BMI Calculator

Calculate your Body Mass Index in seconds and see where you stand on the WHO scale.

⚖️ What is BMI?

BMI stands for Body Mass Index - a numerical value calculated from a person's height and weight that provides a simple, quick way to screen for weight categories associated with potential health risks. It was developed in the 19th century by the Belgian statistician Adolphe Quetelet and later adopted by health organisations worldwide as a standard screening metric.

BMI is used by doctors, nutritionists, and public health researchers because it requires only two easily measurable inputs - height and weight - and produces a single number that can be compared against internationally standardised ranges. It is not a direct measure of body fat; it is a ratio that correlates with body fat at a population level, making it a useful first-pass screening tool rather than a precise diagnostic instrument.

For adults aged 18 and over, WHO defines four main BMI categories: Underweight (below 18.5), Normal or Healthy Weight (18.5 to 24.9), Overweight (25 to 29.9), and Obese (30 or above). The obese category is further divided into Class I (30–34.9), Class II (35–39.9), and Class III (40 and above), also known as severe or morbid obesity.

It is important to understand BMI's limitations. Because it only considers height and weight, it cannot distinguish between fat mass and lean mass. A highly muscular athlete may have the same BMI as someone with excess body fat. Similarly, an older adult who has lost muscle mass may appear at a normal BMI despite having an unhealthy body composition. BMI is best used as part of a broader health assessment that includes waist circumference, blood pressure, blood glucose, and cholesterol levels.

📐 BMI Formula

The formula is simple and straightforward in metric units:

BMI = Weight (kg) ÷ Height (m)²

Weight = Body weight in kilograms (kg)

Height = Height in metres (m) - convert centimetres by dividing by 100

In imperial units (pounds and inches), the formula includes a conversion factor:

BMI = [Weight (lb) ÷ Height (in)²] × 703

703 = Conversion factor to align imperial units with the metric result

Height (in) = Total height in inches (e.g. 5 ft 7 in = 67 inches)

Both formulas produce the same result when the same person's measurements are converted correctly between unit systems.

📖 How to Use This Calculator

Steps to Calculate Your BMI

Choose your unit system using the tabs at the top - Metric (cm and kg) or Imperial (feet, inches, and pounds).

Enter your height either by typing in the field or using the slider. For metric, enter centimetres. For imperial, enter feet and inches separately.

Enter your weight in kilograms (metric) or pounds (imperial).

Click Calculate BMI to see your BMI value, your WHO category, and your position on the BMI scale.

Use the "Healthy Range" shown in the results to understand what weight you would need to be to fall within the 18.5–24.9 normal range at your current height.

💡 Example Calculations

Example 1 — Adult Female, Metric

Height: 162 cm, Weight: 58 kg

Convert height to metres: 162 cm ÷ 100 = 1.62 m

BMI = 58 ÷ (1.62)² = 58 ÷ 2.6244 = 22.1

BMI = 22.1 → Normal / Healthy Weight

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Example 2 — Adult Male, Imperial

Height: 5 ft 10 in, Weight: 210 lb

Convert height: 5 × 12 + 10 = 70 inches total

BMI = [210 ÷ (70)²] × 703 = [210 ÷ 4900] × 703 = 0.04286 × 703 = 30.1

BMI = 30.1 → Obese (Class I) - Healthy weight at this height: 129–174 lb

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❓ Frequently Asked Questions

What is a normal BMI for adults?+

According to the World Health Organization, a BMI between 18.5 and 24.9 is classified as normal or healthy weight for adults. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is classified as obese. These thresholds apply to adults aged 18 and over regardless of sex or age.

Is BMI accurate for everyone?+

BMI is accurate as a population screening tool but has limitations for individuals. It cannot differentiate between fat mass and lean muscle mass. An athlete with significant muscle may show a "overweight" BMI despite having low body fat. Conversely, an older adult who has lost muscle may have a "normal" BMI despite having excess fat. Use BMI alongside waist circumference and other health metrics for a more complete picture.

What is the BMI formula?+

In metric units: BMI = weight(kg) / height(m)². For example, a person who weighs 70 kg and is 1.75 m tall has a BMI of 70 / (1.75 × 1.75) = 70 / 3.0625 = 22.9. In imperial units, the formula is: BMI = (weight in lb / height in inches²) × 703.

How do I lower my BMI?+

Lowering BMI means reducing weight while maintaining or increasing height (which isn't possible for adults). Focus on reducing caloric intake moderately - a deficit of 500 calories per day typically results in about 0.45 kg (1 lb) of weight loss per week. Combine this with regular aerobic exercise and strength training. Consult a doctor before starting any weight loss programme, especially if your BMI is in the obese range.

Does BMI differ by age?+

For adults 18 and over, the same BMI cut-off values apply regardless of age. However, for children and teenagers (under 18), BMI is not interpreted using fixed categories. Instead, it is plotted on age- and sex-specific growth percentile charts. A child with the same numerical BMI as an adult may be in a completely different health category.

