BMI Calculator (Body Mass Index)

BMI (Body Mass Index) is a numerical measure that uses your weight and height to estimate whether your body weight is in a healthy range. It's calculated by dividing your weight in kilograms by your height in meters squared (kg/m²). BMI doesn't directly measure body fat, but research has shown it correlates reasonably well with more direct measures of body fat for most people. It's used as a screening tool to identify potential weight problems that may lead to health issues. The resulting number falls into categories: underweight (below 18.5), normal weight (18.5-24.9), overweight (25-29.9), or obese (30 and above).

To calculate BMI manually using metric units, divide your weight in kilograms by your height in meters squared. For example, if you weigh 75 kg and are 1.80 m tall: BMI = 75 ÷ (1.80 × 1.80) = 75 ÷ 3.24 = 23.1. For imperial units, multiply your weight in pounds by 703, then divide by your height in inches squared. For example, at 165 lbs and 5'9" (69 inches): BMI = (165 × 703) ÷ (69 × 69) = 115,995 ÷ 4,761 = 24.4. You can use our calculator above for instant, accurate results without manual calculations.

According to the World Health Organization (WHO) and Centers for Disease Control (CDC), a healthy or normal BMI for adults is between 18.5 and 24.9 kg/m². This range is associated with the lowest health risks related to weight. Many health professionals consider a BMI of around 22 to be ideal, as it sits comfortably in the middle of the healthy range. However, "healthy" varies by individual factors including age, sex, ethnicity, muscle mass, and overall fitness level. For Asian populations, the healthy range may be considered 18.5-22.9 due to increased health risks at lower BMIs.

No, BMI has significant limitations for certain populations. It may overestimate body fat in athletes and muscular individuals who have high lean mass. It may underestimate body fat in older adults who have lost muscle mass, or in people with conditions causing muscle wasting. It doesn't account for differences in fat distribution (belly fat is more dangerous than hip fat), doesn't differentiate between fat and muscle, and may not accurately reflect health risks across all ethnicities. For these reasons, BMI should be used as a starting point, not the sole measure of health. Waist circumference, body fat percentage, and other measurements provide additional valuable information.

Obesity is divided into three classes based on BMI. Class I Obesity ranges from BMI 30.0 to 34.9 and is associated with moderate health risks and typically responds well to lifestyle modifications. Class II Obesity ranges from BMI 35.0 to 39.9 and carries high health risks, often requiring more intensive intervention. Class III Obesity, also called severe or morbid obesity, is BMI 40.0 or higher and is associated with significantly elevated health risks and often benefits from medical or surgical intervention. Each higher class is associated with progressively greater risks of obesity-related health conditions including heart disease, type 2 diabetes, and certain cancers.

BMI is important because it provides a quick, inexpensive screening tool for weight categories that may indicate health risks. Research consistently shows that both very high and very low BMIs are associated with increased mortality and health problems. High BMI is linked to cardiovascular disease, type 2 diabetes, high blood pressure, certain cancers, sleep apnea, and joint problems. Low BMI is associated with malnutrition, weakened immunity, bone loss, and fertility issues. By tracking BMI, individuals and healthcare providers can identify potential weight-related health risks early and take preventive action. However, BMI should be combined with other health assessments for a complete picture.

For children and teens aged 2-19 years, BMI is calculated the same way but interpreted differently using age and sex-specific percentile charts. This is because body composition varies significantly as children grow and differs between boys and girls. A child's BMI percentile shows how their BMI compares to other children of the same age and sex. The CDC categorizes children as underweight (below 5th percentile), healthy weight (5th to 85th percentile), overweight (85th to 95th percentile), or obese (95th percentile or above). The CDC provides BMI-for-age growth charts for this purpose.

For most adults maintaining a stable weight, checking BMI every few months or at annual health checkups is sufficient. If you're actively working on weight loss or gain, weekly or bi-weekly measurements can help track progress, though daily weigh-ins may cause unnecessary stress due to normal weight fluctuations. When checking BMI regularly, weigh yourself under consistent conditions: same time of day, same clothing (or none), same scale, preferably in the morning after using the bathroom and before eating. This ensures comparable measurements. Remember that BMI changes gradually, so focus on long-term trends rather than short-term fluctuations.

Yes, it's possible to have a BMI in the overweight range and still be metabolically healthy, a concept sometimes called "metabolically healthy obesity." Some individuals with higher BMIs have normal blood pressure, cholesterol, blood sugar, and other health markers. This is more common when excess weight is carried in the hips and thighs rather than the abdomen, when the person is physically active, and when they maintain healthy eating habits. However, research suggests that metabolically healthy obesity may not remain healthy long-term, with many individuals developing metabolic problems over time. Regular health screenings remain important regardless of apparent health status.

If your BMI is outside the normal range, first consider whether BMI accurately reflects your situation (athletes, for example, may have high BMIs from muscle mass). If BMI is genuinely elevated, consult a healthcare provider who can assess your overall health, identify any weight-related issues, and help develop a safe weight management plan. Evidence-based approaches include balanced nutrition (calorie control, whole foods, reduced processed foods), regular physical activity (150+ minutes moderate exercise weekly), behavioral modifications (mindful eating, adequate sleep, stress management), and in some cases, medical intervention. If underweight, your provider will investigate potential causes and recommend appropriate weight gain strategies. Avoid crash diets or extreme measures; sustainable lifestyle changes yield the best long-term results.