# Tree age estimator > Enter a tree's trunk diameter or circumference at 4.5 feet, or a shrub's height, with the species and the site. Get an age range in years, good site to poor. Blue Whale Trees, a native tree nursery in the United States. Web version: https://bluewhaletrees.com/tools/tree-age/ Try [a northern catalpa 13.5 inches across](https://bluewhaletrees.com/tools/tree-age/index.html.md?species=northern-catalpa&by=diameter&trunk=13.5), [a white oak 70 inches around](https://bluewhaletrees.com/tools/tree-age/index.html.md?species=white-oak&by=circumference&trunk=70) or [a spicebush 8 feet tall](https://bluewhaletrees.com/tools/tree-age/index.html.md?species=spicebush&height=8). Enter the species, the trunk's diameter or circumference at 4.5 feet, the site and whether the tree grew in the open or in a forest. For a shrub, enter its height in feet instead, and the estimator runs its height rules backward. The thickest stem's width at the base, if you have it, sets a floor under the age. The estimator gives an age range from a good site to a poor one, with the figure for your site. It runs the growth estimator's trunk rules backward from a 3-foot sapling, so the answer is an estimate to check, not a measured age. Every rule is shown with its source. Without JavaScript the inputs cannot be changed here: the answer shown is for the inputs in the address, or the estimator's own example. These are its rules. The trunk grows by the growth estimator's own rules, from a 3-foot sapling with a half-inch trunk. A slow grower's trunk adds 0.25 inches a year on an average site, a medium one 0.4 and a fast one 0.6, slowing to 0.6 of that as the tree nears its mature height. A poor site grows at 0.65 of the average rate and a good one at 1.3 times it. The age on each site is the first year the trunk reaches the diameter measured, and past 200 years the estimator says 200 years or more. A tree grown in a forest adds trunk at half the open-grown rate, since Purdue Extension finds that woodland trees increase in circumference at about half the rate of open-grown ones. A circumference is divided by pi, 3.1416, for the diameter at breast height, as American Forests' handbook finds it. The common growth-factor formula, 3 to 5 years an inch for most species in Purdue Extension's table, multiplies the diameter by that factor. That formula describes forest-grown trees, whose trunks thicken at about half the open-grown rate, so for a tree grown in the open it often runs older than the estimator's figure, which names the formula only in a note. A shrub is aged by its height instead, since the growth estimator follows a shrub by its height alone. Its height grows 1, 1.5 or 2.75 feet a year by class on an average site and slows to nothing at its mature height. The count starts at 3 feet, or about a third of the mature height for a shrub that matures under 4.5 feet. A shrub at 90 percent of its mature height or taller is at least as old as the years it takes to get there. A shrub's thickest stem, given, sets a floor by the estimator's own rule of thumb. A slow grower's stem takes 10 to 30 years an inch, a medium one's 5 to 15 and a fast one's 3 to 6. Measured rings replace it for rosebay rhododendron, mountain laurel, red osier dogwood and beaked hazelnut. The larger of the two figures stands, since both are minimums. ## Estimate Enter the trunk's diameter or circumference at 4.5 feet, and the age range appears here. The rules it applies: - The trunk grows by the growth estimator's own rules, from a 3-foot sapling with a half-inch trunk, year by year on a poor, an average and a good site. - The age on each site is the first year its trunk reaches yours, up to the estimator's 200 years. - A tree grown in a forest adds trunk at half the open-grown rate, by Purdue Extension. - A circumference is divided by pi, 3.1416, for the diameter. An example: a northern catalpa 13.5 inches across, grown in the open on an average site. Change any input for your own answer. Northern Catalpa, 13.5 in across at 4.5 ft, average site, grown in the open [See Northern Catalpa](https://bluewhaletrees.com/plants/northern-catalpa/index.html.md) Estimate **≈ 24 yr** on your average site · ≈ 19 on a good site, ≈ 37 on a poor one **Fast** growth class · Northern Catalpa The years count from a 3-foot sapling with a half-inch trunk, so the seedling years before that are not counted. The common growth-factor formula, 3 to 5 years an inch for most species in Purdue Extension's table, would call this tree 41 to 68 years old. That formula describes forest-grown trees, whose trunks thicken at about half the open-grown rate, so for a tree grown in the open it runs older than the estimator's figure. *Trunk diameter by year from a 3-foot sapling: one line for each site, yours in bold. The dashed line is your trunk, 13.5 in. The lines reach it in year 19 on a good site, year 24 on an average one and year 37 on a poor one.