Tree-Ring Dating The most common, most accurate way to find the age of a tree is to count the number of rings visible when their trunk is cut horizontally. Each year, most trees add an extra layer of growth to their trunks. Over time, their trunks get thicker and thicker.

How do you date a living tree?

To do this, you need to multiply the diameter of the tree by its species-specific growth factor. First, you measure the circumference of the trunk in inches. Next, calculate the diameter and then multiply the diameter by the species average growth factor. Now you will have a rough age of the living tree!