BMI Calculator

Enter your height and weight to calculate your Body Mass Index.

Last updated: April 2026

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Person stepping onto a bathroom scale to measure their weight for a BMI calculation

BMI categories

The World Health Organisation defines four primary BMI categories for adults. The table below shows each range and what it signals about weight status.

Category BMI range What it means
Underweight < 18.5 May indicate insufficient nutrition or an underlying health condition.
Normal weight 18.5 – 24.9 Associated with the lowest risk of weight-related health problems.
Overweight 25.0 – 29.9 Elevated risk of cardiovascular disease and type 2 diabetes.
Obese class I 30.0 – 34.9 Moderately increased risk; lifestyle changes typically recommended.
Obese class II 35.0 – 39.9 High risk; medical evaluation is usually advised.
Obese class III 40+ Very high risk; also called severe or morbid obesity.

BMI (Body Mass Index) is a number derived from height and weight that gives a quick indication of whether a person's weight falls within a healthy range. It is used in clinical screenings, population health studies, and insurance assessments worldwide. The key thing to understand: it is a screening tool, not a diagnosis — the categories flag when further investigation may be warranted, not that something is definitely wrong.

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Quick reference

Understanding BMI

What BMI measures

BMI is a ratio of weight to height squared. It was designed to normalise weight across people of different heights, so you can compare a 1.60 m person and a 1.90 m person on the same scale. A taller person is expected to weigh more, and the formula accounts for this — but only roughly.

What BMI does not measure

BMI has no knowledge of body composition. Two people with the same BMI could have very different amounts of muscle and fat. A rugby player and a sedentary office worker might both show BMI 27, but their health risk profiles are completely different. BMI also does not measure where fat is stored — visceral fat around the abdomen carries more risk than subcutaneous fat elsewhere.

Age and BMI

The standard adult thresholds (18.5–24.9 etc.) apply to people aged 18 and over. For children and teenagers, BMI is interpreted using age- and sex-specific percentile charts because body composition changes significantly during growth. Older adults (65+) may have a slightly higher optimal BMI due to muscle loss with age.

Common mistakes

Which category should concern you?

Underweight (<18.5): Worth discussing with a doctor, particularly if unintentional. May indicate nutritional deficiency or an underlying condition.

Normal weight (18.5–24.9): No action needed on BMI alone. Maintain with regular activity and a balanced diet.

Overweight (25–29.9): Modest increased risk. Small reductions in weight (5–10%) have meaningful health benefits for many people.

Obese (30+): Consider a consultation with a GP. Risk increases across classes I, II, and III. Medical support is available and effective.

Frequently asked questions

What is a healthy BMI?

For adults, a BMI between 18.5 and 24.9 is considered normal weight. Below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is obese.

Is BMI accurate?

BMI is a useful screening tool but has real limitations. It does not distinguish between fat and muscle, so a muscular athlete and a sedentary person can share the same BMI. It also does not account for where fat is stored, which affects health risk significantly.

What is my BMI if I am 180 cm and 80 kg?

Your BMI is 24.7, which falls in the normal weight range (18.5–24.9).

How is BMI calculated?

BMI is calculated by dividing your weight in kilograms by your height in metres squared. For example: 80 kg ÷ (1.80 × 1.80) = 24.7.

What is a healthy BMI for a woman?

The healthy BMI range is the same for men and women: 18.5 to 24.9. However, women naturally carry a higher percentage of body fat than men at the same BMI, so some health professionals consider the threshold differently depending on age and build.

Who invented the BMI?

BMI was invented by Belgian mathematician Adolphe Quetelet in the 1830s as a statistical tool to study populations — not to assess individual health. It was adopted by the medical community in the 1970s as a convenient screening tool. Critics argue it was never designed for individual diagnosis and has well-known limitations around muscle mass and body composition.

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