* - **The diameter**: American Forests' handbook measures the trunk "at 4.5 feet above mid-slope of the tree's base". Yours is 13.5 inches across. - **The growth class**: its plant page puts Northern Catalpa in the fast class, with a mature height of 70 feet. The classes follow the Missouri Department of Conservation's growth key: "Fast: Has the potential to grow 24 inches or more annually." - **The trunk by year**: the growth estimator's own reading, a nursery rule of thumb rather than a measurement. From a 3-foot sapling with a half-inch trunk, a fast grower's trunk adds 0.6 inches a year on an average site. A poor site grows at 0.65 of that and a good one at 1.3 times it, so 0.39, 0.6 and 0.78 inches from poor to good while the tree is young. The trunk slows to 0.6 of its rate as the tree nears its full height. - **Where it grew**: in the open the trunk keeps the estimator's open-grown rate. Purdue Extension finds that a woodland tree adds circumference at about half that rate, so choose a forest for a tree that grew up crowded. - **The age**: year by year, the trunk reaches 13.5 inches in year 19 on a good site, year 24 on an average one and year 37 on a poor one. - **The first years**: the count starts at the 3-foot sapling and gives it the full rate from its first year, as the growth estimator does. The tree is older by the years it took to reach that size, and a planted one by a year or two more while its roots took hold. - **Why not the common formula**: Purdue Extension calls the growth-factor formula "a very rough estimate of a tree's age". Kevin T. Smith of the Forest Service finds that such tables "assume a consistent linear or straight-line relationship of diameter increment to years of growth". The estimator's trunk slows as the tree nears its full height instead of keeping one steady rate. - **How sure**: Kevin T. Smith of the Forest Service finds that "ring width is highly variable", so a simple average growth rate is a poor guide "even for trees growing under apparently similar conditions". Read the range as the spread to expect, not as limits. Every figure on this page is an estimate. A known planting date, with the tree's age when planted, gives the age outright, as Purdue Extension notes. Failing that, rings counted on a stump, a fallen tree of similar size on the same site or an increment core give a direct count. For a tree you plant, the [growth estimator](https://bluewhaletrees.com/tools/growth/index.html.md) runs the same rules forward. ## How to measure for an age - **Measure at 4.5 feet.** Wrap a tape around the trunk 4.5 feet above the ground, or read its diameter there. On a hill, measure from the middle of the slope, as American Forests' handbook does. - **Go under a branch or a swelling.** Where a branch or a swelling meets the trunk at 4.5 feet, measure the narrowest point below it, as the North Carolina Forest Service does, and note the height. - **Take the largest of several stems.** A tree that splits below 4.5 feet is measured at its largest stem, by the North Carolina Forest Service's guidelines. Two trunks that meet only at the ground are two trees, by American Forests' pith test. - **Divide the inches around by pi.** A tape's reading divided by 3.1416 gives the diameter at breast height, so a trunk 70 inches around is 22.3 inches across. - **Measure a shrub's height.** A shrub is aged by its height, so measure from the ground to the top of its tallest stem, in feet. One near its full height is at least as old as the range, since its height then says little more. - **Pick the site by its soil.** Deep, moist and fertile ground is a good site. Compacted, shallow or dry ground is a poor one, as a street tree's often is. For a shrub, shade counts as a poor site too. - **Say where it grew.** A tree that grew up in a forest or a crowded stand adds trunk at about half the rate of an open-grown one, by Purdue Extension. Choose the setting it spent most of its life in. - **Read the range, not one figure.** The answer runs from a good site to a poor one, counted from a 3-foot sapling, or for a shrub from its starting height. The growth estimator's rates are nursery rules of thumb, so a real tree can fall outside the range. - **Expect other sites to say older.** Many sites multiply the diameter by a growth factor, most of them 3 to 5 years an inch in Purdue Extension's table, and the formula describes forest-grown trees. A tree grown in the open adds diameter far faster, so it is usually younger than they say. - **Check it against a ring count.