Translation guide
In Japanese, 'diameter' is most commonly expressed as 直径 (ちょっけい). This is the standard term used in mathematics, science, and everyday contexts. There is also a less common term 口径 (こうけい), which specifically refers to the diameter of a circular opening, such as the caliber of a gun or the aperture of a lens.
The straight line passing through the center of a circle or sphere, or its length.
The standard word for 'diameter' in almost all contexts, from geometry to everyday measurements.
The diameter of a circular hole, tube, or opening, especially in technical contexts like firearms, optics, or plumbing.
直径 (ちょっけい) is the general term for diameter of any circle or sphere. 口径 (こうけい) is a specialized term for the inner diameter of a tube, barrel, or opening. Use 直径 unless you are specifically talking about the caliber of a gun, the aperture of an optical instrument, or a similar technical measurement.
円の直径は10cm、パイプの口径は5cmです。
The diameter of the circle is 10 cm, and the inner diameter of the pipe is 5 cm.
この円の直径を測ってください。
Please measure the diameter of this circle.
地球の直径は約1万2700キロメートルです。
The diameter of the Earth is about 12,700 kilometers.
Used specifically for the inner diameter of cylindrical objects, such as gun barrels, pipes, or lenses. Not used for solid circles.
このライフルの口径は7.62mmです。
The caliber of this rifle is 7.62mm.
望遠鏡の口径が大きいほど、多くの光を集められます。
The larger the aperture of a telescope, the more light it can gather.
Can also be used for openings, but 口径 is more precise in technical contexts. 直径 is acceptable in everyday language.
パイプの直径は5センチです。
The diameter of the pipe is 5 centimeters.