The girth of a tree is the measurement of its circumference at a certain height above the ground. It is usually taken at breast height, which is about 1.3 meters for most trees. The girth of a tree can be used to estimate its age, health, and biomass. To measure the girth of a tree, one can use a flexible tape measure or a string that can be wrapped around the trunk and then measured with a ruler. Alternatively, one can use a formula that relates the diameter of a tree to its girth, such as girth = pi * diameter.