The girth of a tree is usually much easier to measure than the height, as it is a simple matter of stretching a tape round the trunk, and pulling it taut to find the circumference.
As a general standard, tree girth is taken at “breast height”. This is converted to and cited as dbh (diameter at breast height) in tree and forestry literature. Breast height is defined differently in different situations, with most forestry measurements taking girth at 1.3 m above ground, while those who measure ornamental trees usually measure at 1.5 m above ground; in most cases this makes little difference to the measured girth. On sloping ground, the “above ground” reference point is usually taken as the highest point on the ground touching the trunk, but in North America a point, that is the average of the highest point and the lowest point the tree trunk appears to contact the soil, is usually used. Some of the inflated old measurements may have been taken at ground level. Some past exaggerated measurements also result from measuring the complete next-to-bark measurement, pushing the tape in and out over every crevice and buttress. The measurements could also be influenced by deviation of the tape measure from a horizontal plane (which might seem called for if the trunk does not grow straight up), and the presence of features such as branches, spikes, etc.
Modern trends are to cite the tree’s diameter rather than the circumference. The diameter of the tree is calculated by finding the mean diameter of the trunk, in most cases obtained by dividing the measured circumference by π; this assumes the trunk is mostly circular in cross-section (an oval or irregular cross-section would result in a mean diameter slightly greater than the assumed circle). Accurately measuring circumference or diameter is difficult in species with the large buttresses that are characteristic of many species of rainforest trees. Simple measurement of circumference of such trees can be misleading when the circumference includes much empty space between buttresses. See also Tree girth measurement
Baobabs (genus Adansonia) store large amounts of water in the very soft wood in their trunks. This leads to marked variation in their girth over the year (though not more than about 2.5%), reaching maximum at the end of the rainy season, and minimum at the end of the dry season.
List of Stoutest Living Single-Trunk Trees by Species
|Tree Species||Tree Diameter||Tree name||Tree Location|
|Montezuma cypress (Taxodium mucronatum)||11.62||38.1||Árbol del Tule||Santa Maria del Tule, Oaxaca, Mexico|
|Baobab (Adansonia digitata):||10.64||34.9||Sunland Baobab||Sunland Farm, Limpopo, South Africa|
|Coast redwood (Sequoia sempervirens)||8.90||29.2||Jupiter||Redwood National Park, California, United States|
|Giant sequoia (Sequoiadendron giganteum)||8.85||29.0||General Grant||General Grant Grove, California, United States|
|Za (Adansonia za)||8.85||29.0||The Ampanihy Baobab||North of Morombe, southwest Madagascar|
|Chinese camphor tree (Cinnamomum camphora)||8.23||27.0||Kamou no Okusu||Kamou, Kagoshima, Japan|
|Eucalyptus regnans||6.52||21.4||Big Foot||Geeveston, Tasmania, Australia|
|Western redcedar (Thuja plicata)||5.94||19.5||Quinault Lake Cedar||Olympic National Park, Washington, United States|
|Eucalyptus delegatensis||5.82||19.1||Troll||Hermons Road, Tasmania, Australia|
|Sitka spruce (Picea sitchensis)||5.39||17.7||Quinault Lake Spruce||Olympic National Park, Washington, United States|
|Kauri (Agathis australis)||5.33||17.5||Te Matua Ngahere||Waipoua Forest, New Zealand|
Measurements become ambiguous when multiple trunks (whether from an individual tree or multiple trees) grow together. The Sacred Fig grows adventitious roots from its branches, which become new trunks when the root reaches the ground and thickens; a single sacred fig tree can have hundreds of such trunks. The multi-stemmed Hundred Horse Chestnut was known to have a circumference of 57.9 m (190 ft) when it was measured in 1780.
There are known more than 50 species of trees exceeding the diameter of 4.45 m or circumference of 14 m.