BMI Calculator Online
BMI calculator online free — calculate your Body Mass Index in metric or imperial and understand what it means. Free, no signup, everything runs in your browser.
Your Measurements
Your BMI Result
BMI Categories
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
About BMI
Body Mass Index (BMI) is a simple calculation using your height and weight to estimate whether you are at a healthy weight. It is widely used as a screening tool to identify potential weight-related health risks.
Limitations: BMI does not account for muscle mass, bone density, age, sex, or body fat distribution. Athletes may have a high BMI without excess body fat, and older adults may have a normal BMI while carrying more fat than is healthy.
This tool is for informational purposes only and is not medical advice. Consult a qualified healthcare professional for personalized guidance.
How to Calculate Your BMI
- 1Select your measurement system: Metric (kg and cm) or Imperial (lb and ft/in). The input fields adjust automatically.
- 2Enter your height and weight. In metric mode: height in centimetres, weight in kilograms. In imperial mode: height in feet and inches, weight in pounds.
- 3Click Calculate. Your BMI value, category (underweight/normal/overweight/obese), and healthy weight range for your height appear instantly.
Understanding Body Mass Index
Body Mass Index is a number derived from a simple formula: weight in kilograms divided by the square of height in metres (BMI = kg/m²). It was developed in the 1830s as a statistical tool, not a medical diagnostic. Despite its limitations, it remains widely used in clinical settings as a first-pass screening tool for weight-related health risks.
The World Health Organization defines four primary adult BMI categories, each associated with different population-level health risk profiles. Individual health depends on many more factors than BMI alone — physical fitness, diet quality, waist circumference, blood pressure, and blood glucose are all important indicators.
BMI Categories (WHO Adult Standards)
| BMI Range | Category | Associated Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Possible malnutrition, bone density issues |
| 18.5 – 24.9 | Normal weight | Lowest population-level risk range |
| 25.0 – 29.9 | Overweight | Moderately increased risk |
| 30.0 – 34.9 | Obese Class I | High risk |
| 35.0 – 39.9 | Obese Class II | Very high risk |
| 40.0 and above | Obese Class III | Extremely high risk |
What BMI Can and Cannot Tell You
BMI Has Limitations
BMI does not distinguish between muscle mass and fat mass. Highly muscular individuals often fall in the "overweight" or "obese" range despite having low body fat. It is a screening tool, not a definitive diagnosis.
Different Thresholds for Asian Populations
Several Asian health authorities recommend lower BMI thresholds (overweight ≥23, obese ≥27.5) as research shows higher metabolic risk at lower BMI compared to European populations.
Not Applicable to Children
The standard adult BMI formula is not appropriate for children under 18. Paediatric BMI uses age- and sex-specific percentile charts that account for growth stages.
BMI Doesn't Measure Fat Distribution
Waist circumference is a better indicator of abdominal visceral fat, which carries greater health risk than subcutaneous fat. A high BMI with a large waist indicates greater metabolic risk.
Use It as a Starting Point
If your BMI falls outside the normal range, consider discussing it with a healthcare professional. They can assess blood pressure, glucose, cholesterol, and fitness for a fuller picture.
Metric vs Imperial — Same Result
The calculator converts between measurement systems before computing. You get the same BMI whether you enter metric or imperial measurements — they are mathematically identical.
Frequently Asked Questions
What is a healthy BMI for adults?
The World Health Organization defines the healthy BMI range for adults as 18.5 to 24.9. A BMI below 18.5 is classified as underweight; 25.0 to 29.9 is overweight; 30.0 and above is obese. These thresholds apply to the general adult population but may differ for specific ethnic groups, older adults, and athletes.
How is BMI calculated?
The formula is: BMI = weight (kg) ÷ height (m)². For example, a person who is 70 kg and 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.86. In imperial units: BMI = (weight in pounds × 703) ÷ (height in inches)².
Is BMI accurate for everyone?
BMI is a population-level statistical tool and is less accurate at the individual level. It tends to overestimate body fat in muscular individuals (e.g. athletes) and underestimate it in older adults who have lost muscle mass. It also does not account for fat distribution, which is a more direct predictor of metabolic risk.
What is a healthy weight range for my height?
The calculator shows a healthy weight range based on your height for a BMI between 18.5 and 24.9. For example, a person 175 cm tall has a healthy weight range of approximately 56.7 kg to 76.4 kg.
Does BMI differ for men and women?
The standard BMI formula and categories are the same for men and women. However, women typically have a higher body fat percentage than men at the same BMI because men tend to carry more muscle mass. Some clinicians apply different norms, but the WHO uses the same universal thresholds for both sexes.
My BMI is in the overweight range — should I be concerned?
A BMI in the 25–29.9 range indicates modestly elevated population-level risk. It does not mean you are unhealthy. Factors such as physical fitness, diet, blood pressure, and cholesterol are better predictors of individual health outcomes. Consult a doctor for a comprehensive assessment rather than acting on BMI alone.
Can I calculate my child's BMI here?
This calculator uses the adult BMI formula, which is not appropriate for children and teenagers. For children (ages 2–18), BMI is assessed using CDC or WHO growth charts that adjust for age and sex. Consult your paediatrician for a child's weight assessment.
How often should I check my BMI?
BMI changes slowly with changes in weight and is meaningful to track every few months rather than daily. Daily weight fluctuations of 1–2 kg from water retention, meals, and exercise are normal and do not represent true fat gain or loss.