The "clearance" of all osmotically active particles can be calculated in manner similar to the clearance of individual substances:
Cosm = Uosm x V / Posm
This is termed the osmolar clearance, and can be viewed as the volume of plasma cleared of osmotically active particles per unit time
The difference between the urine formation rate (V) and the osmolar clearance (Cosm), is termed the free water clearance, and can be viewed as the volume of pure water subtracted from (positive free water clearance) or added to (negative free water clearance) the plasma per unit time: