BMI Calculator
Calculate your Body Mass Index in metric or imperial units and see your weight category.
Ages 2-120
BMI Results
📊 Your BMI vs. WHO Categories
📋 WHO BMI Classification Table
Your category is highlighted based on your current BMI.
| BMI Range | Category |
|---|---|
| < 16.0 | Severe Thinness |
| 16.0 – 16.9 | Moderate Thinness |
| 17.0 – 18.4 | Mild Thinness |
| 18.5 – 24.9 | Normal← You |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese Class I |
| 35.0 – 39.9 | Obese Class II |
| ≥ 40.0 | Obese Class III |
You're in the Healthy Range!
Your BMI indicates a normal, healthy weight.
Stay Active
150+ minutes of moderate exercise per week
Balanced Diet
Rich in fruits, vegetables, and whole grains
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7-9 hours of quality sleep every night
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Disclaimer: This tool is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making any health-related decisions or starting any diet or exercise programme.
How to Use the BMI Calculator
- Choose metric (kg/cm) or imperial (lbs/ft/in).
- Enter your age, gender, weight, and height.
- Your BMI score, category, BMI Prime, Ponderal Index, healthy weight range, and personalised health insights all appear instantly.
- View the animated gauge, comparison chart, WHO classification table, and health risk breakdown for a complete picture.
Formula
Metric: BMI = weight (kg) ÷ height (m)²
Imperial: BMI = 703 × weight (lbs) ÷ height (in)²
BMI Prime: BMI ÷ 25 (ratio to upper normal limit)
Ponderal Index: weight (kg) ÷ height (m)³ — more reliable for very tall/short people
History & Interesting Facts
💡 Did You Know?
Arnold Schwarzenegger at his peak (1974 Mr. Olympia) had a BMI of approximately 33 — firmly in the "obese" category according to WHO guidelines. Yet he had only 4–5% body fat. This is the most famous example of why BMI alone is an incomplete health metric.
Origin & History
BMI was invented in 1832 by Belgian polymath Adolphe Quetelet, who called it the "Quetelet Index." Quetelet was a statistician and astronomer, not a physician — and he created it purely as a tool to describe population distributions, explicitly stating it should NOT be used for individual assessment. For 140 years, the index was largely ignored as a medical tool. In 1972, American physiologist Ancel Keys re-examined it in a study of 7,500 men across five countries and coined the term "Body Mass Index," showing it correlated better with body fat than other simple measurements. The WHO adopted it as a global obesity screening tool in 1995.
Fascinating Facts
- 1
Japan uses a BMI of ≥25 as the threshold for obesity — significantly lower than the WHO's global threshold of 30. This reflects the fact that Japanese people tend to develop metabolic complications at lower BMI values.
- 2
Globally, about 2.5 billion adults are overweight (BMI ≥25) and 890 million are obese (BMI ≥30) as of 2022 — more than double the rate in 1990.
- 3
At a BMI below 16, starvation-related organ failure becomes a serious medical risk. The body begins breaking down muscle and organ tissue for energy when fat stores are depleted.
- 4
The BMI scale ranges from approximately 10 (extreme emaciation) to 70+ (extreme obesity). The highest reliably recorded BMI in a living person was approximately 188 in Jon Brower Minnoch in 1978.
- 5
Despite being invented for population statistics, BMI is now used in insurance premiums, military fitness standards, and medical eligibility criteria for procedures like joint replacement surgery — applications its creator never intended.
- 6
A 2023 policy statement from the American Medical Association warned against using BMI as the primary measure of health, calling it "an imperfect proxy" that can produce misleading results when used in isolation.
Frequently Asked Questions
What is a healthy BMI?
A BMI between 18.5 and 24.9 is considered normal weight by the WHO. Below 18.5 is underweight; 25–29.9 is overweight; 30–34.9 is Class I obese; 35–39.9 is Class II; 40+ is Class III (severe) obesity. Different thresholds apply in some countries — Japan uses ≥25 as obese.
What is BMI Prime?
BMI Prime is the ratio of your BMI to the upper limit of normal (25 kg/m²). A value less than 0.74 means underweight, 0.74–1.00 is normal, above 1.00 is overweight, and above 1.20 is obese. It lets you see at a glance how far you are from the healthy ceiling.
What is the Ponderal Index?
The Ponderal Index (PI) uses height³ instead of height², making it more reliable for very tall or very short individuals. A normal PI is around 11–15 kg/m³. Unlike BMI, the PI doesn't overestimate adiposity in tall people or underestimate in short people.
Are there BMI limitations?
Many. BMI doesn't measure body fat directly — it simply compares weight to height. A muscular athlete with very little fat can have a high BMI (labelled obese), while an inactive person with little muscle but high visceral fat can have a normal BMI. For a fuller picture, use BMI alongside body fat %, waist circumference, and other markers.
Is BMI different for different ethnicities?
Yes, significantly. South and East Asian populations have higher health risks at lower BMIs. The WHO recommends a separate overweight threshold of 23 for Asian populations. Additionally, Black individuals may have higher bone density, meaning their BMI slightly overstates fat relative to the general formula.
My BMI is over 25 — what should I do?
Don't panic! BMI is a starting point. Check your waist circumference (>94cm for men / >80cm for women raises risk), get a body fat assessment, and speak with your GP. If you genuinely have excess body fat, even a 5–10% weight reduction delivers substantial health improvements — use the Health Plan link below to build a personalised diet and workout plan.
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