Optimum moisture content refers to the moisture content of soil at which it can be compacted to the maximum dry unit weight by a given compactive effort. It is a measure of the compaction level of soils and is determined by performing the Proctor Test in a laboratory. The Optimum Moisture Content (OMC) is the moisture content at which the maximum dry density is attained. The determination of maximum dry density and optimum moisture content of the soil is important in highway construction, earth dam construction, and other civil engineering structures.

The OPT is the water content of the soil in which it can be compacted the most. If there is too much water, there would be too much pore water pressure during compression to compact any further. If there is too little water, the soil would naturally resist compaction via shear strength/friction/effective stress. The OPT is important because if tillage is carried out on fields that are wetter or drier than the OPT, many problems can be caused, including soil structural damage, through the production of large clods, and an increase in the content of readily dispersible clay which is indicative of the soil stability.

The Optimum Moisture Content (OMC) is the moisture content at which the soil attains maximum dry density. This OMC value is with respect to the specific amount of compaction energy applied to the soil. The relationship between maximum dry density and optimum moisture content of soil can be obtained from the soil compaction curve obtained from the Standard Proctor Compaction Test. The maximum peak point of the soil compaction curve obtained is called the Maximum dry density value. The water content corresponding to this point is called the Optimum water content (O.W.C) or optimum moisture content (O.M.C) .

In summary, optimum moisture content is the moisture content of soil at which it can be compacted to the maximum dry unit weight by a given compactive effort. It is determined by performing the Proctor Test in a laboratory and is important in highway construction, earth dam construction, and other civil engineering structures.