BMI Calculator

Body Mass Index (BMI) is calculated by dividing your weight in kilograms by the square of your height in metres: BMI = weight (kg) ÷ height (m)². It is used by the NHS as a screening tool to indicate whether your weight is in a healthy range for your height.

The NHS classifies BMI as follows: under 18.5 is underweight, 18.5–24.9 is a healthy weight, 25–29.9 is overweight, and 30 or above is obese. These ranges apply to most adults but may be less accurate for very muscular people, pregnant women, older adults and certain ethnic groups.

This calculator supports both metric (kg/cm) and imperial (stones, pounds, feet, inches) inputs and converts automatically. It shows your BMI value, NHS category and a visual indicator of where you fall on the scale.

How BMI is calculated. Formula: weight (kg) ÷ height (m) squared. Example: 70kg ÷ (1.75m)² = 22.9 BMI. Imperial: (weight in pounds × 703) ÷ height (inches)². WHO categories for adults: under 18.5 underweight, 18.5-24.9 healthy, 25-29.9 overweight, 30-34.9 obese class I, 35-39.9 obese class II, 40+ obese class III (severe). 80% of UK adults fall in 20-35 BMI range. Quick mental math: healthy weight in kg = height in cm − 100 (rough estimate).

BMI limitations and when it misleads. BMI doesn't distinguish muscle from fat. Muscular athletes can have BMI 28+ with low body fat. 'Skinny fat' adults can have BMI 22 with poor metabolic health. Elderly people lose muscle (sarcopenia), inflating BMI scores. Tall people (>1.9m) may register as 'overweight' at healthy proportions. Use waist circumference (>94cm men / >80cm women = raised risk regardless of BMI) for better cardiometabolic risk assessment.

BMI for different ethnicities — NHS adjustment. Since 2013, NHS uses adjusted BMI for South Asian, Chinese, Black African, African-Caribbean: overweight from 23 (vs 25 White European), obese from 27.5 (vs 30). Reason: these ethnicities develop diabetes type 2 and cardiovascular disease at lower BMIs due to body composition (higher visceral fat proportionally). NICE guideline NG7 recommends using ethnic-specific thresholds.

BMI for children and teens. UK90 BMI-for-age centile charts replace adult categories. Under 2nd centile underweight, 2-91st healthy, 91-98th overweight, 98th+ obese. Children's BMI fluctuates with growth — chubby toddlers often slim down. Don't rely on BMI alone for under-5s. NHS Healthy Weight Calculator gives age and sex-appropriate centiles. If concerned about your child's weight, see GP or paediatric dietitian rather than self-assessing via BMI.