CodeMaster AcademyThe formula behind the number
BMI comes from a single equation devised by Belgian statistician Adolphe Quetelet in the 1830s: weight in kilograms divided by height in meters, squared. Enter 70 kg and 1.75 m and you get 70 ÷ (1.75 × 1.75) = 22.9. This calculator runs that same equation behind the scenes, along with the imperial equivalent (weight in pounds × 703, divided by height in inches squared), so you can switch units without doing the conversion arithmetic yourself.
How the World Health Organization reads the result
Once you have a number, the WHO classification splits adults into four bands: below 18.5 is underweight, 18.5 to 24.9 is the "normal weight" range, 25 to 29.9 is overweight, and 30 and above is classified as obese. These thresholds were built from population-level mortality and disease-risk data, which is why doctors use BMI as a first-pass screening signal rather than a diagnosis on its own.
Where BMI gets it wrong
The formula only knows two numbers — height and weight — so it can't tell muscle from fat. A rugby prop or a competitive weightlifter can land in the "overweight" band purely from muscle mass, while someone with a normal BMI but very low muscle mass and high visceral fat ("skinny fat") can carry real metabolic risk the number doesn't show. BMI also doesn't adjust for age, sex, or ethnicity, even though body-fat distribution varies across all three. Waist circumference, waist-to-hip ratio, or a DEXA scan give a fuller picture when BMI alone isn't conclusive.
Using this calculator
Pick metric or imperial with the toggle at the top, enter your height and current weight, and press Calculate — the result and its category appear immediately, calculated locally in your browser. Nothing you enter is sent anywhere or stored after you close the tab, which matters for a tool handling personal health figures.
A few practical notes
- Weigh yourself at a consistent time of day — first thing in the morning, before eating, tends to be the most repeatable baseline.
- The adult BMI bands don't apply to children or teenagers; pediatric assessment uses age-and-sex percentile growth charts instead.
- Pregnant users should not rely on standard BMI categories, since expected weight gain during pregnancy isn't reflected in the formula.
- Treat a borderline or unexpected result as a prompt to talk to a doctor, not as a verdict — BMI is a screening tool, not a diagnosis.
Questions people ask
Why did I get a different BMI on another site?
The formula is standardized, so differences usually come down to rounding or a units mix-up (e.g., entering pounds while the tool is set to kilograms). Double-check your unit selection if a result looks off.
Is a BMI of 25 automatically unhealthy?
Not necessarily — it's the low end of "overweight" on the WHO scale, but individual health depends on far more than one ratio, including fitness level, diet, and other risk factors a single number can't capture.
Does this tool store my height and weight?
No. The calculation happens in your browser using JavaScript; nothing is transmitted to a server or saved after you leave the page.
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Page last reviewed: July 2026