Background: Obesity and poor respiratory function are associated with morbidity and mortality. Obesity affects lung function, however the impact of all degrees of obesity on lung function need to be explored in different populations and genders. Aims and Objectives: The authors investigated the relation of BMI, waist circumference, physical activity with lung function in Hail City, Saudi Arabia. Materials and Methods: This analysis included 359 females aged 18–44 years with no known preexisting serious illness and who had complete anthropometric (height, weight and waist circumference) and forced vital capacity (FVC) using simple spirometry and chest measurements. Physical activity was measured using “International Physical Activity Questionnaire” (IPAQ). Results: Both FVC and predicted FVC along with chest expansion measurements were linearly and inversely related across the entire range of body mass index (BMI) and waist circumference (WC) and positively associated with physical activity in study subjects even after adjusting for age confirming our hypothesis. However, BMI and physical activity explained the greatest proportion of variance for both FVC and chest expansion in regression analysis as compared to WC. Conclusion: In the general adult female population, obesity may play a role in the impairment of lung function even from BMI ≥ 35 kg/m2 while even moderate physical activity can positively affect lung function.