Girth in plants, specifically in trees, is a measurement of the circumference of the tree trunk. It is one of the most ancient, quickest, and simplest of foresters measures of size and records of growth of living and standing trees. Girth is a measurement of the distance around the trunk of a tree measured perpendicular to the axis of the trunk. In the United States, it is measured at breast height, or at 4.5 feet (1.4 m) above ground level. Elsewhere in the world, it is measured at a height of 1.3 meters (4.3 ft), 1.35 meters (4.4 ft), 1.4 meters (4.6 ft), or 1.5 meters (4.9 ft). The girth or circumference of the trunk is the most often measured parameter of monumental trees. For most trees in temperate climates, a growth ring is formed each year, so the gradually increasing circumference is an indicator of tree age. While a tree is alive, it increases in girth annually, although some trees can collapse due to old age or can have different girths in dry and wet seasons.

Measuring tree girth directly is a common educational technique allowing students to learn about their local environment in a practical manner. It is often used in the primary setting to introduce topics such as measuring, using numbers, and simple calculation. Simple techniques can be used to estimate tree age. Remote girth measurements are also possible.

In summary, girth in plants, specifically in trees, is a measurement of the circumference of the tree trunk, which is used to estimate tree age and is measured at breast height or at a specific height above ground level depending on the country.