A tree caliper is a special caliper to measure the diameter at breast height of a tree. When used in forestry, the term "caliper" can refer to the diameter of a tree's trunk at breast height itself. The measurement is generally made at 4.5 feet (1.4 m) to 5 feet (1.5 m) above the soil.
"Caliper measurements on young trees are taken 6 inches above the soil. Once a tree's caliper exceeds 4 inches, the tree is measured at a height of 12 inches." 
There are a considerable number of designs of the tool.