BMI Calculator

About the BMI Calculator

A BMI calculator (Body Mass Index calculator) is the world's most widely used health screening tool for assessing whether a person's body weight is in a healthy range relative to their height. Requiring only two inputs — height and weight — it produces a single standardised number that healthcare providers, insurance companies, and public health agencies use worldwide to classify weight status and communicate health risk to patients and populations. The calculation is used by the CDC, WHO, NHS, and virtually every primary care physician globally as the standard starting point for discussions about weight-related health conditions, fitness goals, and medical eligibility. Our free BMI calculator works in both metric (kg and cm) and imperial (lbs and feet/inches), instantly shows your WHO BMI category, calculates the healthy weight range for your specific height, and explains what your result means in a real-world health context. Knowing your body mass index helps you understand your relative risk for conditions strongly associated with excess weight — including type 2 diabetes, cardiovascular disease, hypertension, sleep apnoea, non-alcoholic fatty liver disease, certain cancers, and joint problems — as well as the serious health risks of being underweight, including bone density loss and immune compromise. While BMI has well-documented limitations (it does not directly measure body fat percentage, cannot distinguish muscle mass from fat mass, and does not account for fat distribution patterns), it remains the clinically accepted population-level screening tool precisely because it requires only two easy measurements and correlates reliably with metabolic health outcomes across large population studies. In Australia, the UK, Canada, New Zealand, and the USA, BMI is embedded in routine preventive health screening, annual wellness checks, and national public health monitoring surveys such as the NHANES in the US and the Australian Health Survey. Public health agencies use population-level BMI data to track obesity prevalence trends over time, allocate resources, and design intervention programmes. For individuals, the BMI calculator is a starting point — not a final verdict. A reading in the overweight or obese range should prompt a conversation with your healthcare provider who can assess body fat distribution, metabolic markers, blood pressure, and lifestyle factors in full context. Conversely, a normal BMI does not guarantee good metabolic health; some normal-weight individuals have high visceral fat and significant cardiovascular risk. Think of BMI as the first data point in a broader conversation about your health, not the conclusion of it. This tool is suitable for adults aged 18 and over. Children and teenagers require age- and sex-specific BMI-for-age percentile charts, as growth patterns change rapidly during development.

How It Works

The BMI formula is: BMI = weight in kilograms divided by height in metres squared. In imperial units: BMI = 703 multiplied by weight in pounds, divided by height in inches squared. Example: a person weighing 75 kg at 170 cm has BMI = 75 divided by (1.70 x 1.70) = 75 divided by 2.89 = 25.95 — placing them in the Overweight category under WHO adult classification. The WHO adult BMI classification is as follows: Underweight is below 18.5; Normal or healthy weight is 18.5 to 24.9; Overweight is 25.0 to 29.9; Obese Class I is 30.0 to 34.9; Obese Class II is 35.0 to 39.9; Obese Class III (severe obesity) is 40.0 and above. To calculate the healthy weight range for a given height, multiply 18.5 and 24.9 by height in metres squared. For 170 cm (1.70 m): healthy weight minimum = 18.5 x 2.89 = 53.5 kg; healthy weight maximum = 24.9 x 2.89 = 71.9 kg. In pounds for 5 feet 7 inches (67 inches): healthy range = 18.5 x (67 x 67) divided by 703 = 118 lbs to 24.9 x 4489 divided by 703 = 159 lbs. For Asian and South Asian populations, the WHO has recommended adjusted thresholds: overweight at BMI of 23.0 and obese at 27.5, because epidemiological data shows these populations develop metabolic complications at lower BMI values than populations from which the original WHO thresholds were derived. Always use your ethnic background as context when interpreting BMI results alongside your healthcare provider.

Tips & Best Practices

• ✓BMI systematically underestimates body fat in elderly people where age-related muscle loss keeps weight normal despite elevated fat percentage, and overestimates it in very muscular individuals such as athletes and bodybuilders. Always interpret BMI alongside waist circumference and body fat percentage measurements for a complete picture.
• ✓Asian and South Asian populations face higher metabolic risk — including type 2 diabetes and cardiovascular disease — at lower BMI values. The WHO recommends using 23.0 as the overweight threshold for adults of Asian descent instead of the standard 25.0. If you are of South Asian, East Asian, or Southeast Asian heritage, discuss adjusted thresholds with your doctor.
• ✓Children and teenagers require BMI-for-age percentile charts specific to their age and sex, not adult category cutoffs. A BMI of 22 is very healthy for a 40-year-old adult but may indicate overweight for a 10-year-old child depending on their growth stage. Use paediatric-specific tools and consult your child's paediatrician.
• ✓Research consistently shows that even modest weight loss of 5 to 10 percent of body weight in overweight individuals produces clinically significant improvements in blood pressure, fasting blood glucose, LDL cholesterol, and sleep apnoea severity — without needing to reach a normal BMI to see meaningful health benefits.
• ✓Waist circumference adds critical context that BMI cannot provide: health risk rises significantly above 88 cm (35 inches) for women and 102 cm (40 inches) for men regardless of BMI. A person with a normal BMI but large waist circumference may carry as much cardiometabolic risk as someone classified as overweight.
• ✓Bariatric surgery eligibility standards across the USA, UK, Australia, Canada, and New Zealand typically require BMI at or above 40, or BMI at or above 35 with at least one serious obesity-related medical condition such as type 2 diabetes or severe obstructive sleep apnoea. Some programmes now accept BMI as low as 30 with multiple comorbidities.
• ✓The BMI scale was developed in the 1830s by Belgian statistician Adolphe Quetelet from data on European men and was never originally intended as an individual clinical diagnostic tool. Use it as the starting point for a broader conversation with your healthcare provider, not as a definitive verdict on your health.
• ✓Obesity is officially classified as a chronic, multifactorial disease by the American Medical Association, WHO, and most major medical bodies worldwide. It involves genetic, hormonal, environmental, socioeconomic, and behavioural factors — not simply personal choices about food and exercise. Compassionate, evidence-based care is the appropriate response to elevated BMI.

Who Uses This Calculator

Individuals use the BMI calculator as an accessible, zero-cost starting point for understanding their weight status and its health implications — particularly as preparation before scheduling a doctor's appointment to discuss weight management or before starting a new fitness programme. Primary care physicians screen every adult patient with BMI at annual well-visits because it is embedded in electronic health record systems and reimbursed as a standard preventive service in the USA, Australia, and the UK. Health insurance companies use BMI as a factor in life insurance underwriting and some health insurance products. Bariatric surgery programmes use BMI to screen candidates: most guidelines require BMI at or above 40, or BMI at or above 35 with at least one serious obesity-related medical condition, to establish eligibility for surgical intervention. Clinical researchers use BMI as both an eligibility criterion and a study covariate in trials involving metabolic health, cardiovascular disease, cancer incidence, and medication dosing. Public health departments use population-level BMI data from national surveys to track obesity prevalence trends and allocate healthcare resources. Fitness professionals and personal trainers use BMI as one of several baseline measurements when onboarding new clients alongside body fat percentage and waist circumference. Employers in safety-sensitive industries in some countries may use BMI as part of occupational health assessments. Parents concerned about their child's growth use the paediatric version of BMI-for-age percentile charting during school health checks and paediatric appointments.