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 for their height. BMI requires only two inputs — height and weight — and 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 such as central versus peripheral adiposity), it remains the clinically accepted population-level screening tool precisely because it requires only two easy measurements and correlates reliably with metabolic health outcomes in large population studies.

How It Works

The BMI formula: 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. WHO adult BMI classification: Underweight = below 18.5; Normal (healthy) weight = 18.5 to 24.9; Overweight = 25.0 to 29.9; Obese Class I = 30.0 to 34.9; Obese Class II = 35.0 to 39.9; Obese Class III (severe obesity) = 40.0 and above. Healthy weight range calculation: divide the BMI target by height squared. For 170 cm (1.70 m): healthy weight minimum = 18.5 x (1.70 x 1.70) = 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.

Tips & Best Practices

• ✓BMI systematically underestimates body fat in elderly people (age-related muscle loss keeps weight normal despite elevated fat percentage) and overestimates it in very muscular individuals — always interpret BMI alongside other health metrics.
• ✓Asian and South Asian populations face higher metabolic risk (type 2 diabetes and cardiovascular disease) at lower BMI values — WHO recommends using 23.0 as the overweight threshold for adults of Asian descent, not the standard 25.0.
• ✓Children and teenagers require BMI-for-age percentile charts, not adult category cutoffs — a BMI of 22 is very healthy for a 40-year-old adult but overweight for a 10-year-old child.
• ✓Research consistently shows that even modest weight loss of 5-10% of body weight in overweight individuals produces clinically significant improvements in blood pressure, fasting blood sugar, LDL cholesterol, and sleep apnoea severity.
• ✓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 what their BMI shows.
• ✓Bariatric surgery eligibility: most insurance programmes and surgical guidelines 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.
• ✓The BMI scale was developed in the 1830s from data on European men and was never originally intended as an individual health tool — use it as a starting point for a broader conversation with your healthcare provider, not as a definitive verdict.
• ✓Obesity is officially classified as a chronic, multifactorial disease by the American Medical Association, WHO, and most major medical bodies — it involves genetic, hormonal, environmental, and behavioural factors, not just food and exercise choices.

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. Health insurance companies use BMI as a factor in life insurance underwriting and some health insurance products, making it directly relevant to premium costs. Bariatric surgery programmes use BMI to screen candidates, establish eligibility for specific procedures, and document the medical necessity required for insurance reimbursement. Clinical researchers use BMI as both an eligibility criterion and a study covariate in nearly every trial involving metabolic health, cardiovascular disease risk, cancer incidence, or medication dosing. Public health departments use population-level BMI data from national surveys like the NHANES to track obesity prevalence trends, allocate healthcare resources, and evaluate public health intervention effectiveness over time. Fitness professionals and personal trainers use BMI as one of several baseline measurements when onboarding new clients alongside body fat percentage, waist-to-hip ratio, and fitness assessments.