BMI Calculator
Calculate your Body Mass Index and see what it means for your health
Results & Details
// Weight Targets
What Is BMI?
Body Mass Index (BMI) is a simple numerical measure of a person's weight in relation to their height. It is widely used as a screening tool to categorise weight status and identify potential health risks associated with being underweight or overweight.
BMI Formula
BMI Categories (WHO)
The World Health Organisation defines: Underweight (below 18.5), Normal weight (18.5–24.9), Overweight (25–29.9), Obesity Class I (30–34.9), Obesity Class II (35–39.9), Obesity Class III (40 and above).
Limitations of BMI
BMI does not distinguish between muscle and fat mass. Athletes and highly muscular individuals often have a high BMI despite low body fat. Similarly, BMI does not account for age-related changes in body composition, fat distribution (waist circumference is an important additional measure), or differences between ethnic groups. It is a useful screening tool but should not be used as a sole diagnostic measure.
BMI for Children
BMI categories for children and teenagers are age- and sex-specific, as body composition changes significantly with development. This calculator uses adult BMI categories. For children under 18, consult a healthcare professional for age-adjusted interpretation.
BMI: What It Measures, What It Misses
Built and verified by Andrius R. · Updated June 2026
Body Mass Index is a screening tool, not a diagnosis. It does one thing well — flag whether weight is high or low relative to height across large populations — and several things badly. Knowing both sides makes the number above far more useful.
The formula, both ways
Metric: BMI = weight (kg) ÷ height (m)². A person who is 70 kg and 1.75 m: 70 ÷ (1.75)² = 70 ÷ 3.0625 = 22.9.
Imperial: BMI = 703 × weight (lb) ÷ height (in)². A person who is 154 lb and 5′9″ (69 in): 703 × 154 ÷ 4,761 = 22.7.
(The tiny difference is rounding — 154 lb is not exactly 70 kg.)
The standard categories (WHO, adults)
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obesity (classes I–III at 30, 35, 40) |
Some health bodies use lower thresholds for people of South Asian, Chinese and some other ethnic backgrounds (overweight from ~23), because health risks appear at lower BMI in these groups.
Where BMI genuinely fails
- Muscle vs fat. BMI can't tell them apart. Many rugby players, sprinters and weightlifters are "overweight" or "obese" by BMI while carrying very little fat.
- Fat location. Visceral fat around the organs is far more strongly linked to health risk than fat elsewhere — and BMI is blind to where weight sits. Waist circumference or waist-to-height ratio captures this better; a common guideline is keeping waist under half your height.
- Older adults. Muscle loss with age can keep BMI "healthy" while body composition worsens.
- Children and teens. Adult cutoffs don't apply; pediatric BMI is read against age- and sex-specific percentile charts.
So why is it still used everywhere?
Because it costs nothing, needs only a scale and a tape measure, and at the population level it correlates reasonably well with body-fat-related health risks. For an individual, the honest framing is: a BMI of 23 doesn't certify health, and a BMI of 27 doesn't certify a problem — but values drifting toward either extreme are a reason to look closer, ideally with a clinician and better measurements (waist, blood pressure, blood lipids, body composition).
If your BMI is outside the healthy range
One measurement is information, not a verdict. Sustainable change comes from the boring fundamentals — a modest calorie adjustment (our calorie calculator can size it), resistance training to protect muscle, sleep, and time. Crash approaches reliably reverse. And if you're an athlete reading an "overweight" result: the tool is mislabeling muscle, and a body-fat estimate (see the body fat calculator) will tell you far more.
From the Blog
// Beyond BMI
Waist circumference is a stronger predictor of metabolic risk. Men: under 94 cm, women: under 80 cm is generally healthy.
// Muscle vs Fat
BMI cannot distinguish muscle from fat. A muscular athlete may show "overweight" BMI despite excellent health.
// Healthy Range
A BMI of 18.5–24.9 is associated with the lowest risk of weight-related health conditions in the general adult population.
// Trend Matters
The direction your BMI is moving matters as much as the number. A rising BMI over years is a more useful health signal.