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Body Mass Index (BMI) is a simple calculation using your height and weight to estimate whether you're in a healthy weight range. The formula divides your weight in kilograms by the square of your height in meters. The World Health Organization (WHO) classifies BMI into four categories: underweight (below 18.5), normal (18.5 to 24.9), overweight (25 to 29.9), and obese (30 and above).
BMI doesn't distinguish between muscle and fat mass. Athletes with high muscle mass may have a high BMI without excess body fat. For a more complete picture, consider using our Body Fat Calculator or Fat-Free Mass Index (FFMI) Calculator, which accounts for lean mass relative to height.
BMI is a useful screening tool for the general population to quickly assess whether bodyweight falls within a healthy range. It works best for people who do not carry unusually high amounts of muscle mass. If you strength train regularly, pair BMI with a body fat measurement for a more accurate picture. BMI is also commonly used by healthcare providers as a first step in evaluating weight-related health risks.
The most common mistake is treating BMI as a definitive health indicator rather than a screening tool. A high BMI does not automatically mean someone is unhealthy, especially if they carry significant muscle. Another frequent error is measuring height incorrectly or using outdated weight data. Always measure in the morning under consistent conditions for the most reliable results. Finally, do not compare your BMI to others without considering differences in age, gender, and body composition.
A BMI between 18.5 and 24.9 is considered normal weight by WHO standards. This range is associated with the lowest risk of weight-related health issues. However, individual health depends on many factors beyond BMI alone.
No. BMI does not differentiate between muscle and fat. Someone who strength trains heavily may be classified as overweight or obese by BMI while having a low body fat percentage. In these cases, the Fat-Free Mass Index (FFMI) Calculator is a better metric.
The standard BMI formula is the same for all adults regardless of age or gender. However, body composition naturally shifts with age - older adults tend to carry more fat at the same BMI. Women also typically have higher body fat percentages than men at equal BMI values.
Checking BMI once a month is enough for most people. If you are actively losing or gaining weight, tracking weekly can help you monitor progress. Pair it with other metrics like waist circumference or body fat percentage for a more complete picture.