Osmolal clearance

It is the amount of urine per minute that would be excreted if the urine was isosmotic with plasma.  

It is calculated by :

osmolal clearance = urine excreted in one minute X osmolality of urine/osmolality of plasma. 

For example, if urine is hypotonic compared to plasma, the osmolal clearance will be greater than lesser than actual urine volume. Also if the urine is hypertonic, osmolal clearance will be greater than urine volume.