI in global screening guidance.
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How accurate BMI is for one person
BMI misclassifies a substantial minority of individuals when measured against DXA body fat. It has moderate sensitivity and misses excess adiposity in many normal-weight adults.
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Quetelet, Keys and how BMI became the standard
BMI began as Adolphe Quetelet's 19th-century statistical description of the average man. Ancel Keys named it in 1972, and the WHO adopted it as a screening standard in 1995.
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The BMI categories and where the cut-offs come from
The WHO adult BMI categories were fixed by an expert committee in the 1990s using epidemiological cut-points, not physiological thresholds. Every band names its source and year.
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What BMI is and how it is calculated
What What BMI is and how it is calculated claims, what owners report, and the one trade-off that decides whether it is the right machine.
Weight and energy balance
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Basal metabolic rate, explained
Basal metabolic rate is measured under strict conditions; prediction equations estimate it with individual error typically ±10–20%, and some groups face systematic bias.
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How fast weight can safely change
Clinical guidelines recommend modest weekly rates; the first week's rapid drop is mostly water and glycogen, and very fast loss carries documented gallstone risk.
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Metabolic adaptation and plateaus
Metabolic adaptation is the reduction in energy expenditure beyond what mass loss predicts; plateaus have multiple causes including adherence decay and NEAT reduction.
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Total daily energy expenditure
Total daily energy expenditure has four components; the physical activity multiplier is the weakest link, introducing individual error typically ±15–25% against doubly labelled water.
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What a calorie deficit actually does
The 3500-calorie-per-pound rule is a static 1958 estimate; dynamic models show energy expenditure falls as mass is lost, and the first week's loss is mostly water.
When BMI reads wrong
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Athletes and muscular builds
How far BMI overstates health risk in athletes and muscular builds, with measured examples from published studies and alternative body composition methods.
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BMI and pregnancy
Pre-pregnancy BMI categories, IOM gestational weight gain ranges by category, and why BMI is not interpreted during pregnancy—with every threshold sourced to its issuing body and year.
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BMI in older adults
Why the healthiest BMI range shifts upward in older adults, how sarcopenia complicates BMI interpretation, and what measurements add useful information.
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Ethnicity and body composition
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Very short and very tall people
BMI divides weight by height squared, but bodies are three-dimensional. For very short and very tall people, this arithmetic distorts risk classification—here is by how much and what to use instead.
