Health

BMI Calculator

Calculate your Body Mass Index instantly

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Enter your weight and height above to calculate your BMI

Not medical advice. Consult a healthcare professional before acting on these results.

What is BMI and How Is It Calculated?

Body Mass Index (BMI) is a simple numerical measure calculated from a person's height and weight. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet as a population-level screening tool and remains one of the most widely used health metrics today.

The BMI formula is straightforward:

BMI = weight (kg) ÷ height² (m²)

In imperial units, the formula adjusts to: BMI = (weight in lbs × 703) ÷ height² (in inches²). For example, someone who weighs 180 lbs and is 5'10" tall (70 inches) would have a BMI of (180 × 703) ÷ (70 × 70) = 25.8.

The Four BMI Categories Explained

The World Health Organization (WHO) defines four main BMI categories for adults:

  • Underweight (BMI below 18.5): May indicate nutritional deficiency, hormonal issues, or other health conditions. Associated with increased risk of osteoporosis and immune system impairment.
  • Normal weight (BMI 18.5–24.9): Generally associated with the lowest health risks for most adults. This is the target range for maintaining good health.
  • Overweight (BMI 25–29.9): Associated with increased risk of conditions like type 2 diabetes, heart disease, and hypertension. Lifestyle changes can reduce risk significantly.
  • Obese (BMI 30 and above): Associated with substantially higher risk of serious health conditions. Medical guidance is typically recommended.

Where BMI Falls Short

BMI is a useful screening tool, but it has well-documented limitations. Because it only considers total body weight relative to height, it cannot distinguish between muscle mass and fat mass. This leads to some well-known issues:

  • Athletes and muscular individuals often register as "overweight" or even "obese" despite having very low body fat percentages. Bodybuilders, rugby players, and elite athletes frequently fall into these categories purely due to muscle mass.
  • Older adults may have a "normal" BMI but carry excess fat while having lost muscle mass — a condition called sarcopenic obesity that BMI cannot detect.
  • Ethnicity differences: Research shows that Asian populations tend to have higher body fat percentages at the same BMI as Western populations. Organizations like the World Health Organization have proposed lower BMI thresholds (23.0 for overweight, 27.5 for obesity) for Asian populations.
  • Sex differences: Women typically carry more essential fat than men at the same BMI. A BMI of 22 represents different body compositions in men vs. women.

Better Health Metrics to Use Alongside BMI

Healthcare providers rarely rely on BMI alone. Here are other measurements that provide a fuller picture:

  • Waist circumference: Abdominal fat is a stronger predictor of metabolic disease than BMI. Health thresholds are generally 35 inches (88 cm) for women and 40 inches (102 cm) for men.
  • Waist-to-hip ratio: Measures fat distribution. A ratio above 0.90 for men and 0.85 for women indicates abdominal obesity.
  • Body fat percentage: More accurate than BMI. Healthy ranges are typically 10–20% for men and 18–28% for women.
  • Blood pressure, cholesterol, blood glucose: These clinical measurements directly assess metabolic health risk, regardless of BMI.

How to Reach a Healthy BMI

If your BMI is outside the healthy range, the goal is not to hit a specific number — it is to improve your overall health. Evidence-based approaches include:

  • A moderate calorie deficit (300–500 calories/day below maintenance) combined with regular physical activity for weight loss
  • Resistance training to build muscle and improve body composition regardless of weight changes
  • 150+ minutes per week of moderate aerobic activity (CDC/WHO recommendation)
  • Working with a registered dietitian for a sustainable nutrition plan
  • For underweight individuals, a structured increase in caloric intake with adequate protein is typically recommended

A BMI reduction of just 1–2 units can significantly reduce risk of metabolic conditions. The goal is sustainable, gradual change rather than rapid weight loss.

BMI vs. BMI for Children

This calculator is designed for adults (18 and over). BMI interpretation is different for children and teenagers because body composition changes significantly during development. For children aged 2–19, BMI is plotted on CDC growth charts and expressed as a percentile relative to other children of the same age and sex. A BMI at or above the 95th percentile is considered obese; 85th–94th percentile is overweight.

Frequently Asked Questions

What is BMI and what does it measure?
BMI (Body Mass Index) is a numerical value calculated from your height and weight. It provides a rough estimate of body fat and is used to screen for weight categories that may lead to health problems. BMI does not directly measure body fat — it is a screening tool, not a diagnostic one.
What are the BMI categories?
The standard BMI categories (WHO) are: Underweight = below 18.5, Normal weight = 18.5–24.9, Overweight = 25–29.9, Obesity Class I = 30–34.9, Obesity Class II = 35–39.9, Obesity Class III = 40 and above. A "healthy" BMI is typically considered to be between 18.5 and 24.9.
Is BMI accurate for everyone?
BMI has known limitations. It does not account for muscle mass, bone density, age, sex, or ethnicity. Athletes with high muscle mass often have a "overweight" BMI despite being very fit. Older adults may have a healthy BMI but high body fat. Asian populations may have health risks at lower BMI thresholds. Always consult a healthcare provider for a proper health assessment.
How do I calculate BMI?
The BMI formula is: BMI = weight (kg) ÷ height² (m²). For example, a person weighing 70 kg at 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9. In imperial units: BMI = (weight in lbs × 703) ÷ height² (inches²).
What is a healthy BMI for adults?
For most adults, a BMI between 18.5 and 24.9 is considered healthy. However, optimal ranges vary by age, sex, and ethnicity. For example, some health organizations recommend a lower threshold (23.0) for Asian adults due to different fat distribution patterns at comparable BMI levels.
What should I do if my BMI is outside the healthy range?
If your BMI is outside the 18.5–24.9 range, consult a doctor or registered dietitian. BMI is just one data point — your doctor will consider other factors like waist circumference, blood pressure, blood tests, and lifestyle before making any recommendations. Small, sustainable lifestyle changes (diet and exercise) are usually the first step for overweight individuals.
Does BMI apply to children?
BMI for children and teens is calculated differently — it is plotted on growth charts specific to age and sex, and expressed as a percentile rather than a fixed number. This calculator is designed for adults. For children, consult a pediatric BMI calculator or your child's doctor.
What is my healthy weight range?
Your healthy weight range is the range of weights that produce a BMI between 18.5 and 24.9 for your height. For example, for a person 5'9" (175 cm) tall, the healthy weight range is approximately 57 kg (125 lbs) to 77 kg (170 lbs). This calculator shows your specific healthy weight range based on your height.

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