Complete Guide to BMI: How to Calculate and Interpret Your Body Mass Index
Body Mass Index (BMI) is one of the most widely used health screening tools in the world. This guide explains how BMI is calculated, what your number actually means, where the metric falls short, and how to use it alongside other indicators to get a fuller picture of your health.
BMI is a simple ratio of your weight to your height. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet — which is why it is sometimes called the Quetelet index — and was adopted as a population health screening tool in the 20th century. Today the World Health Organization (WHO) uses BMI as the standard reference for classifying underweight, normal weight, overweight, and obesity in adults.
Despite its age and simplicity, BMI remains useful precisely because it requires no equipment more sophisticated than a scale and a tape measure. You can calculate it in a few seconds, and the result correlates broadly with body fat percentage and risk of weight-related health conditions. That said, BMI is a screening tool, not a diagnosis. It works best for sedentary adults of average build and starts to mislead at the extremes — for athletes, the elderly, pregnant people, and those with very different body compositions.
The BMI formula
BMI is calculated by dividing your weight in kilograms by the square of your height in meters: BMI = kg / m². If you prefer imperial units, the equivalent formula is BMI = (lbs / in²) × 703. Both produce the same number; only the units differ.
Squaring the height — rather than using it linearly — is what makes BMI roughly independent of how tall you are, because mass tends to scale with the square of height across most adult body shapes. This is also where some of BMI's limitations come from: very tall people often score slightly higher, and very short people slightly lower, than their actual body fat would suggest.
WHO BMI categories for adults
The WHO classifications below apply to adults aged 20 and over. Children and teenagers use age- and sex-specific percentile charts instead, because their bodies are still developing.
| BMI range | Category | General health risk |
|---|---|---|
| Below 18.5 | Underweight | Increased risk of nutrient deficiencies and osteoporosis |
| 18.5 – 24.9 | Normal weight | Lowest associated health risk |
| 25.0 – 29.9 | Overweight | Mildly elevated risk for type 2 diabetes and heart disease |
| 30.0 – 34.9 | Obesity class I | Moderately elevated risk |
| 35.0 – 39.9 | Obesity class II | High risk |
| 40.0 and above | Obesity class III | Very high risk; sometimes called severe obesity |
How to use this calculator
- 1
Enter your height
Choose either centimeters or feet/inches and enter your standing height. Take the measurement without shoes, with your back straight against a wall.
- 2
Enter your weight
Enter your weight in kilograms or pounds. Weighing yourself in the morning, after using the bathroom and before eating, gives the most consistent reading.
- 3
Read your BMI and category
The calculator will instantly compute your BMI and place you into one of the WHO categories. The number is for reference only — see the limitations section below before drawing conclusions.
Where BMI breaks down
BMI does not distinguish between fat, muscle, bone, or water. A bodybuilder with very low body fat can easily score in the obese range simply because muscle is denser than fat. Conversely, an older adult who has lost muscle mass to sarcopenia may have a normal BMI while carrying an unhealthy amount of visceral fat — a condition sometimes called 'normal weight obesity.'
Population-level cutoffs also do not work equally well for every ethnic group. Research has shown that people of South Asian, East Asian, and Pacific Islander descent tend to develop diabetes and cardiovascular disease at lower BMIs than people of European descent. Several countries — including Japan, China, and India — use lower cutoffs for overweight (often 23) and obesity (often 25 or 27.5) to reflect this.
BMI is not appropriate during pregnancy, for children under 20 (use the CDC growth charts instead), or for people with limb amputations or severe scoliosis. If any of these apply to you, talk to a healthcare provider about more relevant measurements.
Better metrics to use alongside BMI
If you are using BMI to think about your health, pair it with one or two of these more direct measurements:
- •Waist circumference — abdominal fat is a stronger predictor of cardiovascular risk than BMI alone. Above 94 cm (37 in) for men or 80 cm (31.5 in) for women is considered elevated.
- •Waist-to-height ratio — keep your waist under half your height. This is one of the simplest and most predictive single metrics for cardiometabolic risk.
- •Body fat percentage — measured with skinfold calipers, bioelectrical impedance scales, or DEXA scans. More accurate than BMI but requires equipment.
- •Resting heart rate, blood pressure, and lipid panel — your numbers on these tell you far more about your cardiovascular health than any anthropometric measurement.
Extended FAQ
Is the same BMI healthy for men and women?
The WHO categories are identical for men and women, but the underlying body composition is not. Women naturally carry more body fat at the same BMI, while men carry more muscle. The cutoffs work as a population-level screen for both sexes despite this difference.
Does BMI account for age?
It does not. As people age, they tend to lose muscle mass and gain visceral fat even if their weight stays the same — meaning BMI can underestimate body fat in older adults. Some clinicians use slightly higher BMI ranges (24–29) as the 'optimal' window for adults over 65.
Is the BMI shown by this calculator stored anywhere?
No. This calculator runs entirely in your browser. Your height and weight are never sent to any server, never logged, and disappear when you close the tab.
How accurate is BMI compared to a DEXA scan?
BMI agrees with DEXA-measured body fat percentage about 70–80% of the time at a population level, but it can be off by 5–10 percentage points for individuals — especially athletes, older adults, and people of Asian descent. If you want a real measurement of body fat, a DEXA scan is the gold standard.
What should I do if my BMI is in the overweight or obese range?
Don't panic, and don't take BMI alone as a diagnosis. Speak with a healthcare provider, get your waist measured, and ask about a basic metabolic panel. Sustainable changes — moving more, sleeping enough, and eating more whole foods — generally outperform aggressive short-term diets.