Introduction Pulmonary function tests (PFTs) are critical tools in evaluating respiratory health by measuring lung capacity, airflow, and overall pulmonary performance. These tests provide insights into various respiratory disorders and help assess the impact of different physiological and environmental factors on lung function. Among the numerous factors influencing pulmonary function, body mass index (BMI) has emerged as a significant determinant Methods A cross sectional study was done at Department of Physiology The sampling was purposive for selection of specified population based on their body weight. The selection of obese and underweight was deviant purposive technique. Sample size was calculated by using open epi sample size calculator version 3.01. The parameters included were mean and standard deviation of FEV, 108±1.2 of the subjects having duration of obesity less than five years and 107±1.2 of subjects having duration of obesity more five-ten years,9 The 95% confidence interval and 90% power was considered for sample size calculation. Minimum needed sample size came up as 31 participants in each group of BMI category. A total of 180 participants were calculated. Results The present study included 200 participants (120 males and 80 females). All participants aged 18-20 years who are apparently healthy. Male and female according to BMI categorized into two groups, i.e., BMI < 25 and BMI > 25. Respiratory parameters such as FVC, FEV1, FEV1/FVC, and PEFR are found to be significantly lower in female participants in comparison to male participants as shown in Table 1. The correlation of different pulmonary function parameters with BMI of the male and female participants are presented in Tables 2 and 3. It is observed that respiratory parameters such as FVC, FEV1, FEV1/FVC, and PEFR of both male and female participants correlated positively with BMI < 25 but there were substantial lung function losses with increasing BMI > 25, i.e., PFTs were negatively correlated Conclusion Obesity influences the respiratory function enhancing dyspnoea and increasing both cardiac load and respiratory muscle fatigue of the thoracic wall and the diaphragm due to the higher pressure exerted by intrabdominal adipose accumulation. In our study the results showed that increase in BMI had an inverse relationship with FVC, FEV1 and PEFR in obese when compared to the normal weight subjects.