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BMI Calculator

Body Mass Index gives a quick idea of whether your weight is in a healthy range for your height. Pick your unit system and enter your details below.

Last updated: 11 Nov 2026 Read time: 5 min Category: Health Clinically reviewed
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What BMI Is

Body Mass Index, usually shortened to BMI, is a ratio that compares a person's weight to their height. It was devised in the 1830s by the Belgian statistician and astronomer Adolphe Quetelet, originally as the Quetelet Index for use in demographic studies rather than medicine. It was renamed BMI by physiologist Ancel Keys in a 1972 paper and later adopted by the World Health Organization in the 1990s as a global screening metric for underweight, overweight, and obesity in adult populations.

BMI matters because excess body weight is a well-documented risk factor for type 2 diabetes, cardiovascular disease, certain cancers, osteoarthritis, and sleep apnoea. It is cheap to measure, reproducible, and can be calculated at home with just a scale and a tape measure. However, BMI treats everyone the same regardless of muscle mass, bone density, fat distribution, age, or ethnicity. That makes it a useful population-level indicator but an imperfect tool for any single individual. Clinicians combine BMI with waist circumference, blood pressure, and metabolic markers before drawing conclusions.

The Formula

In metric units: BMI = weight (kg) / height (m)^2. In imperial units: BMI = 703 x weight (lb) / height (in)^2. The constant 703 converts pounds and inches into the metric result so the numeric scale matches. Worked example: a person weighing 70 kg at 1.75 m tall has BMI = 70 / (1.75 x 1.75) = 70 / 3.0625 = 22.86, which falls squarely in the WHO normal range.

A second worked example in imperial units: 165 lb at 5 feet 9 inches, which is 69 inches. BMI = 703 x 165 / (69 x 69) = 703 x 165 / 4761 = 24.37, still within the normal range but close to the overweight threshold.

How to Use This Tool

  1. Pick Metric (kg, cm) or Imperial (lb, ft/in) depending on which units you are most comfortable with.
  2. Enter your height. For imperial users, put feet and inches in separate fields.
  3. Enter your weight.
  4. Click Calculate BMI.
  5. Read the numeric BMI value and the category badge: underweight, normal, overweight, or obese.
  6. Compare with the Healthy Range reference of 18.5 to 24.9 for adults.

WHO Adult BMI Categories

  • Below 18.5 (Underweight): may indicate undernutrition, hyperthyroidism, or an underlying illness.
  • 18.5 to 24.9 (Normal weight): associated with the lowest all-cause mortality in large cohort studies.
  • 25.0 to 29.9 (Overweight): moderately raised risk of hypertension and metabolic syndrome.
  • 30.0 to 34.9 (Obese class I): clearly raised risk of type 2 diabetes and cardiovascular disease.
  • 35.0 to 39.9 (Obese class II): high risk, often with existing comorbidity.
  • 40.0 and above (Obese class III): very high risk, sometimes called severe or morbid obesity.

Common Mistakes to Avoid

  • Rounding height to the nearest inch when your true height is between two values. A 2 cm error at 170 cm shifts BMI by about 0.5 points.
  • Weighing after a large meal or drink. Daily weight fluctuates by 1 to 2 kg from water alone.
  • Relying on BMI alone during a heavy strength training phase. Waist circumference is a better indicator of visceral fat.
  • Treating a single BMI reading as a diagnosis. Trend over months matters more than a one-off value.
  • Applying adult BMI bands to children or teenagers. Growth charts and percentiles are more appropriate.

Limitations and Alternatives

BMI does not distinguish between fat, muscle, bone, and water. A 185 cm rugby player with 95 kg of lean muscle has a BMI of about 27.8, which the formula labels overweight even though body fat is low. Conversely, an older adult who has lost muscle through sarcopenia may have a healthy BMI but a high body fat percentage. These edge cases motivate the use of complementary measurements.

Common alternatives include waist circumference (an abdominal measurement that correlates well with visceral fat; a waist of more than 102 cm in men or 88 cm in women signals raised risk), waist-to-height ratio (should ideally be under 0.5), body fat percentage from skinfold calipers or bio-impedance scales, and clinically, DEXA scans. For people of South Asian, East Asian, and Southeast Asian descent, the WHO 2004 expert consultation recommended lower cutoffs (overweight at 23, obesity at 27.5) given that cardiometabolic risk emerges at lower BMI in these populations.

Important BMI is a screening tool and not a diagnosis. If your BMI is outside the normal range, or if you have concerns about your weight, blood sugar, or blood pressure, please consult a qualified healthcare provider. Do not make medical decisions based on this calculator alone.

Privacy

Your height and weight are processed entirely in the browser. Nothing is sent to our servers, cookies, or any third party.

Frequently Asked Questions

Does BMI work for children and teens?

This calculator uses adult ranges. For children and teens under 20, BMI is interpreted against age and sex specific percentile charts published by the CDC and WHO. A paediatrician or school nurse can map a child's BMI to the correct percentile band.

Is BMI accurate for muscular people?

Not always. A bodybuilder or elite athlete can measure as overweight or even obese by BMI despite having low body fat because muscle is denser than fat. Combine BMI with waist circumference, body fat percentage, or guidance from a clinician for a complete picture.

Can pregnant women use this calculator?

Standard BMI ranges do not apply during pregnancy because weight includes the baby, placenta, and extra fluid. Antenatal clinics track weight gain against pre-pregnancy BMI using separate Institute of Medicine guidelines.

Is there a different BMI cutoff for people of Asian descent?

Yes. The WHO 2004 expert consultation noted that cardiovascular risk begins at lower BMI values in many Asian populations. Public health bodies in India, China, Japan, and Singapore use an overweight cutoff of 23 and obesity cutoff of 27.5. This calculator reports the standard WHO adult bands.

How accurate is BMI as a health indicator?

BMI is a screening tool, not a diagnosis. It correlates with population level disease risk but can misclassify individuals, particularly those with very high muscle mass, older adults with reduced muscle, and some ethnic groups. Clinicians complement BMI with waist circumference, blood pressure, and metabolic markers.

Is a BMI of 25 dangerous?

A single reading of 25 is not dangerous by itself. It marks the WHO threshold where population average health risk begins to rise, especially for type 2 diabetes and cardiovascular disease. Consistent lifestyle habits matter more than any single number.

Are my inputs stored?

No. Your height and weight stay on your device. All math runs in the browser and nothing is transmitted or logged.