BMI Calculator
Calculate your Body Mass Index
Formula
BMI = weight(kg) / height(m)²
Imperial: BMI = weight(lb) × 703 / height(in)²
BMI Categories
| BMI | Category |
|---|---|
| <18.5 | Underweight |
| 18.5-24.9 | Normal |
| 25-29.9 | Overweight |
| 30-34.9 | Obese I |
| 35-39.9 | Obese II |
| ≥40 | Obese III |
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Complete Guide to BMI Calculator
Body Mass Index (BMI) is a simple, widely-used screening tool that estimates body fat based on your height and weight. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, BMI has become a standard metric used by healthcare professionals worldwide to quickly assess whether someone is at a healthy weight.
How BMI Works
BMI calculates the ratio of your weight to your height squared. The resulting number places you in one of several categories: underweight (below 18.5), normal weight (18.5-24.9), overweight (25-29.9), or obese (30 and above). These categories were developed based on research showing increased health risks at higher BMI levels.
BMI Limitations You Should Know
While BMI is useful for population-level health assessments, it has significant limitations for individuals:
- Muscle vs. Fat: BMI cannot distinguish between muscle and fat. Athletes and bodybuilders often have "overweight" BMIs despite being very fit.
- Age Factors: Older adults may have more fat and less muscle at the same BMI as younger adults.
- Ethnic Differences: Research shows BMI may underestimate health risks in Asian populations and overestimate them in Black populations.
- Fat Distribution: BMI doesn't account for where fat is stored. Belly fat (visceral fat) is more dangerous than fat stored elsewhere.
When to Use BMI
BMI is best used as a starting point for health discussions, not a definitive diagnosis. Consider combining it with other measurements like waist circumference, body fat percentage, and blood pressure for a more complete health picture.
Health Risks by BMI Category
Higher BMI is associated with increased risk of type 2 diabetes, heart disease, high blood pressure, certain cancers, sleep apnea, and osteoarthritis. However, many "overweight" individuals with healthy lifestyle habits have better health outcomes than sedentary "normal weight" individuals.
Frequently Asked Questions
A healthy BMI is between 18.5 and 24.9 for most adults. This range is associated with the lowest risk of weight-related health problems. However, optimal BMI can vary based on age, ethnicity, and muscle mass.
No, BMI often misclassifies athletes as overweight because muscle weighs more than fat. If you exercise regularly and have significant muscle mass, consider measuring body fat percentage instead for a more accurate assessment.
Children and teens use BMI-for-age percentiles instead of fixed categories. A child's BMI is compared to other children of the same age and sex using CDC growth charts. The 85th-95th percentile is overweight; 95th+ is obese.
No. Research shows Asian populations may have higher health risks at lower BMIs, so some guidelines suggest using 23 as the overweight cutoff for Asians. The standard BMI categories may overestimate risks for Black populations.
Yes. 'Normal weight obesity' occurs when someone has a healthy BMI but high body fat and low muscle mass. This is common in sedentary people who don't exercise. Waist circumference and body fat percentage provide additional health insights.
Aim for gradual weight loss of 0.5-1 kg per week through a modest calorie deficit (500-750 calories below TDEE), increased physical activity, and sustainable dietary changes. Focus on whole foods, adequate protein, and strength training to preserve muscle.
A BMI of 40 or higher is classified as Class III obesity (formerly 'morbid obesity'). A BMI of 35+ with obesity-related health conditions also qualifies. At these levels, medical intervention including surgery may be recommended.
Checking monthly is sufficient for tracking progress. Daily fluctuations are normal due to water retention, food intake, and other factors. Focus on long-term trends rather than day-to-day changes.