The following are some instructions on how to accurately estimate the size of your tree:

Tree Circumference

Use a tape measure to measure around the circumference of the tree as in the following examples:

First, measure the circumference of the tree 1m from the ground.

For multi-stem trees measure the circumference of each stem at 1m and add them together.

Canberra Tree Services - measurement J

Then measure the circumference of the tree around the base.

Canberra Tree Services - measurement HCanberra Tree Services - measurement G

Tree Height

Using a stick or any other reasonably straight object, position the object so that it is at arms length from your body. Maintaining this stance, position your body so that the top and the bottom of the object, when viewing at arms length, appear to match the top and bottom of the tree.

At this point, take a tape measure from your feet through to the base of the tree. This will provide a reasonable estimate of the height of the tree.

Canberra Tree Services - measurement A

Canberra Tree Services - measurement C

Canberra Tree Services - measurement B

Tree Width

Canopy width is measured by taking a tape measure from beneath the furthest point of the canopy through to beneath the furthest point of the canopy on the opposite side of the tree.

Further Info

Read here for more information.