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Tree Age Calculator

Estimate how old a tree is without cutting it, from the trunk circumference or diameter at breast height and the species growth factor (oak, maple, pine, birch and more) — the ISA arborist method.

Units

Measurement

Trunk size at breast height (1.4 m / 4.5 ft)

Species

Custom growth factor

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How it works

DBH = circumference ÷ π · age (years) ≈ DBH (inches) × species growth factor · 1 inch = 2.54 cm · measure at breast height, 4.5 ft (1.37 m) above the ground

Arborists estimate the age of a living tree with the growth-factor method promoted by the International Society of Arboriculture and arboreta such as the Morton Arboretum: each species adds roughly the same diameter per year in typical conditions, so the diameter at breast height times a species factor gives an age. Fast growers such as cottonwood have small factors (about 2), slow growers such as white oak larger ones (about 5). Factors were derived for trees in the US Midwest; trees in forests, poor soils or dry climates grow more slowly and may be much older, so treat the result as an estimate — counting rings on a core is the only accurate method.

Worked example

A red oak with a circumference of 150 cm has a diameter of 150 ÷ π = 47.7 cm, which is 18.8 inches. With a growth factor of 4.0 its age is about 18.8 × 4.0 = 75 years — on average its trunk widened by 6.4 mm a year.

Sources: The Morton Arboretum — How old is my tree? · International Society of Arboriculture — Trees Are Good

Frequently asked questions

How can I tell how old a tree is without cutting it down?

Measure the trunk circumference at 1.4 m (4.5 ft), divide by π to get the diameter, convert to inches and multiply by the species growth factor. An increment borer can extract a thin core to count rings exactly.

What is a tree growth factor?

The average number of years a species takes to add one inch of trunk diameter. For example silver maple 3, red oak 4, sugar maple 5.5 and shagbark hickory 7.5.

How accurate is the growth factor method?

It is a rough estimate, often within 20–30% for open-grown trees. Shade, competition, drought and soil can make the true age very different.

Where exactly should I measure?

At 4.5 ft (1.37 m, often rounded to 1.4 m) above ground on the uphill side. For trees that fork below that height, measure the narrowest point below the fork.

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This calculator is for educational purposes. Double-check important results before acting on them.