BMI Calculator

Calculate your Body Mass Index instantly. See your BMI category, where you fall on the scale, your ideal weight range, and exactly how much to gain or lose to reach a healthy BMI.

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What Is BMI and Why Is It Used?

Body Mass Index (BMI) is one of the most widely used screening tools in medicine and fitness. Developed in the 1830s by Belgian mathematician Adolphe Quetelet, it provides a simple ratio of weight to height that correlates, at a population level, with body fatness and associated health risks.

BMI is used by healthcare providers as a quick first-pass assessment. It costs nothing, requires no equipment, and takes seconds to calculate. For population studies and clinical screenings, it remains the standard despite its well-known limitations.

The BMI Formula Explained

The formula is straightforward: divide your weight in kilograms by your height in metres, squared.

Metric: BMI = weight (kg) ÷ height (m)²
Imperial: BMI = [weight (lbs) ÷ height (in)²] × 703

For example, a person who is 175 cm (1.75 m) and weighs 70 kg has a BMI of 70 ÷ (1.75²) = 22.9, which falls in the Normal weight range. The result is a unitless number, the same whether you use metric or imperial inputs.

BMI Categories and What They Mean

The four standard adult BMI categories, as defined by the World Health Organization, are:

Underweight, below 18.5: May indicate malnutrition, eating disorders, or other health issues. Can affect immune function, bone density, and reproductive health.
Normal weight, 18.5 to 24.9: Associated with the lowest risk of weight-related disease in population studies. Not a guarantee of good health, but a positive indicator.
Overweight, 25 to 29.9: Elevated risk of cardiovascular disease, hypertension, and type 2 diabetes. Many people in this range are otherwise healthy, particularly if they are muscular.
Obese, 30 and above: Significantly elevated health risks. Further subdivided into Class I (30–34.9), Class II (35–39.9), and Class III (40+).

Limitations of BMI You Should Know

BMI is a screening tool, not a diagnosis. Its most significant limitations include:

Muscle mass: Bodybuilders and athletes often have "overweight" or "obese" BMIs despite very low body fat. Muscle weighs more than fat per unit volume.
Age: Older adults tend to have more body fat at the same BMI as younger adults, since muscle mass naturally decreases with age.
Sex: Women naturally carry more body fat than men at the same BMI. The same BMI may represent different health profiles for men vs women.
Ethnicity: Asian populations show increased cardiometabolic risk at BMI values below the standard overweight threshold (23 vs 25).
Fat distribution: BMI tells you nothing about where fat is stored. Visceral fat (around the organs) carries higher health risk than subcutaneous fat.

How to Move Your BMI Into a Healthy Range

If your BMI falls outside the 18.5–24.9 range, the path to a healthier number is straightforward in principle, though the execution requires consistency.

For those with a BMI above 25, a moderate calorie deficit of 300–500 kcal/day combined with resistance training produces the best long-term results. This rate of loss (0.25–0.5 kg/week) preserves muscle while reducing fat, which improves body composition beyond what the BMI number alone captures.

For those with a BMI below 18.5, a gradual calorie surplus of 200–300 kcal/day, combined with strength training, can add healthy lean mass. Focus on protein-rich whole foods and progressive resistance exercise to ensure weight gained is mostly muscle rather than fat.

Frequently Asked Questions

What is BMI?

BMI (Body Mass Index) is a numerical value calculated from your height and weight. It provides a general indication of whether your weight falls within a healthy range for your height. BMI is calculated by dividing weight in kilograms by height in metres squared (kg/m²).

What is a healthy BMI range?

For adults, the standard BMI categories are: Underweight (below 18.5), Normal weight (18.5–24.9), Overweight (25–29.9), and Obese (30 and above). A BMI between 18.5 and 24.9 is considered a healthy weight range for most adults.

Is BMI an accurate measure of health?

BMI is a useful screening tool but has limitations. It does not account for muscle mass, bone density, age, sex, or fat distribution. Athletes and muscular individuals may have a high BMI despite being very lean, while older adults can have a normal BMI with excess body fat. Always consider BMI alongside other health markers.

What is the difference between BMI and body fat percentage?

BMI uses only height and weight, making it quick and easy to calculate but less precise. Body fat percentage directly measures how much of your body is fat vs lean tissue. Two people with identical BMIs can have very different body compositions, a reason why body fat calculators are often used alongside BMI.

Does BMI apply to children?

For children and teenagers (ages 2–19), BMI is calculated the same way but interpreted differently using age- and sex-specific percentile charts, not fixed cut-offs. The adult categories above apply only to those 20 years and older.

Can BMI be different for different ethnic groups?

Yes. Research shows that people of Asian descent may have higher health risks at lower BMI values. Some organisations recommend a lower "overweight" threshold of 23 for Asian adults. Latin American, Middle Eastern, and South Asian populations may also have different risk profiles at the same BMI.

How do I lower my BMI?

Reducing BMI to a healthy range involves losing body fat through a moderate calorie deficit (eating 300–500 kcal/day below your TDEE) combined with resistance training to preserve or build muscle. Aim for 0.25–0.5 kg of weight loss per week for sustainable, muscle-preserving results.

What BMI is considered obese?

A BMI of 30 or above is classified as obese. This is further divided into Class I (30–34.9), Class II (35–39.9), and Class III or "severe obesity" (40 and above). At these levels, the risk of cardiovascular disease, type 2 diabetes, and other conditions increases significantly.

How it's calculated: this tool uses established, published equations. See the full calculator methodology for the formulas and assumptions behind it.

Results are estimates for general guidance, not medical advice. See our disclaimer, and speak to a qualified healthcare professional before making medical or dietary changes.