BMI Calculator and Weight Category Estimate

Calculate BMI using metric or imperial units

Body mass index uses height and weight to produce an adult screening estimate. Enter kilograms and centimetres or pounds, feet, and inches. The result includes an educational weight-category explanation.

What a BMI result can and cannot show

BMI does not distinguish muscle from body fat and is not a diagnosis. Interpretation can differ for children, pregnancy, older adults, athletes, and some population groups. Use the result as a starting point for a conversation with a qualified health professional.

WHO BMI categories for adults

Frequently asked questions

What is BMI and how is it calculated?

BMI (Body Mass Index) is a measure of body fat based on height and weight. It is calculated by dividing weight in kilograms by height in metres squared (kg/m²). For imperial units, the formula is (weight in pounds ÷ height in inches²) × 703. For example, a person weighing 70 kg who is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.

What are the BMI categories according to WHO?

The World Health Organization defines four BMI categories: Underweight (BMI below 18.5), Normal weight (BMI 18.5–24.9), Overweight (BMI 25.0–29.9), and Obese (BMI 30.0 and above). Obesity is further classified as Class I (30–34.9), Class II (35–39.9), and Class III — morbid obesity (40 and above).

Is BMI accurate for everyone?

BMI is a useful population-level screening tool but has well-known limitations for individuals. It does not distinguish between muscle mass and fat mass, so highly muscular athletes may have a high BMI without excess fat. It may also underestimate fat in older adults who have lost muscle. For children, a different age- and sex-specific BMI-for-age percentile system is used. Always combine BMI with other clinical assessments for a full health picture.

What BMI is considered overweight?

A BMI of 25.0 to 29.9 is classified as overweight by the WHO. Being overweight is associated with an increased risk of conditions including type 2 diabetes, hypertension, cardiovascular disease, and certain cancers. However, risk depends on many factors beyond BMI alone, including waist circumference, fitness level, and genetics.

Is BMI accurate for athletes and bodybuilders?

No. BMI is not accurate for athletes or bodybuilders because muscle tissue is denser than fat. A heavily muscular athlete may have a BMI above 25 or even above 30, which would technically classify them as overweight or obese, despite having very low body fat. In these cases, body fat percentage measurement (via DEXA scan, hydrostatic weighing, or skinfold calipers) is a more reliable assessment.

How is BMI calculated in metric units (kg and cm)?

To calculate BMI in metric units: divide your weight in kilograms by your height in metres squared. First convert your height from centimetres to metres by dividing by 100. For example: weight = 80 kg, height = 175 cm = 1.75 m. BMI = 80 ÷ (1.75 × 1.75) = 80 ÷ 3.0625 = 26.1. This would be classified as overweight.

What is the difference between BMI and body fat percentage?

BMI is an indirect estimate calculated from height and weight alone. Body fat percentage directly measures what proportion of your total body weight is fat tissue. A normal BMI (18.5–24.9) does not always mean healthy body fat levels — this condition, called 'normal weight obesity' or 'skinny fat', is associated with metabolic risks similar to obesity. Body fat percentage is a more direct measure but requires specialised equipment to assess accurately.

What is a healthy BMI range for women vs. men?

The WHO BMI categories (underweight/normal/overweight/obese) apply equally to adult men and women. However, women naturally carry a higher percentage of body fat than men at the same BMI. Some researchers suggest that women's optimal BMI range may differ slightly, and that waist-to-hip ratio is an additional important measure for women. Clinically, the same BMI cut-offs are used for both sexes in most healthcare guidelines.

Is the BMI calculation the same for children?

No. For children and adolescents (aged 2–19), BMI is assessed using BMI-for-age percentile charts rather than fixed cut-off numbers. This is because normal body fat varies with age and sex during growth. A child is classified as underweight (below 5th percentile), healthy weight (5th–84th), overweight (85th–94th), or obese (95th percentile or above).

What is morbid obesity BMI?

Morbid obesity (also called Class III obesity or severe obesity) is defined as a BMI of 40 or above. At this level, the risk of serious weight-related health conditions — including type 2 diabetes, heart disease, sleep apnoea, joint problems, and certain cancers — is substantially elevated. BMI of 35–39.9 with a serious weight-related health condition is also classified as Class II severe obesity and may qualify for bariatric surgery consideration.

Can BMI be used to assess disease risk?

BMI is one of several tools used to estimate cardiovascular, metabolic, and cancer risk. Higher BMI is associated with increased risk of type 2 diabetes, hypertension, dyslipidaemia, coronary artery disease, stroke, sleep apnoea, and certain cancers (breast, colon, endometrial). However, BMI alone does not determine individual disease risk — waist circumference, blood pressure, blood glucose, and lipid levels all contribute to a complete risk assessment.

What should I do if my BMI is outside the healthy range?

If your BMI is below 18.5 or above 24.9, speak with your GP or a registered dietitian. They can perform a full clinical assessment, including waist circumference, blood tests, and lifestyle evaluation. Small, sustainable changes to diet and physical activity are more effective long-term than rapid weight loss or gain. Avoid crash diets or extreme exercise programmes without medical supervision.

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