BMI Calculator
Free BMI calculator based on WHO guidelines. Enter your height and weight to get your BMI, category, and ideal weight range.
Calculate your Body Mass Index and find your healthy weight range.
The BMI calculator helps you quickly determine whether your weight falls within a healthy range for your height. Whether you are managing your weight for the first time or tracking progress over time, understanding your Body Mass Index gives you a useful starting point for conversations with healthcare professionals and for setting personal health goals.
What is BMI?
Body Mass Index, commonly abbreviated as BMI, is a numerical value derived from a person’s weight and height. The formula divides weight in kilograms by the square of height in metres:
BMI = weight (kg) ÷ height (m)²
The index was first proposed by Belgian mathematician and statistician Adolphe Quetelet in 1832 as a way to study the proportions of human body weight across large populations. Because it is simple, cheap, and non-invasive, it became the standard screening metric adopted by the World Health Organization and health systems worldwide.
BMI matters because carrying excess body weight is associated with an increased risk of type 2 diabetes, cardiovascular disease, hypertension, certain cancers, and musculoskeletal problems. Conversely, being underweight can signal malnutrition, hormonal imbalances, or other conditions that merit medical attention. BMI gives clinicians a fast, standardised way to flag individuals who may benefit from further assessment.
How to Use This Calculator
- Select your unit system — choose metric (kilograms and centimetres) or imperial (pounds and inches).
- Enter your weight — use your current body weight, ideally measured in the morning before eating.
- Enter your height — remove shoes and stand straight for an accurate measurement.
- Read your results — the calculator displays your BMI value, the corresponding WHO weight category, and the range of weights considered healthy for your height.
No account or personal data is required. All calculations happen locally in your browser.
Examples
Example 1 — Normal weight adult: A person weighing 70 kg and standing 175 cm tall has a BMI of approximately 22.9. This falls within the “Normal weight” range (18.5–24.9).
Example 2 — Overweight range: A person weighing 90 kg and 170 cm tall has a BMI of approximately 31.1, placing them in the “Obese class I” range. To reach a healthy BMI, their target weight would be roughly 53–76 kg.
Example 3 — Imperial units: A person weighing 154 lb and 5 ft 9 in (69 inches) tall has a BMI of about 22.7 — well within the normal range.
BMI Categories
The World Health Organization defines the following adult BMI categories:
| BMI Range | Category | Health Implications |
|---|---|---|
| Below 18.5 | Underweight | Risk of malnutrition, osteoporosis, anaemia |
| 18.5 – 24.9 | Normal weight | Lowest risk for weight-related conditions |
| 25.0 – 29.9 | Overweight | Moderately increased risk of cardiovascular disease |
| 30.0 – 39.9 | Obese | Substantially increased risk; medical evaluation recommended |
| 40.0 and above | Severely obese | Highest risk; specialist care typically required |
These thresholds apply to adults aged 18 and over. Children, adolescents, and pregnant women require different assessment methods.
Limitations of BMI
BMI is a useful screening tool, but it has well-documented limitations that are important to understand:
Muscle vs. fat: BMI measures total body mass relative to height but cannot distinguish between lean muscle and adipose tissue. A competitive weightlifter may have a BMI of 30 and be classified as obese despite having very low body fat. Always interpret BMI alongside other measures such as waist circumference or body fat percentage.
Age and sex differences: Older adults tend to have more body fat at the same BMI compared to younger adults. Women generally carry more body fat than men at equivalent BMI values. Some researchers argue for sex-specific and age-adjusted thresholds.
Ethnicity: Research has shown that individuals of Asian descent may face elevated health risks at lower BMI values (some guidelines suggest a threshold of 23 for “overweight” in Asian populations). Conversely, some studies suggest that people of sub-Saharan African descent may tolerate higher BMI values with lower metabolic risk.
Pregnancy: Standard adult BMI thresholds should not be applied during pregnancy. Weight gain during pregnancy is expected and healthy; separate gestational weight gain guidelines apply.
Children and adolescents: Paediatric BMI must be interpreted using age- and sex-specific growth charts, not adult cut-offs.
Frequently Asked Questions
Is BMI accurate for athletes? No, not reliably. Athletes with high muscle mass are often classified as overweight by BMI alone. A more complete picture comes from combining BMI with body fat percentage measurement (via DEXA scan, bioelectrical impedance, or skinfold callipers).
Should I worry if my BMI is borderline? A single measurement at the boundary of two categories is rarely cause for alarm. Trends over time, your overall lifestyle (diet, exercise, sleep), and other clinical markers (blood pressure, cholesterol, blood glucose) are far more informative than a single number.
How often should I calculate my BMI? For most adults, checking every three to six months is sufficient to track meaningful weight trends. More frequent checking can foster unhealthy preoccupation with weight.
Can I use this calculator for my child? This calculator uses adult BMI thresholds. For children under 18, please use an age- and sex-specific paediatric BMI tool or consult your child’s doctor.
Does losing weight always improve BMI-related health risks? Generally yes, even modest weight loss (5–10% of body weight) in people with elevated BMI has been shown to improve blood pressure, blood sugar, and cholesterol levels. However, the goal should be sustainable healthy behaviours rather than achieving a specific BMI number.