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Meaning & use

diameter

noun·Academicmathematics

Last updated

UK/daɪˈæmɪtə/
US/daɪˈæmɪtɚ/

Meanings

  1. nouncountable

    A straight line running from one side of a circle to the other through its middle, or the length of that line.

    Grammar: Often used with a measurement: a diameter of ten centimeters.

    Patterns

    • a diameter of something
    • something in diameter

    The diameter of the pizza is thirty centimeters.

    Draw a diameter across the circle and mark its midpoint.

    The pipe has a diameter of two inches.

  2. nouncountable

    The width of any round or roughly round object, such as a tree trunk or a planet.

    Grammar: Follows the measurement with in diameter: five meters in diameter.

    Patterns

    • something is X in diameter

    The old oak is almost two meters in diameter.

    Jupiter has a far greater diameter than Earth.

    Choose a wire with a smaller diameter for this job.

Word forms

pluraldiameters

Common combinations

in diameter
The crater is about a kilometer in diameter.
a diameter of
The hole needs a diameter of at least five millimeters.
inner diameter
Check the inner diameter of the pipe before you buy a connector.
outer diameter
The outer diameter of the tire is printed on its side.
double the diameter
A circle with double the diameter has four times the area.

Synonyms and how they differ

width
Width can describe any flat shape, while diameter applies only to circles and round things.
breadth
Breadth is a general, slightly formal word for side-to-side measure, with no link to circles.
caliber
Caliber is used for the bore of a gun or the width of a bullet, a narrower use than diameter.

Word family

diametric adjective
relating to a diameter, or completely opposed
diametrically adverb
completely, as in diametrically opposed
radius noun
half of a diameter
circumference noun
the distance around a circle

Commonly confused: diameter or radius? diameter or circumference?

diameter vs radius

A radius runs from the center to the edge and is half the diameter; do not use the two interchangeably in a formula.

diameter vs circumference

Circumference is the distance around the outside, not the distance across.

Practice

A circle has a radius of 6 cm. What is its ___?

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