BMI Calculator
Calculate your Body Mass Index, see which of the 8 WHO categories you fall into, and compare your result against population percentiles, with full context on what the number actually means.
BMI Categories
Underweight
0 – 18.5Normal weight
18.5 – 25Overweight
25 – 30Obesity class I
30 – 35Obesity class II
35 – 40Obesity class III
40 – 50Obesity class IV (Super)
50 – 60Obesity class V (Super-Super)
60+BMI is a population screening tool, not a diagnosis. It doesn't account for muscle mass, bone density, or fat distribution. Use it as a starting point, not a final word on your health.
BMI is a population screening tool, not a diagnosis. Consult a clinician for personalized advice.
The Basics
What is BMI?
The Formula
Weight ÷ Height²
BMI is calculated by dividing your body weight in kilograms by your height in metres squared. It's a simple ratio that gives a single number; easy to calculate and universally understood, and consistent across populations. That simplicity is both its strength and its main limitation.
A Screening Tool
Population-level signal
BMI was designed in the 1830s by a statistician, not a doctor, to describe the average body of a population, not to assess individuals. It's used by healthcare systems worldwide because it costs nothing, requires no equipment, and correlates with health risk across large groups. For the individual, it's a starting point.
Know Its Limits
Muscle, age, and ethnicity matter
BMI doesn't distinguish between fat and muscle, so a heavily muscled athlete can score "obese" while being metabolically healthy. It also doesn't account for where fat is stored, visceral fat around the abdomen carries more health risk than subcutaneous fat, or how those risks differ by age and ethnic background.
Understanding the Scale
Categories Explained
The WHO defines eight BMI categories, from Underweight through to Obesity class V. Each threshold represents a point where statistical health risk changes meaningfully across large population studies.
Underweight (below 18.5) is associated with nutritional deficiency, reduced immune function, bone loss, and hormonal disruption. It's as important to address as obesity, though it receives less public attention.
Normal weight (18.5–25) is where the population average health risk is lowest. That said, a person can sit in this range with poor metabolic health; high visceral fat, low muscle mass, and an inactive lifestyle all matter independently of BMI.
Overweight (25–30) carries mildly elevated risk for some individuals, but for people with high muscle mass or certain ethnic backgrounds, the clinical significance varies. Context always matters.
Obesity classes I–V represent progressively elevated risk for cardiovascular disease, type 2 diabetes, sleep apnoea, joint disease, and all-cause mortality. Class III and above (BMI 40+) is where medical intervention is most commonly recommended.
BMI Category Scale
A Note on Thresholds
These category boundaries are statistical averages, not biological switches. Risk increases gradually across the scale, moving from 24 to 26 doesn't represent a cliff edge. What matters most is the trend direction over time.
What BMI Misses
BMI Limitations
Muscle vs Fat
The athlete problem
Muscle tissue is denser than fat. A well-trained athlete with 12% body fat can have the same BMI as a sedentary person with 30% body fat; yet their metabolic health, cardiovascular risk, and longevity outlook are completely different. BMI simply can't tell them apart. This is why the calculator pairs BMI with body fat percentage.
Fat Distribution
Where fat sits matters more
Two people with identical BMIs can have very different health profiles depending on where their fat is stored. Visceral fat, the fat packed around internal organs in the abdominal region, drives insulin resistance, inflammation, and cardiovascular risk far more than subcutaneous fat stored under the skin. BMI captures none of this.
Ethnicity & Age
Thresholds aren't universal
BMI thresholds were derived primarily from studies of European populations. Research shows that people of South and East Asian descent face higher metabolic risk at lower BMI values, some health organisations recommend lower cut-offs for these groups. Older adults also tend to carry more fat at the same BMI as younger people, making the scale less reliable with age.
The Bigger Picture
BMI vs Other Metrics
BMI is most useful when combined with other measurements. No single metric gives the full picture, but together, a handful of easy-to-track numbers can tell you a great deal about where you are and what to prioritise.
Body fat percentage is the most direct measure of body composition. Unlike BMI, it distinguishes between fat and lean mass, making it far more meaningful for athletes and anyone doing resistance training. The body fat calculator on this site gives you that number.
Waist circumference is a simple proxy for visceral fat, the fat that surrounds your organs and drives metabolic risk. Men above 94 cm and women above 80 cm are at increased risk; above 102 cm and 88 cm respectively, the risk is substantially higher.
Waist-to-height ratio divides your waist circumference by your height. A ratio below 0.5 is the general target, it adjusts for body size in a way that BMI doesn't, and some research suggests it's a better predictor of cardiometabolic risk than BMI alone.
Use BMI as your entry point. Then layer in body fat %, waist measurement, and how you feel and perform day-to-day. That combination gives you a far clearer picture than any single number.
Metric Comparison
Waist Circumference Targets
Men: below 94 cm is healthy, 94–102 cm is increased risk, above 102 cm is high risk.
Women: below 80 cm is healthy, 80–88 cm is increased risk, above 88 cm is high risk.
These cut-offs are WHO guidelines for European populations, lower thresholds apply for South and East Asian individuals.