BMI Calculator

Body Mass Index is your weight in kilograms divided by the square of your height in metres, so BMI equals weight (kg) divided by height (m) squared. The NHS uses it as a screening tool to indicate whether your weight sits in a healthy range for your height. Someone of 70kg and 1.75m has a BMI of 22.9. In imperial units the sum is weight in pounds multiplied by 703, then divided by height in inches squared.

The NHS classifies adult BMI in four bands: under 18.5 is underweight, 18.5 to 24.9 is a healthy weight, 25 to 29.9 is overweight, and 30 or above is obese. The WHO splits that top band further, treating 30 to 34.9 as obese class I, 35 to 39.9 as class II and 40 and over as class III. About 80% of UK adults fall somewhere in the 20 to 35 range. A rough mental check is that a healthy weight in kilograms sits close to your height in centimetres minus 100.

This calculator accepts metric inputs in kilograms and centimetres or imperial ones in stones, pounds, feet and inches, and converts between them. It returns your BMI value, the NHS category it falls into and a visual indicator of where you sit on the scale. It also shows the healthy weight range for your height, which for someone of 175 cm runs from 56.7 to 76.6 kg.

The number has known blind spots. BMI cannot tell muscle from fat, so a muscular athlete can register 28 or more while carrying very little fat, and a sedentary adult at 22 may carry a great deal of it along with poor metabolic health. Older people lose muscle through sarcopenia, which distorts the reading upwards. Very tall people above 1.9m can be classed as overweight at healthy proportions. Waist circumference is the better guide to cardiometabolic risk, with more than 94cm in men or more than 80cm in women counting as raised risk whatever the BMI says.

Ethnicity changes where the thresholds sit. Since 2013 the NHS has used adjusted BMI cut-offs for people of South Asian, Chinese, Black African and African-Caribbean background, treating 23 as the start of overweight rather than 25, and 27.5 as the start of obesity rather than 30. The reason is body composition, because these groups develop type 2 diabetes and cardiovascular disease at lower BMIs, carrying proportionally more visceral fat. NICE guideline NG7 recommends using the ethnic-specific thresholds.

Children and teenagers are assessed differently again. Adult categories do not apply, and the UK90 BMI-for-age centile charts are used instead: below the 2nd centile is underweight, the 2nd to 91st is healthy, the 91st to 98th is overweight and above the 98th is obese. A child's BMI moves about as they grow, and chubby toddlers often slim down without intervention, so BMI alone is a poor guide under 5. The NHS Healthy Weight Calculator gives age and sex-appropriate centiles, and a GP or paediatric dietitian is the right place to take a worry about a child's weight.