⚖️

BMI Calculator

Hot

Calculate your Body Mass Index using metric or imperial units.

What is the BMI Calculator?

A BMI (Body Mass Index) calculator estimates whether your weight is in a healthy range for your height, using a simple ratio between the two. It works using the formula BMI = weight (kg) ÷ height (m)², with the result compared against standard World Health Organization category thresholds: under 18.5 is underweight, 18.5–24.9 is healthy weight, 25–29.9 is overweight, and 30 or above is obese. To calculate it manually in metric, divide your weight in kilograms by your height in metres squared; in imperial, convert your height to inches and weight to pounds first, then use BMI = (weight in lb ÷ height in inches²) × 703. BMI is a general screening tool and doesn't account for muscle mass, frame size or ethnicity — it's not a diagnosis, so speak to a GP for personalised advice.

How it works

Body Mass Index (BMI) is calculated by dividing your weight in kilograms by the square of your height in metres (kg/m²). It's a simple ratio designed to give a quick, population-level screening indicator of whether someone's weight is in a healthy range relative to their height — not a direct measure of body fat, muscle, or overall health.

UK context

BMI bands (underweight, healthy weight, overweight, obese) used across the NHS and internationally are based on population studies, and the same bands are generally applied to adults regardless of frame size, muscle mass or ethnicity — a well-recognised limitation. The NHS and some other health bodies apply adjusted BMI thresholds for some South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean groups, reflecting evidence that health risks can begin at a lower BMI for these groups.

Tips

BMI doesn't distinguish between muscle and fat, so athletes and very muscular people can show a "high" BMI despite having low body fat — it's a screening tool, not a diagnosis.
BMI bands used for adults don't apply to children — child BMI is assessed differently, using age- and sex-specific growth charts.
If you're concerned about your weight or BMI result, a GP or healthcare professional can give a fuller assessment that accounts for your individual circumstances.

Frequently asked questions

How is BMI calculated?

BMI is calculated as weight in kilograms divided by height in metres squared (kg/m²). Calculators using imperial units (stone/pounds and feet/inches) convert to metric first, then apply the same formula.

Is BMI accurate for everyone?

BMI is a useful general population screening tool but has known limitations — it doesn't account for muscle mass, frame size, age, or ethnicity, so it can be misleading for athletes, older adults, or people outside the population the standard bands were derived from.

Are BMI bands the same for everyone?

The standard adult BMI bands are widely used, but the NHS and other health bodies note that health risks can begin at a lower BMI for some South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean groups, and may apply adjusted thresholds accordingly.

Does BMI apply to children the same way as adults?

No — children's BMI is assessed using age- and sex-specific growth charts and percentiles rather than the fixed adult bands, since healthy body composition changes significantly through childhood growth.

A quick note

BMI is a general screening tool, not a diagnosis, and doesn't account for muscle mass, frame size or ethnicity. Speak to a GP or healthcare professional for personalised advice about your weight and health.