BMI Calculator — Body Mass Index for Adults

This free BMI calculator works out your Body Mass Index from your height and weight in metric or imperial units. BMI, defined by the World Health Organization as weight in kilograms divided by height in metres squared (kg/m²), is a quick screening measure that sorts adults into underweight, healthy, overweight, and obese ranges. It is a population-level guide rather than a diagnosis and cannot tell muscle from fat. This tool calculates everything locally in your browser for complete privacy.

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✨ Pro Tips for Best Results

  • Waist Measurement: Use BMI alongside waist circumference. A waist over 35 inches (women) or 40 inches (men) indicates higher health risks regardless of BMI.
  • Muscle Mass: If you lift weights regularly, your BMI may be high due to muscle, not fat. Focus on how your clothes fit.
  • Age Factor: For older adults (65+), a slightly higher BMI (25-27) may actually be protective against frailty and bone loss.

BMI vs. Body Fat Percentage: Which is better?

BMI is a proxy measure. It doesn't measure body fat directly but correlates with it. Body fat percentage is more accurate but requires specialized equipment like skinfold calipers or DEXA scans.

For most people, BMI is a sufficient "first step" to identify potential weight-related health risks. However, if you are a high-performance athlete or bodybuilder, BMI will likely categorize you as "overweight" or "obese" because muscle is much denser than fat. In these cases, focus on body fat percentage and metabolic markers like blood pressure and blood sugar.

How is BMI calculated?

Body Mass Index (BMI) is a numerical value derived from your weight and height, used as a screening tool for weight categories. The formula is BMI = weight (kg) / height (m)2. For imperial units: BMI = 703 × weight (lbs) / height (in)2.

Standard WHO categories: Underweight <18.5, Normal weight 18.5–24.9, Overweight 25–29.9, Obese ≥30. BMI is widely used because it requires no equipment and correlates reasonably well with body fat at the population level. However, it has known limitations: it cannot distinguish muscle from fat (athletes often show "overweight" BMI), doesn't account for fat distribution, and has different risk thresholds for different ethnicities (Asian populations face increased health risk at BMI >23). BMI is best used as one data point among many — including waist circumference, blood pressure, and blood markers — when assessing overall health.

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BMI scale (kg/m²)UnderHealthyOverObese18.52530
BMI screening bands. The healthy range (18.5–24.9) is highlighted; Asian-ancestry guidelines often lower the overweight threshold to 23.

What the BMI categories mean

Body Mass Index is weight in kilograms divided by height in metres squared (kg/m²). The WHO bands are: under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight, and 30+ obese. It is a population-level screening tool, not a diagnosis — a quick, cheap proxy for body fat that works well across large groups.

Where BMI breaks down

Because it only knows height and weight, BMI cannot distinguish muscle from fat. Muscular athletes routinely score 'overweight' despite low body fat, while older adults can score 'healthy' while carrying excess fat and little muscle. It also ignores fat distribution — visceral belly fat is more dangerous than the same weight on hips and thighs, which is why waist circumference is a useful companion measure.

Different cut-offs for different populations

People of South Asian, Chinese and other Asian descent face elevated diabetes and heart risk at lower BMIs, so many guidelines lower the overweight threshold to 23 and obesity to 27.5 for these groups. Treat BMI as one signal among several, not a verdict.

⚠️ Common Mistakes to Avoid

Frequently asked questions

What is BMI and how is it calculated?

BMI (Body Mass Index) is your weight in kilograms divided by your height in metres squared (kg/m²). It is a quick screening number, defined by the World Health Organization, that estimates whether your weight is in a healthy range for your height.

What is a healthy BMI range for adults?

For most adults the healthy range is a BMI of 18.5 to 24.9. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is classed as obese.

How accurate is BMI as a measure of health?

BMI is an accurate population-level screening tool but only a rough individual guide. Because it uses just height and weight, it cannot distinguish muscle from fat, so it can misclassify muscular athletes as overweight and older adults with low muscle as healthy.

What are the limitations of BMI?

BMI ignores body composition, fat distribution, age, sex, and ethnicity. Visceral belly fat is more dangerous than the same weight carried on the hips, which is why waist circumference is a useful companion measure.

Is BMI calculated differently for different ethnicities?

The formula is the same, but the risk thresholds differ. People of South Asian, Chinese, and other Asian descent face higher health risk at lower BMIs, so many guidelines lower the overweight cut-off to 23 and obesity to 27.5 for these groups.

BMI vs body fat percentage — which is better?

Body fat percentage is a more precise measure of health because it distinguishes fat from muscle, but it needs special equipment. BMI is free, instant, and good enough for general screening; use both together for a fuller picture.

Is BMI accurate for children or pregnant women?

No. Children use age- and sex-specific BMI percentile charts rather than fixed adult ranges, and BMI is not meaningful during pregnancy. This calculator is intended for non-pregnant adults.

Reviewed by the ToolsmithPro editorial team · Last updated June 2026. Every calculation and conversion runs entirely in your browser — your inputs are never uploaded, stored or shared. Formulas and methodology are documented on our about page; spot an error? tell us and we'll fix it.