BMI Calculator
Calculate Body Mass Index (BMI) and Ponderal Index with ethnicity-specific health guidelines.
BMI Visual Placement
Understanding Body Mass Index & Health Ranges
BMI estimates body fat from weight and height: metric BMI is weight in kilograms divided by height in meters squared, and the imperial formula multiplies pounds ÷ inches² by 703. The standard categories are underweight (<18.5), normal (18.5–24.9), overweight (25–29.9), and obese (≥30). The healthy weight range shown is the weight at which your BMI falls in the normal category for your height.
For Asian and South Asian populations, studies show that metabolic risk increases at lower BMIs, meaning an adjusted threshold of 23.0 for overweight and 27.5 for obesity is highly recommended. The Ponderal Index (Rohrer's Index) scales weight by the cube of height (kg/m³), correcting height bias in extremely short or tall users. All math is performed locally.
Built and maintained by Meet Shah · Last updated
What this tool is used for
- Getting a BMI figure for a form that asks for one.
- Seeing which category a height and weight combination falls in.
- Tracking a figure over months rather than reacting to one reading.
- Converting between metric and imperial while calculating.
- Getting a number to discuss with a clinician.
How it works in practice
A worked example
The same measurements are being checked against the Western guidelines and against the South and East Asian ones.
Weight 76 kg Height 175 cm
Western / Standard Guidelines YOUR BMI 24.8 Normal weight Healthy weight range for your height: 56.7 – 76.3 kg South Asian / East Asian Guidelines YOUR BMI 24.8 Overweight (Increased Risk) Healthy weight range for your height: 56.7 – 70.1 kg To achieve a normal BMI (18.5 – 22.9) for your height, you would need to lose 5.9 kg.
One body, one number, two legitimate verdicts, and the guideline selector is the only thing that changed. The index itself does not move — weight in kilograms over height in metres squared — but the line drawn through it does, because the health risks associated with a given index appear at a lower value in South and East Asian populations. The World Health Organization published the lower thresholds for that reason, and the healthy range shifts with them: the same height allows six fewer kilograms.
The edge case that catches people
The formula divides by height squared, and people are not two-dimensional. Mass scales closer to the cube of a linear dimension, so the same build reads high for a tall person and low for a short one — which is a known defect of a measure invented in the 1830s to describe populations rather than individuals. That is why this page will also show a ponderal index, which divides by height cubed and gives 13.1 for the same measurements. Neither number can tell muscle from fat, and neither knows where the weight sits.
When not to use this tool
Not for a child, not for an athlete, and not during pregnancy. Children are assessed on percentile charts against age and sex, because a fixed threshold is meaningless while a body is still growing; a rower or a sprinter will read overweight on muscle alone; and pregnancy makes the input a moving target with no useful interpretation. Waist measurement predicts metabolic risk better than this does for almost everyone, and it takes a tape measure. Treat a figure here as one crude input to a conversation, never as a finding.
Frequently Asked Questions
- Where does the 703 in the imperial formula come from?
- It is a unit conversion baked into a constant. BMI is kg/m², and converting pounds per square inch into that gives 0.45359237 ÷ 0.0254² ≈ 703. So the imperial formula is the metric one with the conversion pre-multiplied, not a different measure.
- What do the standard categories mean?
- Under 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, 30 and above obese. Those cut-offs come from WHO population studies of mortality risk, which is what they describe — a band, not a diagnosis, and least meaningful at the edges of any individual's case.
- Why does the tool offer an Asian cut-off?
- Because the WHO issued lower thresholds for Asian populations — overweight from 23 and obese from 27.5 — after evidence that cardiometabolic risk rises at a lower BMI. Using the standard cut-offs there systematically under-identifies risk, which is why the category is population-dependent.
- Why does BMI misclassify muscular people?
- Because it measures mass against height and cannot tell what the mass is. Muscle is denser than fat, so a trained athlete routinely reads overweight or obese while carrying very little fat. That is a limitation of the ratio itself, not a calibration problem.
- Why does BMI use height squared when people are three-dimensional?
- Because it is empirical rather than geometric. Quetelet found in the 1830s that weight scales closer to height squared than cubed across real populations, since taller people are not proportionally wider. The square is the exponent that best flattened the correlation with height, which is the whole design goal.
Common errors and gotchas
- Treating BMI as a measure of body composition, which it explicitly is not — it cannot distinguish muscle from fat.
- Applying adult thresholds to children, who need age and sex percentile charts instead.
- Ignoring that the thresholds differ by population, with lower cut-offs recommended for some.
- Reading a category as a diagnosis rather than a screening indicator.
- Comparing a BMI figure across populations whose recommended thresholds differ.