An oak is a tree or shrub of the genus Quercus, a group of an estimated 430 closely related species, most of them in the Northern Hemisphere. Most oaks are large trees 20–30 meters (66–98 feet) tall, measured from the ground to the top of the crown.

Size

A tree's size is usually given as its height. An oak's height depends on its species and on where it grows, and some species are small shrubs. In Europe, the Woodland Trust gives 20–40 meters (66–130 feet) for the English oak. In eastern North America, the U.S. Forest Service describes the white oak as often 24–30 meters (80–100 feet) tall, with a trunk 91–122 centimeters (36–48 inches) across at chest height.

Where an oak grows changes its shape as well as its height. A white oak growing in the open has a short, stocky trunk and a wide, spreading crown. In a forest, it grows a tall, straight trunk with a compact crown. The Forest Service's 1990 guide to the white oak reports individual trees as tall as 46 meters (150 feet), with trunks as wide as 244 centimeters (96 inches), and as old as 600 years.

Size comparisons

An oak 25 meters (82 feet) tall, in the middle of the usual range, is about 15 times the height of a person 1.7 meters (5 feet 7 inches) tall.
It is about eight times as tall as a basketball hoop, whose rim is 3.05 meters (10 feet) above the floor.
It is about three and a half times as tall as the Apollo Lunar Module, which stood 7.0 meters (22 feet 11 inches) on its landing legs.
An English oak's acorns are 2–2.5 centimeters (0.8–1 inch) long. A 25-meter oak is about 1,000 times as tall as one of its 2.5-centimeter acorns.

Acorns

An oak's fruit is the acorn, a nut that sits in a small cup. White oaks usually bear acorns between the ages of 50 and 200 years, though trees growing in the open can start at 20. Good crops come only every 4 to 10 years. In one good year, a single 69-year-old white oak in Virginia, 21 meters (69 feet) tall, produced more than 23,000 acorns.

Most acorns never sprout, because animals such as jays, mice and squirrels eat them. In white oaks, sometimes only 18% of a crop is sound, and the rest is damaged or destroyed by animals and insects. White oak acorns that survive sprout soon after they fall, in September or October.

Oaks, wildlife and people

In the United Kingdom, oaks support more than 2,300 other species, more than any other native tree, according to the Woodland Trust. Oak forests are home to hundreds of kinds of insects, birds nest in holes and cracks in the bark, and bats roost in old woodpecker holes.

People have prized oak timber for thousands of years, for building, flooring, wine barrels and firewood. Oaks are slow to grow: an English oak can take up to 150 years before it is ready for use in building. Tannin from oak bark has been used to turn hides into leather since at least Roman times.

Many oaks are now at risk. A 2020 assessment of the world's oak species estimated that 31% are threatened with extinction. It found the most oak species in Mexico, 164, followed by China with 117 and the United States with 91.

Sources

The Red List of Oaks 2020. Carrero et al., The Morton Arboretum and Botanic Gardens Conservation International, 2020.
White Oak. Rogers, Silvics of North America, U.S. Forest Service, 1990.
English Oak (Quercus robur). Woodland Trust.
Official Basketball Rules 2026: Basketball Equipment. FIBA, 2026.
Apollo News Reference: Lunar Module. Grumman Aircraft Engineering Corporation, via the Apollo Lunar Surface Journal.

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