What is a good BMI for a 30-year-old woman?+

The WHO BMI classification is the same for all adults regardless of age or sex: 18.5–24.9 is healthy weight, 25–29.9 is overweight, and 30+ is obese. For a 30-year-old woman who is 162 cm tall, a healthy weight range is approximately 48.5–65.4 kg. However, some research suggests Asian populations may have higher metabolic health risks at lower BMI thresholds - around 23 and above - so consult a doctor for personalised guidance.

What BMI is considered obese?+

A BMI of 30 or above is classified as obese according to the WHO, divided into: Class 1 obesity (30–34.9), Class 2 obesity (35–39.9), and Class 3 / severe obesity (40+). Some health organisations use a lower threshold of 27.5 for Asian populations due to differences in body fat distribution and associated metabolic risks at lower body weights.

Can I have a normal BMI but still be unhealthy?+

Yes. This is sometimes called 'normal weight obesity' or 'skinny fat.' Someone with a BMI of 22 could still have high body fat percentage and low muscle mass if they are sedentary. BMI does not measure body fat distribution. Abdominal fat (measured by waist circumference) is a stronger predictor of cardiovascular risk than BMI alone - a waist over 80 cm (women) or 94 cm (men) signals elevated metabolic risk regardless of BMI.

Is BMI the same for men and women?+

The BMI formula and classification ranges are identical for men and women. However, at the same BMI value, women typically have a higher percentage of body fat than men, and men typically have more muscle mass. This means BMI may slightly underestimate health risk in women and overestimate it in heavily muscled men. Body fat percentage measurement (via calipers, bioimpedance, or DEXA scan) provides a more accurate individual assessment.

🔗 Related Calculators

What is a normal BMI for adults?

According to the WHO, a BMI between 18.5 and 24.9 is classified as normal or healthy weight for adults. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese.

Is BMI accurate for everyone?

BMI is a useful population-level screening tool but has limitations. It does not directly measure body fat percentage or distribution. Athletes with high muscle mass may show a high BMI but have low body fat. Older adults may show a normal BMI but have high body fat. Use it as one of several health indicators.

What is the BMI formula?

BMI is calculated as weight in kilograms divided by height in metres squared: BMI = kg / m². In imperial units: BMI = (pounds / inches²) × 703.

How do I lower my BMI?

A BMI in the overweight or obese range can typically be reduced through a combination of dietary changes and regular physical activity. Reducing caloric intake by 500 calories per day typically leads to about 0.5 kg of weight loss per week. Consult a doctor or registered dietitian for a personalised plan.

Does BMI differ by age?

For adults 18 and over, the same BMI ranges apply regardless of age. However, for children and teenagers, BMI is interpreted using age- and sex-specific percentile charts because body composition changes significantly during growth.

What is a good BMI for a 30-year-old woman?

The WHO BMI classification is the same for all adults regardless of age or sex: 18.5–24.9 is healthy weight, 25–29.9 is overweight, and 30+ is obese. For a 30-year-old woman who is 162 cm tall, a healthy weight range would be approximately 48.5–65.4 kg (BMI 18.5–24.9). However, some research suggests Asian populations may have higher health risks at lower BMI thresholds - around 23 and above.

What BMI is considered obese?

A BMI of 30 or above is classified as obese according to the WHO. This is further divided into: Class 1 obesity (BMI 30–34.9), Class 2 obesity (BMI 35–39.9), and Class 3 / severe obesity (BMI 40+). Some health organisations use a lower threshold of 27.5 for Asian populations due to differences in body fat distribution and associated metabolic risks.

Can I have a normal BMI but still be unhealthy?

Yes. This is called 'normal weight obesity' or 'skinny fat.' Someone with a BMI of 22 could still have high body fat percentage and low muscle mass if they are sedentary. BMI does not measure body fat distribution. Abdominal fat (measured by waist circumference) is a stronger predictor of cardiovascular risk than BMI alone. A waist over 80 cm (women) or 94 cm (men) signals elevated metabolic risk regardless of BMI.

Is BMI the same for men and women?

The BMI formula and classification ranges are identical for men and women. However, at the same BMI value, women typically have a higher percentage of body fat than men, and men typically have more muscle mass. This means BMI may slightly underestimate health risk in women and overestimate it in heavily muscled men. For a more accurate assessment, body fat percentage measurement (using calipers or DEXA scan) is recommended.

📌 Quick Tips

💡BMI is a screening tool, not a diagnostic tool. Muscle mass and bone density also matter.

💡Athletes often have a high BMI due to muscle, not fat. Use BMI alongside waist measurement for better accuracy.

💡Children and teens use age- and sex-specific BMI percentiles - this calculator is for adults 18+.

💡The FAQ section below answers the most common questions about this topic.

💡Results are rounded to a practical number of decimal places for everyday use.