Moisture content is a measure of the amount of water or water vapor contained within a substance. It can be helpful to think of it as the percent by mass of water in a sample of a mixture or form of matter. The moisture content of most substances is a variable factor that can change with weather and temperature.

Knowing the amount of water or moisture content of a substance can determine if a substance is suitable for a specific use. For example, the moisture content in soil directly influences the types of organisms that can live in that soil. The amount of water in the soil can be measured with probes that attach to hand-held computers. When the probe is inserted into the soil and activated, it can provide an instant reading.

In the agricultural processing of foods such as coffee or cocoa beans, knowing the moisture content is key to determining when it is safe to package and ship the product. If coffee or cocoa beans have a moisture content that is too great, they will mold when packaged and will not be safe to consume. Conversely, if the moisture content is too low, coffee and cocoa lose some flavor.

Establishing the ideal moisture content of a substance for a commercial purpose takes some trial and error. Often, as in the case of coffee and cocoa beans, simple techniques may be the most effective in attaining the ideal moisture content. The sun, simple drying racks, and time are factors that may be utilized to reduce the moisture content for many food products.

The term for lowering moisture content in a substance is dehydration. Dehydration can turn an otherwise juicy fruit or vegetable into a little, dry, shriveled model of its former self. Many mushrooms and fungus are sold in a dehydrated form in which nearly all moisture has been removed. When these mushrooms are soaked in water, which is called rehydration, they are very similar in taste, texture and appearance to fresh versions.

Carpenters know the importance of moisture content as it relates to wood. Green wood, or wood that was very recently a standing tree, has a high moisture content and is not suitable for building material. Using wood with a high moisture content can cause bowing, buckling, and even formation of large cracks once the moisture begins to evaporate. On the other hand, rehydrating wood by soaking it can allow it to be bent to create specific shapes. This is useful when constructing wood products that need to be curved, such as guitars. Determining and controlling the moisture content of substances is unique and necessary for many products, and the process borders between art and science.