To predict and treat pleural fluid, fluid assessment is very important. We must assess whether the fluid is free or localized, exudative or transudative, purulent or bloody, whether the fluid is in a bedridden patient or not, whether the fluid is in a person with lung or pleural cancer or not, etc. From there, we can assess the level of impact on respiratory function. In the following article, we will introduce some ways to measure the amount of free pleural fluid on ultrasound.