BMI Calculator — Body Mass Index UK

What Is BMI?

Body Mass Index (BMI) is a simple measurement that uses your height and weight to estimate whether you are a healthy weight. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and has been used by health professionals worldwide ever since. The formula is straightforward: your weight in kilograms divided by your height in metres squared.

The NHS uses BMI as a screening tool to identify weight categories that may lead to health problems. While it is not a perfect measure of health, it provides a useful starting point for conversations between patients and healthcare providers about weight management.

NHS BMI Categories

BMI Limitations

BMI is a useful screening tool, but it has well-documented limitations:

• Muscle vs fat: BMI does not distinguish between muscle mass and body fat. Athletes and people who do regular strength training may have a high BMI despite having low body fat.
• Age: Older adults naturally lose muscle mass, so a "healthy" BMI may underestimate body fat in elderly people.
• Ethnicity: BMI thresholds may not apply equally across all ethnic groups. For people of South Asian, Chinese, and other Asian backgrounds, the NHS recommends a healthy BMI range of 18.5–23 (lower than the standard 18.5–24.9) because these groups tend to carry more body fat at lower BMI levels.
• Children and young people: BMI is calculated differently for under-18s, using age and sex-specific percentile charts rather than fixed categories.
• Pregnancy: BMI categories do not apply during pregnancy.

Waist-to-Height Ratio — A Better Measure?

Many health professionals now recommend using waist circumference or the waist-to-height ratio alongside BMI. Your waist measurement is a good indicator of visceral fat (fat around your internal organs), which is more closely linked to health risks than overall body weight.

• Rule of thumb: Your waist should be less than half your height.
• Men: Waist over 94cm (37in) = increased risk; over 102cm (40in) = high risk
• Women: Waist over 80cm (31.5in) = increased risk; over 88cm (34.5in) = high risk

How to Measure Your Height and Weight Accurately

1. Height: Stand against a flat wall without shoes. Look straight ahead. Place a flat object (like a book) on top of your head touching the wall. Mark the wall and measure from the floor to the mark.
2. Weight: Weigh yourself first thing in the morning, after using the toilet, wearing minimal clothing. Use the same scales each time for consistency.
3. Waist: Find the bottom of your ribs and the top of your hips. Wrap a tape measure around your waist midway between these points. Breathe out naturally before measuring. Do not pull the tape tight.

What to Do If Your BMI Is Outside the Healthy Range

If Underweight (BMI below 18.5)

Being underweight can be a sign of underlying health issues. Speak to your GP, who can check for causes and advise on gaining weight safely. Eat regular meals with nutrient-dense foods, include healthy fats, and consider strength training to build muscle.

If Overweight or Obese (BMI 25+)

The NHS recommends losing weight gradually (0.5–1kg per week) through a combination of dietary changes and increased physical activity. The NHS Choices website offers free tools including the 12-week weight loss plan and the Couch to 5K programme. For those with a BMI over 30, speak to your GP about additional support including weight management programmes and, in some cases, medication or surgery.