Instead of cutting down a tree and count its growth rings or use an increment borer to find out how old a tree is, you can make a fairly good estimate of the age of a tree by using a simple formula published by the International Society of Arboriculture.
First, determine tree diameter in inches measured at 54 inches above ground level (remember that diameter equals circumference divided by 3.14 (pi)).
Next, use the table below, which assigns a growth factor to various tree species. Multiply the diameter in inches by the appropriate growth factor to determine the estimated age of your tree.
Let's use the white oak, for example. It has a growth factor of 5. If our white oak has a diameter of 22 inches, we'd multiply that by 5 to get an estimated age of 110 years (diameter in inches x growth factor = tree age)
Tree species and related growth factors
|Bradford pear (invasive non-native, not recommended)||3|
|Colorado blue spruce||4.5|
|European white birch||5|
|Kentucky coffee tree||3|
|Northern red oak||4|