Enter your height and weight to get your body mass index, ideal weight range and excess weight instantly. Add age and sex and your daily calorie need is included too.
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Body mass index is weight divided by the square of height in metres:
BMI = Weight (kg) ÷ Height² (m²)
For example, for someone 1.70 m tall weighing 85 kg:
That falls in the 25.0-29.9 band, so it is classed as overweight. If height is given in centimetres it must first be divided by 100 to convert to metres; that is the most common mistake in the calculation.
| Class | BMI range | Health risk |
|---|---|---|
| Underweight | below 18.5 | Increased |
| Normal | 18.5 - 24.9 | Low |
| Overweight | 25.0 - 29.9 | Slightly increased |
| Obesity class I | 30.0 - 34.9 | Moderately increased |
| Obesity class II | 35.0 - 39.9 | Markedly increased |
| Obesity class III | 40.0 and above | Very high |
Under the World Health Organization classification for adults, the healthy range is 18.5 to 24.9. Below that is underweight, and above it comes overweight and then obesity.
These numbers do not mean the same thing for everyone. The same BMI carries a higher risk in someone with a large waist than in someone with a narrow one. It can read misleadingly high in people with substantial muscle mass and misleadingly low in older people who have lost muscle.
The ideal weight range comes from the weights corresponding to the healthy BMI limits of 18.5 and 24.9. The formula is simply reversed:
Ideal weight = BMI × Height² (m²)
For someone 1.70 m tall the lower limit is 18.5 × 2.89 = 53 kg and the upper limit 24.9 × 2.89 = 72 kg. The healthy range for that height is therefore roughly 53 to 72 kilograms.
Excess weight is the difference between current weight and the upper limit of that range. In bariatric surgery, success is measured not by total weight lost but by how much of this excess weight is lost; the literature calls it "percentage excess weight loss".
The BMI classification is the same for women and men; there are no separate thresholds. The difference lies not in the limits but in what the same value means.
Women naturally carry a higher proportion of body fat, and that fat sits more around the hips and thighs. In men it tends to accumulate around the abdomen, a distribution that carries a higher risk of heart disease and type 2 diabetes. Waist measurement is therefore what distinguishes two people with the same BMI.
BMI is not used in pregnancy; weight is tracked against the pre-pregnancy value using separate criteria.
The adult classification is not used in children and adolescents. Because body composition changes constantly during growth, the calculated BMI is compared against age- and sex-specific percentile charts.
The general approach is that above the 85th percentile counts as overweight and above the 95th as obesity. The calculator on this page is therefore not suitable for anyone under 18; a paediatrician should assess a child's result.
Fat carried around the abdomen carries a markedly higher risk of heart disease, type 2 diabetes and fatty liver than fat around the hips and legs. Because BMI does not show that distribution, waist circumference is measured separately.
The widely accepted risk thresholds are:
The measurement is taken standing, after breathing out, at the narrowest point between the lowest rib and the hip bone.
The internationally accepted criteria are:
These numbers do not make the decision; they only open the assessment. Age, eating habits, previous weight-loss attempts, psychological readiness and general health are weighed together. Preparation before surgery is multidisciplinary, including endocrinology, dietetic and psychiatric assessment.
For people with a BMI between 27 and 35 who have not reached the surgical threshold, endoscopic options such as the gastric balloon and gastric botox can be considered. Which method suits you can only be determined after examination and the necessary investigations.
Weight in kilograms is divided by height in metres squared. For someone 1.70 m tall weighing 85 kg: 85 / (1.70 x 1.70) = 29.4. If height is known in centimetres it must first be divided by 100.
In adults the range considered healthy is 18.5 to 24.9. From 25 to 29.9 is overweight, and 30 and above is classed as obesity.
The ideal weight range is found by multiplying the square of your height by 18.5 and by 24.9. For someone 1.70 m tall that is roughly 53 to 72 kilograms. The calculator above does this for you.
No. A BMI between 30 and 35 is not on its own an indication for surgery. In this range, diet, physical activity and, where appropriate, medical treatment are considered first.
The generally accepted criterion is a body mass index of 40 or above, or between 35 and 40 with an associated condition such as type 2 diabetes, hypertension or sleep apnoea. These numbers do not decide on their own; the decision follows examination.
BMI does not distinguish muscle from fat. It can read high in people with substantial muscle mass without any excess fat. Body fat percentage is more informative in that situation.
No. In children and adolescents the result is compared against age- and sex-specific percentile charts. The calculator on this page is not suitable for anyone under 18.
Fat carried around the abdomen carries a higher risk of heart disease and type 2 diabetes than fat around the hips and legs. Waist circumference therefore complements BMI.
Disclaimer. This page is provided for general information only. Body mass index is a screening tool; it does not constitute a medical diagnosis, advice or a treatment recommendation, and it is not a substitute for examination by a physician. The values calculated are estimates and do not replace individual assessment. Whatever your result, discuss decisions about your health with your own doctor.