** A known planting date beats any estimate. Rings counted on a stump or an increment core give a direct count, though a core from a big old tree rarely reaches the center. [How old is that tree, really?](https://bluewhaletrees.com/articles/how-old-is-that-tree/index.html.md) explains how to get a real answer. ## Common questions ### How old is a catalpa tree with a 13.5-inch trunk? By the growth estimator's rules run backward, a [northern catalpa](https://bluewhaletrees.com/plants/northern-catalpa/index.html.md) 13.5 inches across that grew in the open is about 19 to 37 years old from a good site to a poor one. On an average site it comes out at about 24 years. ### How do you estimate a tree's age from its trunk diameter? Measure the trunk 4.5 feet up, then grow a sapling of the same class year by year, by the growth estimator's rules, until its trunk reaches yours. Our estimator does this for a good, an average and a poor site, so the answer is a range. ### How do you find a tree's age from its circumference? Divide the circumference at 4.5 feet by pi, 3.1416, for the diameter, as American Forests' handbook does, and read the age from that. A trunk 70 inches around is 22.3 inches across. ### Why does the growth-factor method make a tree seem older than it is? The common formula multiplies the diameter by a factor for the species, most of them 3 to 5 years an inch in Purdue Extension's table, a rate for forest-grown trees. An open-grown tree adds diameter far faster, so it can be half the age the formula gives. ### Is a forest tree older than a yard tree of the same size? Usually, since Purdue Extension finds that woodland trees add circumference at about half the rate of open-grown ones. Our estimator halves the trunk's yearly growth for a tree grown in a forest or a crowded stand. ### How can you tell how old a shrub is? Only roughly, while it is still growing: the estimator grows a shrub of its class year by year, by the growth estimator's rules, until it reaches your height. A shrub near its full height is at least as old as the years that took. ## The data and how to cite this page The figures this page draws on, tree growth rates by species, A to Z, are a file: [CSV](https://bluewhaletrees.com/data/growth-rates.csv) or [JSON](https://bluewhaletrees.com/data/growth-rates.json), one row for one tree or shrub the nursery grows. They are published under the [Creative Commons Attribution 4.0](https://creativecommons.org/licenses/by/4.0/) licence. Use them with a credit to bluewhaletrees.com and a link to [the table's page](https://bluewhaletrees.com/articles/growth-rates/index.html.md), which says what each column means. ### Cite this page Copy a link or a reference: ``` `Tree age estimator, bluewhaletrees.com, updated 2026-10-22` ``` bluewhaletrees.com. Tree age estimator. Updated 2026-10-22. https://bluewhaletrees.com/tools/tree-age/ The trunk's growth follows the [growth estimator](https://bluewhaletrees.com/tools/growth/index.html.md)'s rules, whose classes follow the Missouri Department of Conservation's growth key on its [Tree Growth Information](https://mdc.mo.gov/trees-plants/tree-seedlings/tree-growth-information) page. A slow tree has "an annual growth increment of 12 inches or less", and a fast one "the potential to grow 24 inches or more annually". Its trunk adds 0.25, 0.4 or 0.6 inch a year on an average site, by class. Those trunk rates, the site factors and the slowdown are the estimator's own reading, nursery rules of thumb rather than measurements. Each species' class and mature height come from its plant page, or from its sources for a species we do not grow. Every species we grow, with its class, is in the [growth rate table, A to Z](https://bluewhaletrees.com/articles/growth-rates/index.html.md). A shrub's age comes from the growth estimator's height rule, since the estimator follows a shrub by its height alone. Its height adds 1, 1.5 or 2.75 feet a year by class on an average site, from the height the estimator starts a new estimate at, and slows to nothing at the mature height. Past 90 percent of that height, the estimator's own reading is that height says little about age, so a taller shrub gets the least age that fits. The growth-factor formula is a rule for tree trunks and gives a shrub no figure. A shrub's thickest stem sets a floor by the estimator's own rule of thumb, years an inch of stem by growth class. A slow grower's stem takes 10 to 30 years an inch, a medium one's 5 to 15 and a fast one's 3 to 6. Its slow band rests on rosebay rhododendron's measured rings and on black huckleberry stems up to 15 years old. Its fast band rests on red elderberry, whose shrubs averaged 3.4 cm across at the base at 6 years, as the Forest Service's [review of red