BMI Calculator
Find out your Body Mass Index in seconds — metric or imperial, with your weight category and healthy weight range.
What is it?
Body Mass Index (BMI) is a simple numerical measure that relates a person's body weight to their height. It was developed by the Belgian mathematician Adolphe Quetelet in the 19th century and has since become the most widely used screening tool for classifying body weight in clinical, public health and research settings worldwide. BMI is calculated by dividing weight in kilograms by the square of height in metres (kg/m²). For imperial users, the same formula applies after converting pounds and inches. The result places a person into one of four standard categories defined by the World Health Organization: underweight (below 18.5), normal weight (18.5–24.9), overweight (25–29.9) and obese (30 and above). Our free online BMI calculator supports both metric (kg and cm) and imperial (lbs and feet/inches) inputs, shows your exact BMI to one decimal place, identifies your weight category, and calculates the ideal weight range for your height — so you can see exactly where you stand and how far you are from the healthy range.
How to use it
- Select your preferred unit system: Metric (kg / cm) or Imperial (lbs / ft & in).
- Enter your height using the fields provided.
- Enter your current body weight.
- Your BMI is calculated instantly — no button press needed.
- Read your weight category (Underweight, Normal, Overweight or Obese) on the result card.
- Check the healthy weight range for your height to understand your target zone.
- Use the colour bar to see where your BMI sits relative to all categories at a glance.
Why use this tool
BMI is not a perfect measure of body fatness — it does not account for muscle mass, bone density, age or sex distribution of fat. However, it remains the gold standard for population-level screening because it is fast, free and requires no equipment. For the vast majority of adults it correlates well with direct measures of body fat and with the risk of weight-related health conditions, including type 2 diabetes, hypertension, cardiovascular disease and certain cancers. Knowing your BMI is a useful starting point for a health conversation with a doctor. If your BMI falls outside the normal range, it is a prompt to investigate further — not a diagnosis. Our calculator adds the healthy weight range so you can set concrete, realistic targets. All calculations happen entirely in your browser. No data is ever sent to a server, stored, or linked to an account. You can use the tool as often as you like, on any device, without creating an account or providing an email address.
Frequently asked questions
Is BMI accurate for athletes and muscular people?
Not always. BMI cannot distinguish between muscle and fat, so heavily muscled athletes often register as overweight or obese despite having very low body fat. For athletes, body composition measurements (DEXA, skinfold calipers or bioelectrical impedance) are more informative.
Does BMI apply to children?
Standard BMI categories are designed for adults aged 20 and over. For children and teenagers (ages 2–19), BMI is interpreted using age- and sex-specific percentile charts, not the fixed adult thresholds. This calculator is for adults only.
What is BMI Prime?
BMI Prime is your BMI divided by 25 (the upper boundary of the normal range). A value of 1.0 means you are exactly at the top of the normal range. Below 1.0 is normal or underweight; above 1.0 is overweight or obese. It makes it easy to see how far from the boundary you are.
I am in the normal range but feel unhealthy. Why?
BMI is a screening tool, not a health assessment. Factors like cardiovascular fitness, diet quality, sleep, stress and metabolic health matter enormously and are invisible to BMI. A normal BMI does not guarantee good health, and a slightly elevated BMI in a fit, active person may carry lower risk than the raw number suggests.
What is a healthy BMI for women versus men?
The standard WHO BMI categories apply to both sexes. However, women typically have a higher body fat percentage than men at the same BMI. Some clinicians use slightly adjusted cut-offs for different ethnic populations — for instance, health risks in East Asian adults are elevated at a lower BMI threshold (around 23 instead of 25).