BMI Calculator

BMI measures your weight relative to your height. It divides your weight in kilograms by your height in metres squared and produces a single number that places you in a weight category. What it does not measure is how much of your weight is fat versus muscle versus bone. Two people with identical BMI values can have very different body compositions and very different health risks. Think of it as the initial signal that determines whether a more detailed examination is worthwhile rather than a complete health assessment on its own.

BMI Prime is your BMI divided by 25. A value of 1.0 means your BMI is exactly at the top of the healthy range. Values below 1.0 are in the normal or underweight zone. Values above 1.0 indicate overweight or obesity. The main advantage of BMI Prime is that it makes it instantly clear how far you are from the healthy boundary. A BMI Prime of 1.20 tells you your BMI is 20 percent above the upper normal limit which is more intuitive than knowing your absolute BMI value of 30 and having to recall where the cutoffs sit.

The Ponderal Index divides weight in kilograms by height in metres cubed rather than squared. Cubing height makes the formula scale more accurately with body volume which means it gives more reliable results for people who are very tall or very short. BMI tends to overestimate body fat for tall people and underestimate it for short people. The normal Ponderal Index range for adults is approximately 11 to 14 kg per cubic metre. For most people of average height both BMI and Ponderal Index tell a similar story. The Ponderal Index is most valuable as a cross-check when someone falls at the extremes of the height range.

The World Health Organization defines the healthy adult range as 18.5 to 24.9. Below 18.5 is underweight and the WHO further divides this into mild thinness at 17 to 18.5, moderate thinness at 16 to 17, and severe thinness below 16. From 25 to 29.9 is overweight. A BMI of 30 and above is obese and is divided into Class I at 30 to 34.9, Class II at 35 to 39.9, and Class III at 40 and above. These thresholds apply to adults of European descent. For people of South and East Asian descent the WHO recommends lower action points of 23 for overweight and 27.5 for obesity.

No. Muscle is considerably denser and heavier than fat so athletes and people who train regularly often receive overweight or obese BMI readings despite carrying little excess body fat. A competitive powerlifter or rugby player at peak condition may have a BMI above 30 while being physically very healthy. For people with high muscle mass waist circumference and body fat percentage from bioelectrical impedance or a DEXA scan are far more meaningful measures than BMI or BMI Prime.

Research consistently shows that adults of East and South Asian descent carry more body fat and face higher risk of type 2 diabetes and cardiovascular disease at any given BMI compared with people of European descent. The WHO has formally stated that action points of 23 for overweight and 27.5 for obesity are more appropriate for these populations. Many health authorities across Asia and in countries with large Asian communities have adopted these lower thresholds. Tick the Asian descent checkbox in this calculator to switch to these adjusted values automatically.

Waist circumference measures the fat stored around your abdomen which is the most metabolically dangerous type of fat. Abdominal fat is more strongly linked to insulin resistance and heart disease than fat stored in the hips or thighs. A large waist can signal elevated health risk even when BMI appears normal. For men a waist above 94 cm signals elevated risk and above 102 cm indicates substantially increased risk. For women the thresholds are 80 cm and 88 cm respectively. Using both waist circumference and BMI together gives a substantially more complete picture than either measurement alone.

This calculator uses the standard adult formula which only applies from age 18 onwards. Children and teenagers are assessed using BMI for age percentile charts published by the Centers for Disease Control and Prevention and national health bodies because their height and body proportions change rapidly during development. A number that would indicate underweight in an adult might be entirely normal for a growing 14 year old. For anyone under 18 a paediatrician or school health nurse should apply the correct age-specific growth reference charts.

The most sustainable approach is losing around 0.5 to 1 kilogram per week through a modest calorie reduction combined with regular physical activity. Walking 30 minutes a day and eating more protein and vegetables are practical starting points that produce consistent results. Very low calorie diets produce rapid initial weight loss but most people regain the weight within one to three years because of the metabolic adaptations triggered by severe restriction. If your BMI is above 30 involving a doctor or registered dietitian is the right starting point for a safe and medically appropriate plan.

No. From the mid thirties most adults gradually lose muscle and gain fat even when their weight stays the same on the scale. This means a BMI of 24 at age 60 often represents a higher body fat percentage than the same number did at age 30. Some researchers suggest a BMI up to 27 may be acceptable for adults over 65 because a small reserve of body fat can protect against illness and hospitalisation. For older adults muscle mass and physical strength are often more meaningful health indicators than BMI alone.

The formula and the category thresholds are identical for both sexes. However at any given BMI value women tend to carry a higher percentage of body fat than men because women have more essential fat and typically less skeletal muscle on average. This means two people with the same BMI number can have meaningfully different body compositions depending on their sex. Body fat percentage measurement is a more direct way to account for these sex-based differences when precision is important.

Overweight at 25 to 29.9 carries moderately elevated risk for most people. Obese Class I at 30 to 34.9 is linked to high risk of type 2 diabetes and heart disease and typically calls for structured lifestyle intervention. Class II at 35 to 39.9 carries very high risk and usually involves a combination of medical supervision and lifestyle change. Class III at 40 and above carries extremely high risk and is where evidence-based medical options such as pharmacotherapy or bariatric surgery are considered alongside lifestyle intervention.

Small differences almost always come from measurement variation. Clinical measurements are taken without shoes and with light clothing using calibrated stadiometers and scales that are more precise than home equipment. Rounding your height by even one centimetre shifts the BMI by a few tenths. A difference of less than one BMI point between your number and your doctor's is completely normal and falls within standard measurement variation. If the difference is larger it is worth asking your doctor for the exact recorded measurements and re-entering those figures here.