Forest mensuration involves the measurement and quantification of forests and trees. A comprehensive understanding of the methods and processes employed in these measurements is essential for forest managers. This encompasses measures of linear length, area, volume, and weight. Techniques often involve measuring easily identifiable parts of an object, with the actual object size estimated using intricate mathematical formulas derived from component measurements. Whether measuring something basic or highly complex, precision, bias, and accuracy are critical considerations. Therefore, precision-enhancing methods for different dimensions are pivotal to ensure accurate and reliable measurements. This chapter delves into various tree dimensions, including height, diameter at breast height (DBH), girth, bark thickness, tree form, volume, felled logs, and branch measurements. It also explores factors influencing site productivity, such as site quality and canopy density. These measurements are essential for foresters to establish relationships among dependent tree dimensions. This chapter further highlights the importance of forest inventory in estimating the productivity of larger forest areas. The adage “if you cannot measure it, you cannot manage it” underscores the vital role mensuration plays in effective forest management. Mensuration is crucial for obtaining information about forest resources, with the ultimate goal of providing quantitative data to make informed decisions regarding density, use, and management. The discussion extends to factors influencing site production, with a detailed examination of site quality and canopy density. Establishing relationships between dependent tree components is imperative for foresters. This chapter sheds light on forest inventory, contributing to the estimation of productivity in broader forest regions. Effective forest management relies on accurate data, making mensuration an integral component in the quest for information about forest resources and informed decision-making.