[Note to users: This page has been revised from earlier versions by incorporating field research by the Morton Arboretum. Those species published by the Morton are now listed in a second data table.]
Since the only sure way to age a tree is by cutting a tree down and counting growth rings or by taking an invasive trunk bore and counting rings, this short-cut method will give you an approximation, but an approximation that you should realize is a mid-point of a range of probable years.
Step 1: Determine the tree species. Some species may be grouped such as the White Oak species may be used for other members of the White Oak Group such as Bur Oak and Swamp White Oak.
Step 2: Determine the diameter, in inches, of the tree at 4.5 feet about the stump level. This can be most easily done by measuring the circumference with a tape measure at that height and then dividing the circumference by 3.1416 to arrive at the diameter in inches. [Note the circumference formula of C=2 π r gives the radius, dividing by pi directly gives the diameter]
Step 3: Locate the growth factor for the tree species in the growth factor table and multiply that by the diameter in inches to arrive at the approximate age of the tree.
This first table was published years ago by Forestry.com, but without information on the underlying research. Consider the results to be ballpark but with a wide variance.
Additional factors for the above table:
Forestry people have determined that there are differences in growth between forest trees and landscape specimens. Landscape trees tend to grow faster due to better care, more sun, and less crowding. Therefore landscape trees may not be quite as old as the calculation indicates and forest trees will grow slower and thus may be older than the calculation indicates.
Also, young landscape trees grow faster, you should slightly reduce the growth factor and for very old trees which have grown slowly in old age, you should slightly increase the factor.
And remember - this is an approximate calculation, not the definitive answer.
This table was summarized from data published from research done by the Morton Arboretum's study of old growth forest in the Chicago area. It too is an average of trees of various ages, but based on actual field measurement of forest trees. Forest trees of small diameter trees may be somewhat older than this table calculates.
If you have a Red Oak and find that the circumference at 4.5 feet height is 69 inches, then dividing by 3.1416 gives a diameter of 22 inches. In the second table, the growth factor for Red Oak is 6.7. Multiply 6.7 times the 22 inches and the result is an approximate age of 147 years. However, as this table is based on forest trees, if your tree is a landscape specimen, it will be younger than the table indicates.
Thanks to the Morton Arboretum for providing the 2nd set of Growth Factor data.