BMI Calculator
What BMI measures, what it misses, and the calculators to work it out properly - including the ones that answer better questions than BMI does.
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Reading a Growth Chart: CDC and WHO Charts and Crossing Percentiles
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BMI and Type 2 Diabetes Risk: The Gradient and Why It Starts Lower in Some Groups
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Total Daily Energy Expenditure Explained: TDEE Components and Activity Multipliers
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Waist Circumference Thresholds by Sex and Population: Where the Tape Goes
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BMI Categories and Where the WHO Cut-Offs of 25 and 30 Come From
Guides by category
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BMI and health risk
BMI is associated with mortality and disease risk at the population level, but the relationship is shaped by confounding, reverse causation, and body composition differences.
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Body composition
BMI measures weight-for-height, not body composition. This hub routes you to guides on waist-to-height ratio, body fat percentage, visceral fat, and muscle mass, with every threshold attributed to its source.
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Children and teenagers
How to understand a child's BMI result, growth chart position, and what it means for health, with every threshold attributed to its issuing body and year.
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Measuring it properly
Self-reported numbers produce a BMI roughly 0.5-1.5 kg/m² lower than measured ones. These guides give you the protocol to close that gap, with the error size named for each step.
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The calculators
Route to the right body measurement calculator by what you are trying to understand, with every equation's source and margin of error stated before you enter a number.
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Understanding BMI
Understanding body mass index: a hub routing you to guides on BMI screening, measurement methods, population thresholds, and what BMI can and cannot tell you.
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Weight and energy balance
How basal metabolic rate, total daily energy expenditure, calorie deficit and metabolic adaptation interact—explained through mechanisms, not prescriptions.
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When BMI reads wrong
Which populations standard BMI misclassifies—athletes, older adults, pregnant women, and ethnic groups—and what measurements are used instead, with every threshold sourced.
Latest articles
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BMI Calculator Explained: The Formula It Runs, the Categories It Reports, and the Rounding
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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How to Weigh Yourself So the Number Means Something: Time, Clothing, Frequency
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BMI in Athletes and Muscular Builds: How Far It Overstates Risk, with Measured Examples
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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How Child BMI Percentiles Work and What They Actually Measure
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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BMI and Mortality: The J-Shaped Curve, Confounders, and the Evidence
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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Basal Metabolic Rate Explained: BMR Equations, Error Ranges and Accuracy
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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Waist-to-Height Ratio: The 0.5 Rule and How to Measure Your Waist
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.
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What BMI is and how it is calculated Review: Specifications, Range and Who It Suits
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.
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Weight and Energy Balance Explained: BMR, TDEE, Deficit and Maintenance
How basal metabolic rate, total daily energy expenditure, calorie deficit and metabolic adaptation interact—explained through mechanisms, not prescriptions.
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When BMI Reads Wrong: Populations for Whom Standard BMI Misclassifies Health Risk
Which populations standard BMI misclassifies—athletes, older adults, pregnant women, and ethnic groups—and what measurements are used instead, with every threshold sourced.