Weight loss is often intentional or has a clear reason. If you do not take enough calorie for a long time, you will lose body weight. If you lose 5% of body weight over 6-12 months for no obvious reasons, it suggests underlying pathology.
To calculate your Body Mass Index (BMI), the measured of body weight status, use Boby Mass Index (BMI) Calculator
By answering questions about your condition, you can find the causes.