Calculate your Body Mass Index using UK imperial (stones and pounds) or metric units. Includes NHS weight categories, healthy weight range and ideal weight for your height.
Body Mass Index (BMI) is calculated as weight in kilograms divided by height in metres squared (kg/m²). It is used as a simple screening tool to categorise weight status, though it has important limitations.
NHS BMI categories (adults 18+)
Underweight: BMI below 18.5 — may indicate malnutrition or other health issues
Healthy weight: BMI 18.5–24.9 — associated with lowest health risk
Overweight: BMI 25–29.9 — increased risk of type 2 diabetes, heart disease
Obese (Class I): BMI 30–34.9
Obese (Class II): BMI 35–39.9
Obese (Class III): BMI 40 or above — highest health risk category
BMI for South Asian and East Asian adults
Research suggests South Asian, Chinese, Japanese and other East Asian adults have a higher risk of type 2 diabetes and cardiovascular disease at lower BMI thresholds. Some UK guidelines use adjusted thresholds: overweight at BMI 23+ and obese at BMI 27.5+ for these groups.
Limitations of BMI
BMI does not measure body fat directly and cannot distinguish muscle from fat. Highly muscular people (athletes, bodybuilders) may have a high BMI despite being metabolically healthy. Older adults may have a healthy BMI but high body fat percentage due to muscle loss. Waist circumference is a better predictor of metabolic risk than BMI alone.
Healthy waist sizes (NHS)
Men: Below 94cm (37 inches). Above 102cm (40 inches) = very high risk.
Women: Below 80cm (31.5 inches). Above 88cm (35 inches) = very high risk.
Measure midway between the bottom of the ribcage and the top of the hip bones.
Worked Examples
UK average male
Height5'10" (1.778m)
Weight13st 2lb (83.5kg)
BMI26.4
CategoryOverweight
Healthy range58.5–78.6kg
UK average female
Height5'4" (1.626m)
Weight11st 6lb (72.6kg)
BMI27.5
CategoryOverweight
Healthy range48.9–65.8kg
Healthy weight check
Height5'8" (1.727m)
Weight11st (69.9kg)
BMI23.4
CategoryHealthy weight
Healthy range55.2–74.3kg
Frequently Asked Questions
The NHS defines healthy BMI as 18.5–24.9. Below 18.5 is underweight; 25–29.9 is overweight; 30+ is obese. BMI is a screening tool only — it does not account for muscle mass, bone density, age, or fat distribution. A GP can give a more complete picture of health risk.
No — BMI has significant limitations. It overestimates health risk in muscular individuals (athletes, bodybuilders) and underestimates it in those with low muscle mass (older adults). South Asian, Chinese and other East Asian adults have higher health risks at lower BMIs — some UK guidelines recommend overweight at BMI 23+ for these groups. Waist circumference is often a better predictor of metabolic risk.
The NHS recommends below 94cm (37 inches) for men and below 80cm (31.5 inches) for women. Above 102cm (men) or 88cm (women) indicates very high health risk. Measure at the midpoint between the bottom of the ribcage and the top of the hip bones, after breathing out normally. For South Asian men, a lower threshold of 90cm applies.
The NHS recommends reducing calorie intake by 500–600 kcal/day below your total daily energy expenditure and increasing physical activity to at least 150 minutes of moderate aerobic activity per week. A sustainable rate is 0.5–1kg (1–2 lbs) per week. Crash diets lead to muscle loss and are counterproductive for long-term weight maintenance. For personalised guidance, speak to a GP or registered dietitian.
For children under 18, BMI is assessed differently — it is plotted on a UK growth chart by age and sex, as healthy BMI ranges change significantly during development. This calculator is designed for adults aged 18 and over. For children's weight concerns, speak to a health visitor or GP who can plot on the appropriate growth chart.