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BMI is weight in kilograms divided by the square of height in metres. Categories follow WHO cut-offs: under 18.5 underweight, 18.5-24.9 healthy, 25-29.9 overweight, 30+ obese. BMI is a screening tool, not a diagnosis — athletes with high muscle mass may score high without excess fat.
Body Mass Index is your weight in kilograms divided by the square of your height in metres:
BMI = weight (kg) / height (m)²
If you measure in pounds and inches, use the imperial version:
BMI = 703 × weight (lb) / height (in)²
Worked example (metric): a person weighing 70 kg at 170 cm (1.70 m). Square the height: 1.70 × 1.70 = 2.89. Divide the weight: 70 / 2.89 = 24.2. A BMI of 24.2 falls in the healthy-weight range.
Worked example (imperial): a person weighing 160 lb at 5 ft 7 in (67 in). Square the height: 67 × 67 = 4,489. Divide: 160 / 4,489 = 0.03565. Multiply by 703: 0.03565 × 703 = 25.1. A BMI of 25.1 is just above the healthy range.
The World Health Organization groups adult BMI into four main categories. Obesity is further split into three classes:
| BMI range | Category | |---|---| | Below 18.5 | Underweight | | 18.5 - 24.9 | Healthy weight | | 25.0 - 29.9 | Overweight | | 30.0 - 34.9 | Obesity (class I) | | 35.0 - 39.9 | Obesity (class II) | | 40.0 and above | Obesity (class III) |
These cut-offs apply to adults aged 20 and over. For children and teenagers, BMI is read as a percentile against peers of the same age and sex.
BMI categories explained: the chart above follows the standard WHO bands used by health agencies worldwide.
Use this free BMI calculator to calculate your Body Mass Index in seconds: 1. Enter your weight in kilograms (kg). 2. Enter your height in centimetres (cm). 3. Your Body Mass Index and weight category appear instantly, together with the healthy weight range for your height.
Tip: measure your height without shoes and your weight on a flat, hard surface in the morning for the most consistent readings.
The healthy range (BMI 18.5 - 24.9) translates into a weight band that depends only on your height, not your sex:
| Height | Healthy weight range | |---|---| | 160 cm | 47.4 - 63.7 kg | | 165 cm | 50.4 - 67.8 kg | | 170 cm | 53.5 - 72.0 kg | | 175 cm | 56.7 - 76.3 kg | | 180 cm | 59.9 - 80.7 kg | | 185 cm | 63.3 - 85.2 kg |
The range is wide on purpose: healthy weight is a band, not a single number.
Many people treat this band as the ideal weight range for their height.
BMI is a screening number, not a diagnosis and not medical advice. It cannot tell muscle from fat, so a strong, lean person can land in the overweight range. It also misses where fat is stored, and it can read differently with age, pregnancy, and some ethnic backgrounds.
Use your result as a starting point, not a verdict. Pair it with waist measurement, blood pressure, blood sugar, and cholesterol checks, and talk to a healthcare professional about what your number means for you personally.
For most adults, checking BMI every few months is plenty. Body weight shifts from day to day with water, meals, and activity, so daily checks add noise rather than information. What matters is the trend over time: a steady drift upward or downward tells you far more than any single reading.
For adults aged 20 and over, a healthy BMI is between 18.5 and 24.9. This range is associated with the lowest risk of weight-related health problems for most people.
Divide your weight in kilograms by your height in metres squared: BMI = weight (kg) / height (m)². With pounds and inches, the formula is BMI = 703 × weight (lb) / height (in)².
BMI is a useful screening tool for most adults, but it has limits. It may overestimate body fat in very muscular people and underestimate it in older adults who have lost muscle. It should not be used during pregnancy, and adult cut-offs do not apply to children.
BMI counts all body weight the same — muscle, bone and water included. A lean, strong person can therefore show an overweight BMI despite low body fat. Checking waist size or body fat percentage alongside BMI gives a fuller picture.
The BMI formula and category cut-offs are the same for men and women. Body composition differs, though: women typically carry more body fat than men at the same BMI, so two people with the same number can look quite different.
A BMI of 30.0 or higher is classified as obese. Obesity is further divided into class I (30.0 - 34.9), class II (35.0 - 39.9) and class III (40.0 and above).
They answer different questions. BMI is a quick screen from height and weight alone. Body fat percentage shows how much of your weight is fat, but it needs tools such as calipers, a smart scale or a DEXA scan, and home methods vary in accuracy. Many doctors start with BMI and add other tests when needed.
Every few months is enough for most adults. Body weight changes from day to day, so frequent checks add little information — the long-term trend matters more than any single reading.
No. The calculation runs entirely in your browser, and your height and weight are never uploaded or saved.
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