BMI Calculator
Calculate your Body Mass Index (BMI) and understand your weight category. BMI is a useful measure of underweight, healthy weight, overweight, and obesity.
Understanding Body Mass Index (BMI)
What is BMI?
Body Mass Index (BMI) is a numerical value of your weight in relation to your height. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and has become the most widely used method to assess body weight categories. BMI provides a simple numeric measure to assess whether a person has a healthy body weight for their height.
BMI Formula
BMI = Weight (kg) ÷ [Height (m)]²
Metric Units Example:
Weight: 70 kg, Height: 175 cm (1.75 m)
BMI = 70 ÷ (1.75)² = 70 ÷ 3.0625 = 22.86
Imperial Units Example:
Weight: 154 lbs, Height: 5'9" (69 inches)
BMI = (154 × 703) ÷ (69)² = 108,262 ÷ 4,761 = 22.74
BMI Classification (WHO)
| BMI Range | Category | Health Risk |
|---|---|---|
| < 16.0 | Severe Underweight | Very Severe |
| 16.0 - 16.9 | Moderate Underweight | Severe |
| 17.0 - 18.4 | Mild Underweight | Moderate |
| 18.5 - 24.9 | Normal Weight | Low |
| 25.0 - 29.9 | Overweight | Increased |
| 30.0 - 34.9 | Obese Class I | High |
| 35.0 - 39.9 | Obese Class II | Very High |
| ≥ 40.0 | Obese Class III | Extremely High |
Important Limitations of BMI
Body Composition
BMI doesn't distinguish between muscle mass, bone density, and fat. Athletes with high muscle mass may have a high BMI despite low body fat. Conversely, elderly individuals may have normal BMI despite low muscle mass and high fat percentage.
Demographic Factors
BMI cutoffs may not be appropriate for all ethnic groups. For example, some Asian populations may have health risks at lower BMI thresholds. Additionally, BMI interpretations differ for children, teens, pregnant women, and the elderly.
Fat Distribution
BMI doesn't account for where fat is stored. Abdominal fat (visceral fat) poses greater health risks than fat stored in the hips and thighs. Waist circumference and waist-to-hip ratio can be more predictive of certain health risks than BMI alone.
Alternative Assessment Methods
Body Fat Percentage
Directly measures the proportion of fat in the body. Can be assessed via skinfold measurements, bioelectrical impedance analysis, DEXA scans, or hydrostatic weighing. Often considered more accurate than BMI for assessing health risks.
Waist Circumference
Measures abdominal fat, which is associated with increased health risks. Risk is higher if waist measurement exceeds 40 inches (102 cm) for men or 35 inches (88 cm) for women.
Waist-to-Hip Ratio
Divides waist measurement by hip measurement. Ratios above 0.9 for men and 0.85 for women indicate increased health risk. This measurement accounts for fat distribution patterns.
Body Adiposity Index (BAI)
Uses hip circumference and height to estimate body fat percentage. Formula: BAI = (hip circumference in cm) ÷ (height in m)^1.5 - 18. Developed as an alternative to BMI.
Health Implications of Different BMI Categories
- Underweight (BMI < 18.5): Associated with malnutrition, weakened immune system, fertility problems, osteoporosis, and in severe cases, increased mortality. Often requires medical evaluation to identify underlying causes.
- Normal Weight (BMI 18.5-24.9): Generally associated with optimal health outcomes and lowest risk of weight-related diseases. Maintaining this range through balanced nutrition and regular physical activity is recommended.
- Overweight (BMI 25-29.9): Linked to moderately increased risk of cardiovascular disease, type 2 diabetes, certain cancers, sleep apnea, and osteoarthritis. Risk varies based on fat distribution and overall fitness level.
- Obesity (BMI ≥ 30): Significantly elevated risk of serious health conditions including heart disease, stroke, type 2 diabetes, sleep apnea, fatty liver disease, kidney disease, and certain cancers. Risk increases with higher BMI classifications.
Important Notice
BMI is a screening tool, not a diagnostic tool. It should be used as one component of a broader health assessment in consultation with healthcare providers. Individual factors such as muscle mass, bone density, overall physical fitness, and specific health conditions must be considered alongside BMI when evaluating health status and risks. If you have concerns about your weight or BMI classification, please consult with a healthcare professional.