Calculate your Body Mass Index instantly. See your NHS category, healthy weight range for your height, and what the number actually means.
BMI (Body Mass Index) is a measure of body weight relative to height, calculated as weight (kg) ÷ height (m)². The NHS uses the following ranges: below 18.5 = underweight, 18.5–24.9 = healthy weight, 25–29.9 = overweight, 30 or above = obese. For example, a person who is 1.75m tall and weighs 70kg has a BMI of 70 ÷ (1.75 × 1.75) = 22.9 — within the healthy range. Around 28% of UK adults are classed as obese and a further 36% as overweight. BMI is a screening tool, not a diagnostic measure — it does not account for muscle mass, bone density, or fat distribution. For adults of Asian background, the NHS uses lower thresholds: overweight from 23, and obese from 27.5.
BMI is the most widely used weight screening tool in the UK, but it's also one of the most misunderstood. Here's what the NHS categories actually mean, why BMI has real limitations, and what else to consider alongside it.
BMI divides your weight by your height squared. The result is a single number that places you into one of four NHS categories regardless of age, sex, or build.
May indicate insufficient nutrition or underlying health conditions. Speak to a GP — focus should be on nutrient-dense food and identifying any underlying cause before simply increasing calories.
Within the NHS healthy range. BMI in this range is associated with lower risk of weight-related conditions. However, BMI alone doesn't confirm good health — fitness, diet and other markers also matter.
Increased risk of type 2 diabetes, heart disease and joint problems. A modest calorie deficit of 300–500 kcal/day with regular movement is more sustainable than extreme approaches.
Significantly elevated health risks. NHS support including referral to weight management programmes is available. Small sustainable changes are more effective than crash diets. A GP can help create a plan.
BMI is a population-level screening tool, not a precise individual health measure. Its main limitations are well-documented by the NHS and medical researchers.
BMI doesn't distinguish between muscle mass and fat. A muscular athlete may show as overweight or obese despite having very low body fat. Conversely, a person with a normal BMI may carry significant fat with little muscle — sometimes called "skinny fat."
Where fat is stored matters more than how much. Visceral fat (around the abdomen and organs) carries much greater cardiovascular and metabolic risk than subcutaneous fat elsewhere. Waist circumference is a better proxy for this risk.
South Asian, Chinese, and some other ethnic groups carry higher cardiovascular risk at lower BMI thresholds. The NHS recommends overweight starts at BMI 23 and obese at BMI 27.5 for these groups — lower than the standard thresholds.
Older adults naturally lose muscle mass. A normal BMI in someone over 65 may mask sarcopenia (muscle loss), which carries its own health risks. In older adults, a slightly higher BMI (up to around 27) is sometimes associated with better outcomes.
What to measure alongside BMI: waist circumference (healthy: below 94cm for men, below 80cm for women), waist-to-height ratio (ideally below 0.5), blood pressure, resting heart rate, and fasting blood glucose for a fuller picture of metabolic health.
According to NHS guidelines, a healthy BMI for adults is between 18.5 and 24.9. Below 18.5 is underweight. Between 25 and 29.9 is overweight. A BMI of 30 or above is obese. These thresholds apply to adults aged 18 and over. For people of South Asian, Chinese, Black African or African-Caribbean backgrounds, lower thresholds apply — overweight starts at 23 and obese at 27.5 — because health risks appear at lower BMIs in these groups.
The NHS uses four main BMI categories for adults: Underweight (below 18.5), Healthy weight (18.5 to 24.9), Overweight (25 to 29.9), and Obese (30 and above). The obese category is further divided into Class 1 (30–34.9), Class 2 (35–39.9), and Class 3 (40 and above). For Black, Asian and minority ethnic adults, the NHS uses lower thresholds: overweight at 23+, obese at 27.5+.
BMI is calculated by dividing your weight in kilograms by your height in metres squared. The formula is: BMI = weight (kg) ÷ height (m)². For example, if you weigh 78kg and are 1.75m tall: 78 ÷ (1.75 × 1.75) = 78 ÷ 3.0625 = BMI 25.5. In imperial, the formula is: BMI = (weight in pounds × 703) ÷ height in inches squared. Use the calculator above for instant results in both metric and imperial.
A BMI of 30 or above is classified as obese under NHS guidelines. Class 1 obesity is BMI 30–34.9, Class 2 is 35–39.9, and Class 3 (severe obesity) is 40 and above. For South Asian, Chinese and some other ethnic groups, the obese threshold is lower at 27.5. Obesity is associated with increased risk of type 2 diabetes, heart disease, stroke and some cancers.
The healthy BMI range is the same for men and women in the UK: 18.5 to 24.9. Women naturally carry a higher percentage of body fat than men at the same BMI — this is normal and healthy. The NHS uses the same category thresholds regardless of sex. BMI should be considered alongside waist circumference rather than used as the sole indicator of health.
BMI is a useful first screening tool but has well-documented limitations. It does not distinguish between muscle and fat — a muscular athlete may show as overweight despite very low body fat. It does not account for where fat is stored (visceral abdominal fat carries greater risk than fat elsewhere). It is less accurate for older adults who have lost muscle mass, and uses different appropriate thresholds for different ethnic groups. Use BMI as one data point alongside waist circumference (healthy: below 94cm for men, below 80cm for women) for a fuller picture.
Your healthy weight range depends on your height. For a healthy BMI of 18.5 to 24.9, multiply each figure by your height in metres squared. At 1.60m: healthy range is 47.4kg to 63.7kg. At 1.70m: 53.4kg to 71.9kg. At 1.75m: 56.6kg to 76.3kg. At 1.80m: 59.9kg to 80.7kg. Use the calculator above to get your exact healthy weight range instantly.