BMI stands for Body Mass Index. It's a value derived from an individual's weight and height that provides a general indicator of body fatness and health risk categories.

BMI is calculated by dividing a person's weight in kilograms by the square of their height in meters. The formula is: BMI = weight (kg) / [height (m)]².

- Underweight: BMI less than 18.5
- Normal weight: BMI 18.5 to 24.9
- Overweight: BMI 25 to 29.9
- Obesity: BMI 30 or greater

While BMI is a useful screening tool, it doesn't directly measure body fat. It may not be accurate for athletes with high muscle mass, pregnant women, or the elderly. Always consult a healthcare provider for a comprehensive health assessment.

For most adults, checking BMI every 3-6 months is sufficient unless you're actively trying to change your weight. Significant weight changes or health concerns may warrant more frequent checks.