BMI Calculator
Calculate Body Mass Index with visual scale and category.
What your BMI number really means: and its limitations
Body Mass Index was devised in the 1830s by Belgian mathematician Adolphe Quetelet as a simple ratio to describe the average human body. It divides weight in kilograms by the square of height in metres, producing a dimensionless number. Because weight tends to scale with the square of height in most adults, the ratio stays roughly consistent across different body sizes, making it a convenient population-level screening stat.
The four WHO categories: Underweight (<18.5), Normal weight (18.5–24.9), Overweight (25–29.9), and Obese (≥30): are used by health systems worldwide to flag populations at elevated risk of metabolic disease. However, BMI says nothing about where fat is stored, how much of your weight is muscle, or how cardiorespiratory fit you are. Use it as a first-pass indicator, not a verdict.
How to calculate: step by step
- 1Choose your unit system
Select metric (kg / cm) or imperial (lbs / ft + in) using the toggle at the top. The calculator converts between systems automatically. - 2Enter your weight
Type your body weight in kilograms or pounds. Use a decimal if needed: for example 70.5 kg or 154.3 lbs. - 3Enter your height
For metric, enter height in centimetres. For imperial, use separate feet and inches fields: for example 5 ft 9 in. - 4Read your BMI and category
Your BMI appears immediately along with a colour-coded category: Underweight, Normal weight, Overweight, or Obese, as defined by WHO guidelines. - 5Check your healthy weight range
The calculator also shows the weight range that corresponds to a BMI between 18.5 and 24.9 for your specific height, so you know your target range at a glance.
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Frequently Asked Questions
- What is BMI and how is it calculated?
- BMI (Body Mass Index) is calculated by dividing your weight in kilograms by the square of your height in metres: BMI = kg ÷ m². A result between 18.5 and 24.9 is considered a normal healthy weight by most health organisations.
- What BMI range is considered healthy for adults?
- According to the World Health Organization, a BMI of 18.5–24.9 is classified as normal weight. Below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is obese.
- Is BMI accurate for everyone?
- BMI is a population-level screening tool, not a diagnostic measure. It does not account for muscle mass, bone density, age, or fat distribution. Athletes may have a high BMI with very low body fat. Always consult a healthcare professional for a proper assessment.
- What is BMI Prime?
- BMI Prime is your BMI divided by 25 (the upper limit of the normal range). A value of 1.0 means you are right at the top of the normal range. Below 1.0 is normal or underweight; above 1.0 is overweight or obese.
- How do I calculate BMI in imperial units?
- In imperial: BMI = (weight in pounds × 703) ÷ (height in inches)². Our calculator handles this conversion automatically when you select the imperial option.
- Does BMI differ for men and women?
- The standard BMI formula and thresholds are the same for men and women. However, body fat percentage at the same BMI can differ between sexes. Some health professionals use adjusted thresholds for older adults or different ethnic groups.
- What is a healthy BMI for children?
- Children and teenagers use age- and sex-specific BMI percentile charts rather than the fixed adult thresholds. The calculator on this page is designed for adults aged 18 and over.
- Can I use this calculator for fitness goal planning?
- Yes. The healthy weight range panel shows the weight bracket that corresponds to a normal BMI for your exact height. You can use that as a reference target, though it is advisable to combine it with body composition measurements.
- Is this BMI calculator free?
- Completely free, with no account required. The calculation runs entirely in your browser: no data is sent to any server.
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