BMI Calculator
Our BMI calculator gives you your Body Mass Index in seconds. Enter your weight and height in metric or imperial — get your WHO weight category and an instant health snapshot.
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Your BMI
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How BMI Calculator Works
What Is BMI?
Body Mass Index (BMI) is a simple numerical measure derived from a person's weight and height. It is used by health professionals and the World Health Organization (WHO) as a population-level screening tool to identify potential weight-related health risks.
BMI is not a direct measure of body fat, but it correlates with body fat levels in most adults and provides a fast, cost-free initial assessment of whether someone may be at a healthy weight for their height.
BMI Formula
The standard BMI formula is:
BMI = Weight (kg) ÷ Height (m)²
For example, a person who weighs 70 kg and is 1.75 m tall:
BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9
If you are working in imperial units, convert first: divide pounds by 2.205 to get kilograms, and multiply total inches by 2.54 to get centimetres. The calculator above handles this conversion automatically when you select the Imperial tab.
BMI Weight Status Categories
The WHO classifies adult BMI into four standard weight status categories (for adults aged 18 and over):
| BMI Range | Weight Status | Health Implication |
|---|---|---|
| Below 18.5 | Underweight | May indicate malnutrition or an underlying health condition |
| 18.5–24.9 | Normal weight | Associated with the lowest risk for weight-related health problems |
| 25.0–29.9 | Overweight | Associated with increased risk of cardiovascular disease and type 2 diabetes |
| 30.0 and above | Obese | Associated with significantly elevated risk of serious health conditions |
Worked Examples
Example 1 — Normal weight: A person weighing 68 kg who is 170 cm (1.70 m) tall. BMI = 68 ÷ (1.70)² = 68 ÷ 2.89 = 23.5 — Normal weight.
Example 2 — Overweight: A person weighing 90 kg who is 175 cm (1.75 m) tall. BMI = 90 ÷ (1.75)² = 90 ÷ 3.0625 = 29.4 — Overweight.
Example 3 — Underweight: A person weighing 45 kg who is 165 cm (1.65 m) tall. BMI = 45 ÷ (1.65)² = 45 ÷ 2.7225 = 16.5 — Underweight.
Important Health Disclaimer
BMI is a screening tool, not a diagnostic instrument. It does not account for age, sex, ethnicity, muscle mass, bone density, or fat distribution. Athletes with high muscle mass may register a high BMI without excess body fat. Older adults may have a healthy BMI while still carrying excess fat.
Always consult a qualified healthcare professional before making medical decisions based on your BMI result. BMI is one data point among many — your doctor will consider blood pressure, blood glucose, cholesterol, and lifestyle before making any health recommendations.
Accuracy & Sources
Last reviewed: July 2026. Formula source: World Health Organization (WHO). All calculations run in your browser. No data is sent to any server.
Frequently Asked Questions
BMI (Body Mass Index) is a numerical value calculated from your weight and height. It is widely used by health professionals and the World Health Organization as a population-level screening tool to identify weight categories — underweight, normal weight, overweight, and obese — that may be associated with health risks. BMI is a starting point, not a definitive health verdict.
Divide your weight in kilograms by your height in metres squared. For example: if you weigh 70 kg and are 1.75 m tall, your BMI is 70 ÷ (1.75 × 1.75) = 22.9. Our BMI calculator handles the maths automatically — enter your weight and height in metric (kg and cm) or imperial (lbs, feet, and inches) and click Calculate BMI.
According to the World Health Organization, a BMI of 18.5 to 24.9 is considered normal weight for adults. A BMI below 18.5 is classed as underweight, 25.0 to 29.9 as overweight, and 30.0 or above as obese. These classifications apply to adults aged 18 and over. They do not apply to children, teenagers, or pregnant women.
BMI is a useful population-level tool but has known limitations. It cannot distinguish between fat mass and muscle mass, which means athletes and bodybuilders may register as overweight or obese despite having low body fat. It also does not account for differences in age, sex, or ethnicity. For a more complete picture, combine BMI with other measures such as waist circumference or body fat percentage.
If your BMI indicates underweight, overweight, or obese, speak to a GP or registered dietitian. Do not make significant dietary or exercise changes without professional advice. BMI is one data point — your doctor will consider other factors including blood pressure, blood glucose, cholesterol, and your lifestyle before making any health recommendations.
Yes. Select the Imperial tab on the calculator above and enter your weight in pounds and your height in feet and inches. The calculator converts automatically to metric before calculating. If you prefer to convert manually: divide your weight in pounds by 2.205 to get kilograms, and multiply your height in total inches by 2.54 to get centimetres.
No. Standard adult BMI classifications do not apply to children and teenagers. For children aged 2 to 18, BMI is assessed using age- and sex-specific growth charts (BMI-for-age percentiles), which account for normal growth patterns. Consult your child's GP or paediatrician for an appropriate weight assessment.
For most healthy adults, checking BMI once or twice a year alongside other routine health checks is sufficient. More frequent monitoring may be appropriate for individuals actively managing their weight under medical supervision. BMI should be tracked alongside other health markers — weight alone does not tell the full story.