In the oil industry, mud weight is the density of the drilling fluid and is normally measured in pounds per gallon (lb/gal) (ppg) or pound cubic feet (pcf) . In the field it is measured using a mud scale or mud balance. Mud can weigh up to 22 or 23 ppg. A gallon of water typically weighs 8.33 pounds (or 7.48 ppg).

Mud Weight Can Increased by using Barite ( minimum specific gravity of 4.20 g/cm3) Or Hematite (minimum specific gravity of 5.3 g/cm3).Mud weight can be decreased by dilution or solids control equipment such as an industrial centrifuge, desilter, desander and shale shaker . 
Mud weight use to control the trapped fluids or gas in the formations by adding a hydro static pressure on them, increasing the mud weight = increasing the hydro static pressure. If the hydro static pressure increased over the formations pressure that will make a fracture in the formation leading to lose the mud to the formation.