How to calculate BMI: formula, category chart, and what your result means
Calculate your Body Mass Index with the official WHO formula. Category chart, BMI limitations, and when to see a doctor.
What is BMI and how to calculate it
BMI (Body Mass Index) is a measure relating weight to height to estimate whether a person has a healthy weight.
Formula: BMI = weight (kg) / height (m)²
Example: Person weighing 75 kg, 1.75 m tall: BMI = 75 / (1.75)² = 75 / 3.0625 = 24.5
Imperial: BMI = (weight in lbs / height in inches²) × 703
Calculate your BMI instantly with the NexTools BMI calculator.
WHO BMI category chart
| BMI | Category | Health risk |
|---|---|---|
| Below 18.5 | Underweight | Malnutrition, osteoporosis, weak immune system |
| 18.5 - 24.9 | Normal weight | Minimal risk |
| 25.0 - 29.9 | Overweight | Moderate cardiovascular risk |
| 30.0 - 34.9 | Obesity class I | High risk: type 2 diabetes, hypertension |
| 35.0 - 39.9 | Obesity class II | Very high risk |
| 40+ | Obesity class III | Extreme risk: life expectancy reduced 8-14 years |
2025 stats: 39% of adults worldwide are overweight (WHO). US 73%, Mexico 75%, Japan 27%. Obesity has tripled since 1975.
BMI limitations: what it does NOT tell you
1. Doesn't distinguish muscle from fat. A 90 kg, 1.80 m athlete has BMI 27.8 ("overweight") but may have 10% body fat.
2. Doesn't consider fat distribution. Abdominal (visceral) fat is much more dangerous than hip/thigh fat.
3. Varies by ethnicity. Asians have higher diabetes risk at lower BMI. WHO suggests overweight starts at BMI 23 for Asian populations.
4. Doesn't apply to children. Use age-adjusted BMI percentiles for under-18.
5. Doesn't apply to pregnant women.
Complementary measures: Waist circumference (risk if >94cm men, >80cm women), waist-hip ratio, body fat percentage.
How to use the NexTools BMI calculator
Step 1: Enter weight in kg (or lbs).
Step 2: Enter height in meters/cm (or feet/inches).
Step 3: See your BMI and WHO category instantly.
Visual color indicators show your category. All processing in your browser — health data stays private.
For other percentage calculations, use the NexTools percentage calculator.
BMI by country: 2025 world comparison
| Country | Average adult BMI | % overweight/obese |
|---|---|---|
| USA | 28.8 | 73% |
| Mexico | 28.5 | 75% |
| UK | 27.3 | 64% |
| Germany | 26.3 | 53% |
| Japan | 23.5 | 27% |
| South Korea | 24.1 | 33% |
| India | 22.9 | 24% |
Data: NCD Risk Factor Collaboration / WHO 2025.
When to see a doctor based on your BMI
See a doctor now if: BMI below 16 (severe underweight), BMI above 40, or rapid unexplained weight change (±5 kg in a month).
Schedule a visit if: BMI 30-40, normal BMI but elevated waist circumference, or family history of diabetes/cardiovascular disease.
May not need consultation: BMI 25-29.9 without other risk factors — start with lifestyle changes. BMI 18.5-24.9 with normal complementary measures.
Important: BMI is only initial screening. A healthcare professional considers many more factors.
Alternative formulas to BMI
1. Body Adiposity Index (BAI): Uses hip circumference and height. No scale needed.
2. A Body Shape Index (ABSI): Combines waist, weight, and height. Better mortality predictor than BMI.
3. Waist-to-Height Ratio: Waist / Height. Above 0.5 = elevated risk. Simple and better cardiovascular predictor than BMI.
4. Body fat percentage: Measured via bioelectrical impedance, skinfold calipers, or DEXA. Most direct measure.
For exact age calculations (needed for some adjusted formulas), use the NexTools age calculator.
Common myths about BMI
Myth 1: "Normal BMI = healthy." False. You can have normal BMI but high blood pressure, high cholesterol, or visceral fat (TOFI: Thin Outside, Fat Inside).
Myth 2: "High BMI = unhealthy." Not necessarily. Athletes and muscular people can have high BMI and be perfectly healthy.
Myth 3: "I just need to lower my BMI." What matters are habits: balanced diet, regular exercise, adequate sleep, stress management. Obsessing over a number can lead to eating disorders.
Myth 4: "BMI works the same for all ethnicities." No. Asian populations have higher metabolic risk at lower BMI.
Try this tool:
Open tool→Frequently asked questions
How is BMI calculated exactly
Formula: BMI = weight (kg) / height (meters)². Example: 70 kg, 1.70 m: BMI = 70 / 2.89 = 24.2. Imperial: BMI = (weight in lbs / height in inches²) x 703.
What is the ideal BMI
WHO considers 18.5-24.9 normal weight. However, 'ideal' BMI varies by person, age, sex, ethnicity, and activity level. For Asians, the suggested 'normal' range is 18.5-22.9.
Is BMI reliable for athletes
Not for muscular athletes. BMI doesn't distinguish muscle from fat. A bodybuilder can have BMI 30 ('obese') but only 10% body fat. For athletes, body fat percentage is much better.
At what age can BMI be calculated
Standard WHO BMI categories are for adults 18+. For children/adolescents (2-17), age and sex-adjusted BMI percentiles are used.
Does BMI change with age
Body composition changes with age: muscle is lost and fat is gained. A BMI of 25 at age 30 and 70 represents very different compositions. Some experts suggest BMI 25-27 may be optimal for older adults.
Can I use imperial units (pounds and feet)
Yes. The NexTools calculator accepts weight in kg or lbs, and height in meters/cm or feet/inches. Conversion is automatic.