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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What a Calorie Deficit Actually Does: The Arithmetic, Adaptation and the 3500 Calorie Rule
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Body Fat Percentage Ranges by Sex and Age: What Any Method Can Tell You
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History of BMI: From Quetelet's Average Man to a Global Screening Standard
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TDEE Calculator Equations Mifflin-St Jeor Error Range and Activity Multipliers
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Measuring Height Accurately: The Errors That Move BMI by a Whole Point
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 in Older Adults: Why the Healthiest Range Shifts with Age and the Sarcopenia Problem
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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Reading a Growth Chart: CDC and WHO Charts and Crossing Percentiles
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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BMI and Type 2 Diabetes Risk: The Gradient and Why It Starts Lower in Some Groups
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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Total Daily Energy Expenditure Explained: TDEE Components and Activity Multipliers
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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Waist Circumference Thresholds by Sex and Population: Where the Tape Goes
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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BMI Categories and Where the WHO Cut-Offs of 25 and 30 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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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.