elderberry](https://research.fs.usda.gov/feis/species-reviews/samrac) reports. The rule is an approximation, and measured rings replace it one species at a time, each source's for its own species alone. Katherine Elliott and James Vose, in [Age and distribution of an evergreen clonal shrub in the Coweeta Basin](https://research.fs.usda.gov/treesearch/41289) of 2012, cut 487 rosebay rhododendron stems 10 cm above the ground and counted their rings. They found that "10 cm stems ranged in age from 30 to 120 years old", rings of about 0.42 to 1.67 mm a year. Robert Chastain, in [Distribution and dynamics of the evergreen understory layer in central Appalachian highland forests](https://hdl.handle.net/1903/1713) of 2004, counted the rings of 225 mountain laurel stems cut about 30 cm up. The dissertation's plotted stems give rings of 0.28 to 0.96 mm a year. James Lutz and others, in their 2017 [data on four Yosemite shrubs](https://doi.org/10.15142/T3WK55), counted the rings at the base of 41 red osier dogwood stems and 49 California hazelnut stems. California hazelnut is the western subspecies of beaked hazelnut. The hazelnut's rings run 0.5 to 3.25 mm a year. The dogwood's run 0.39 to 10.25 mm, the widest from a single stem 4.1 cm across at 2 years. The Forest Service's [Fire Effects Information System](https://www.fs.usda.gov/database/feis/) finds that stems renew while a clone lives on: black huckleberry stems averaged 5.7 years, and smooth sumac's were 1 to 15 years old in clones up to 22 by 40 meters. Purdue Extension's [How Old Is My Tree?](https://www.purduelandscapereport.org/article/how-old-is-my-tree/), by Lindsey Purcell in 2018, gives the growth-factor method many sites use. Measure the circumference at 4.5 feet, divide it by 3.14 for the diameter, then "Multiply the diameter of the tree by the growth factor as determined by species". The article calls the formula "a very rough estimate of a tree's age", and the estimator names it only in a note, since its factors describe forest-grown trees. The article also finds that woodland trees "increase in circumference at about half of the rate of full, open-grown tree found in a park or residential landscape". The estimator follows that for a tree grown in a forest. Kevin T. Smith of the USDA Forest Service, in [Big trees, old trees, and growth factor tables](https://research.fs.usda.gov/treesearch/55655) of 2018, finds that the tables "assume a consistent linear or straight-line relationship of diameter increment to years of growth". He also finds that "ring width is highly variable". The estimator's trunk slows as the tree nears its full height instead of growing at one steady rate, and its answer is a range. The sources disagree on a trunk that splits below 4.5 feet. The National Champion Tree Program scores the smaller of its waist below the split and its functional circumference, as the [big-tree measurer](https://bluewhaletrees.com/tools/big-tree/index.html.md) does. The North Carolina Forest Service's [guidelines for its champion trees](https://www.ncagr.gov/divisions/nc-forest-service/how-to-measure-champion-tree-basic/open) measure "the larger stem" of a tree that forks below breast height, and a multi-stemmed tree "on the largest stem". The estimator follows North Carolina for an age, its own reading, since the waist and the functional circumference count the wood of stems that grew side by side as one trunk's. The measuring rules follow American Forests' Champion Trees Measuring Guidelines Handbook of 2014. It wraps the tape "at 4.5 feet above mid-slope of the tree's base" and finds the diameter by "dividing CBH by the constant pi". Size is a poor clock, as [How old is that tree, really?](https://bluewhaletrees.com/articles/how-old-is-that-tree/index.html.md) explains, and every figure here is an estimate. The address carries the tree age estimator's inputs, so a link opens it filled in, and Copy link on a result writes one: species (a plant's slug, the last part of its page's address, such as white-oak, or slow, medium or fast for a tree or shrub without one), size (shrub, small-tree, medium-tree or large-tree, for a tree or shrub without a species), by (diameter or circumference, for how the trunk was measured), trunk (the trunk's measure in inches at 4.5 feet), height (a shrub's height in feet, in place of the trunk), stem (a shrub's thickest stem across its base in inches, for a floor on its age), stems (one, or several for a trunk that splits below 4.5 feet), site (poor, average or good), grew (open, or forest for a tree grown in a forest or a crowded stand). For example, [/tools/tree-age/?species=northern-catalpa&by=diameter&trunk=13.5](https://bluewhaletrees.com/tools/tree-age/index.html.md?species=northern-catalpa&by=diameter&trunk=13.5).