BMI Calculator

BMI calculator: what your number actually means

Body Mass Index (BMI) is the most widely used screening number for body weight in clinical and public-health practice. Our calculator gives you the BMI value, the WHO weight category, and the weight range that would put you in the healthy band for your height. Below is a complete explanation of what the number does and does not tell you, the ethnicity-specific cut-offs, and the limitations every BMI calculator should mention but most don't.

The BMI formula

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

Example: a 70 kg adult who is 1.70 m tall has a BMI of 70 ÷ (1.70 × 1.70) = 24.2. The height is squared, which means small changes in height meaningfully shift the score — people often forget to use their height in metres, not centimetres.

WHO standard BMI categories (adults)

• Underweight: BMI under 18.5
• Healthy weight: BMI 18.5 to 24.9
• Overweight: BMI 25.0 to 29.9
• Obesity class I: BMI 30.0 to 34.9
• Obesity class II: BMI 35.0 to 39.9
• Obesity class III (severe): BMI 40.0 and above

Lower thresholds for Asian populations

The WHO recommends modified cut-offs for many Asian populations because cardio-metabolic risk (diabetes, heart disease) appears at lower BMIs than in European populations:

• Overweight: BMI 23.0–27.4
• Obese: BMI 27.5 and above

If you are of South Asian (Pakistan, India, Bangladesh, Sri Lanka), East Asian (China, Japan, Korea), or Southeast Asian descent, the adjusted thresholds are the more clinically relevant ones to use. Many doctors in those regions diagnose "overweight" at BMI 23 — not 25.

What BMI gets right

Across very large populations, BMI correlates well with body fat and with risk of weight-related illness: type 2 diabetes, cardiovascular disease, sleep apnoea, certain cancers, and joint disease. For an average sedentary adult, a BMI above 30 is almost always associated with higher health risk than a BMI in the 20–25 range.

What BMI gets wrong (and how to compensate)

BMI was designed by a Belgian statistician in the 1830s as a tool for studying populations, not individuals. It has well-known blind spots:

• Muscle mass: a muscular person can register as overweight or obese despite very low body fat. Most elite athletes have BMIs above 25.
• Body shape: two people with identical BMI can have very different fat distributions. Visceral (belly) fat is more dangerous than subcutaneous (under-skin) fat. Waist circumference catches this; BMI does not.
• Age: body composition shifts with age. An older adult with a BMI of 23 can have a higher fat percentage than a 25-year-old with a BMI of 25.
• Height extremes:very short and very tall people's BMIs are slightly distorted by the squared-height denominator.
• Pregnancy: BMI does not apply during pregnancy. Use gestational weight-gain charts instead.

For a personal health picture, pair your BMI with two other measurements: waist circumference (under 94 cm / 37 in for men, 80 cm / 31.5 in for women indicates lower cardio-metabolic risk) and waist-to-height ratio (under 0.5 is the target). These three numbers together tell a much more complete story than any one alone.

Healthy weight range for your height

The calculator above shows the weight range that would put you in the 18.5–24.9 healthy band. For a 1.70 m adult that range is roughly 53–72 kg; for a 1.80 m adult it is 60–81 kg. The width of the healthy range expands with height because of the squared denominator — taller people have more allowed variation.

If you want to change your BMI

For most adults, a sustainable loss of 0.5–1% body weight per week is the maximum that preserves muscle mass and is realistic to maintain. Faster loss almost always rebounds. The most predictive lifestyle changes — across hundreds of randomised trials — are: strength training 2–3× per week, walking 7,000+ steps daily, eating 1.6 g of protein per kg of target body weight per day, and sleeping 7+ hours nightly.

Before starting any deliberate weight-change programme, especially if your BMI is under 18 or over 35, talk to a doctor or registered dietitian. A BMI value is a screening tool, not a diagnosis.

Related calculators

• Percentage Calculator — for quick math on weight-change goals.
• Age Calculator — for accurate age in years, months, and days.
• Weight Converter — convert between kg, lb, st, oz, and more.