About the BMI Calculator
Body Mass Index (BMI) is a screening measure that estimates body fat from height and weight. It is calculated as weight in kilograms divided by the square of height in metres (kg/m²) and grouped into categories defined by the World Health Organization. BMI is the most widely used population health metric because the inputs are universally available and the formula is simple, even though it has well-documented limitations for individuals.
This calculator supports both metric (kg, cm) and imperial (lb, ft, in) inputs and reports the BMI value alongside the WHO category. It also explains where BMI is informative and where it isn't.
Who the standard cutoffs apply to
The WHO thresholds listed above the calculator are for adults, and they are identical for men and women — BMI makes no sex adjustment. They do not transfer to everyone. Children and adolescents are assessed against age- and sex-specific percentile charts instead, because a growing body changes shape faster than a fixed cutoff can track. Several health authorities also use lower thresholds for people of South and East Asian descent, who show elevated cardiometabolic risk at BMI values the WHO defaults still call normal; Japan, China, and India each publish their own adjusted cutoffs. Pregnancy invalidates the measure entirely, and for adults over roughly 65 the evidence points the other way — a BMI slightly above the "normal" band is associated with better outcomes than one at the bottom of it.
Where BMI is misleading
BMI was designed for population studies, not individual diagnosis. It does not distinguish muscle from fat — a muscular athlete may register as "overweight" or even "obese" without carrying excess body fat. It also ignores fat distribution: abdominal fat carries higher cardiometabolic risk than peripheral fat, but BMI cannot tell. It is less accurate for very tall or very short people, and the standard cutoffs may not apply equally well across ethnic groups (some Asian populations face elevated metabolic risk at lower BMI values than the WHO defaults suggest).
When BMI is informative
At the population level, BMI tracks long-term health outcomes well. For most adults of average build, a BMI well outside the normal range is a useful starting point for a conversation with a clinician — particularly when combined with waist circumference, blood pressure, and fasting lab work. It is not a standalone diagnostic, but it is a perfectly reasonable first-pass screening number.
How to use the BMI Calculator
Pick a unit system
Metric (kg and cm) or imperial (lb and ft/in).
Enter height and weight
The BMI updates instantly. There is no submit button.
Read the category
Both the numeric BMI and the WHO category appear, colour-coded for quick interpretation.
Worked examples
Example 1
Input: 70 kg, 175 cm
Result: BMI 22.9 — Normal weight
A typical "normal range" example.
Example 2
Input: 154 lb, 5\'9"
Result: BMI 22.7 — Normal weight
Same person using imperial inputs.
Example 3
Input: 95 kg, 180 cm
Result: BMI 29.3 — Overweight
Just below the obesity threshold (30).
Real-world use cases
- A quick personal health check-in alongside other measurements.
- Filling forms (gym, insurance, occasional medical paperwork) that ask for BMI.
- Tracking BMI alongside weight over time during a diet or training programme.
- Comparing one's BMI category against WHO population reference ranges.
Tips & common mistakes
- BMI is one indicator. Pair it with waist circumference, blood pressure, and lab work for a more complete picture.
- Athletes and very muscular people: expect BMI to overestimate body fat. Use body composition measurement (DEXA, bioimpedance) for accuracy.
- For children and teens, use age- and sex-specific percentile charts, not the adult cutoffs.
- Always consult a qualified healthcare professional for an actual assessment of body composition and health risk.
Frequently asked questions
Is BMI accurate for me personally?
For most non-athletic adults, BMI is a reasonable rough indicator. For very muscular or very lean individuals, it can over- or under-estimate body fat substantially.
What BMI is "ideal"?
WHO classifies 18.5–24.9 as normal weight, but no single number is "ideal" for every person. Health depends on more than BMI alone.
Do BMI cutoffs differ by ethnicity?
Yes — some health authorities recommend lower overweight cutoffs (e.g. 23 instead of 25) for South and East Asian populations because metabolic risk rises at a lower BMI in those groups.
Should I rely on BMI for medical decisions?
No. BMI is a screening tool. Decisions about health, diet, and exercise should involve a qualified clinician.
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Last updated: October 2026 · All processing happens locally in your browser.