To measure the size of a tree for planning or ecological purposes, the standard method used in the UK is to calculate the Diameter at Breast Height (DBH). This refers to the width of the tree’s trunk measured 1.5 metres above ground level.
To take an accurate DBH measurement, first stand at the base of the tree. If the ground is sloping, you should measure from the highest point of ground that meets the base of the tree. From this point, use a measuring tape or stick to find a point 1.5 metres vertically up the trunk.
At this height, use a flexible tape measure to wrap around the tree and measure the circumference of the trunk. If your tape gives the diameter directly, you can use that figure. Otherwise, take the circumference in centimetres and divide it by 3.14 (π) to calculate the diameter. For example, if the circumference is 157 cm, the diameter would be approximately 50 cm (or 500 mm).
If the tree has branches, burls, or other irregularities at 1.5 metres, simply measure just below the obstruction, at the nearest straight section of trunk. If the tree has multiple stems (i.e. it forks below 1.5 metres), each stem should be measured individually at 1.5 metres, and the diameters combined using the formula set out in BS5837 (sum the square of each stem diameter, then take the square root of the total).
Measurements should be rounded to the nearest 10 mm. For surveys involving multiple trees, it is also good practice to label or tag each tree to keep records consistent.
This method ensures a consistent and reliable way of assessing tree size, which is essential for root protection calculations, tree categorisation, and assessing planning impact under the relevant UK standards.
