BMI Calculator

What this calculator does

• Calculates body mass index from weight in kilograms and height in metres.
• Places the result in the standard adult categories.
• Is intended as a starting point for a conversation, not as a health assessment.

How the calculation works

BMI divides weight in kilograms by height in metres squared. Squaring the height is what makes the measure roughly independent of size, so that tall and short people with similar body composition land in a similar place. The standard adult categories put a healthy weight at 18.5 to 24.9, overweight at 25 to 29.9, and obesity from 30 upwards. Those cut-offs come from population-level associations between BMI and health outcomes, which is exactly what BMI is good for and exactly why it is weak for any individual. It uses only two numbers, so it cannot distinguish muscle from fat, cannot see where fat is carried, and takes no account of age, sex or build. NICE recommends lower thresholds for people from South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds — overweight from 23 and obesity from 27.5 — because cardiometabolic risk appears at a lower BMI in these groups. This calculator applies the standard thresholds only.

Step by step

1. Take weight in kilograms.
2. Square the height in metres.
3. Divide the weight by that figure.
4. Compare the result against the standard adult categories.

Worked example

An adult weighing 82 kg and standing 1.78 m tall works out their BMI.

What was entered

The arithmetic

1. Height squared is 1.78 × 1.78 = 3.1684.
2. BMI is 82 ÷ 3.1684 = 25.88.
3. That sits just inside the overweight band, which starts at 25.
4. The result is barely above the boundary, and a boundary drawn from population data says very little about one person a fraction over it.
5. Someone with high muscle mass could reach the same figure with low body fat, which is the clearest illustration of what BMI cannot see.

What the calculator returns

Key assumptions

• The categories applied are the standard adult thresholds.
• Height and weight are entered accurately in metres and kilograms.

Limitations

• BMI cannot distinguish muscle from fat. Athletic or muscular people are routinely classified as overweight while carrying low body fat.
• It says nothing about where fat is carried, and central adiposity carries more cardiometabolic risk than the same weight distributed elsewhere. A waist measurement adds information BMI cannot.
• This calculator applies the standard thresholds only. NICE recommends lower cut-offs for several ethnic backgrounds, so the category shown may understate risk for some people.
• It is not designed for children and young people, who are assessed against age and sex-specific centiles instead.
• It is not valid during pregnancy, and is unreliable for older adults and for people with certain medical conditions.
• This is information, not a diagnosis or a health assessment. A GP or practice nurse can interpret it alongside everything else that matters.

Common questions

Related calculators

• BMR Calculator — Estimate the energy your body uses at rest, which is a different question from body composition.
• TDEE Calculator — A waist-based measure adds the information about fat distribution that BMI cannot capture.

Official sources

Every figure in this guide was checked against the sources below. Where a source could not confirm a figure, it is marked as requiring verification rather than presented as settled.

• Overweight and obesity management (NG246): identifying and assessing overweight, obesity and central adiposity — NICEHealthy weight 18.5–24.9, overweight 25–29.9, obesity from 30. For South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds, overweight is 23–27.4 and obesity 27.5 or above
• Calculate your body mass index (BMI) for adults — NHSPeople from an Asian, Black African, African-Caribbean or Middle Eastern background have a higher chance of developing health problems at a lower BMI