Meaning
The kanji 径 (ケイ / こみち) holds two distinct but related meanings. In literary and classical writing, it describes a narrow path or lane — the intimate trail threading through a bamboo grove, garden, or mountain forest. In scientific and technical contexts, the same character forms the core of diameter (直径) and radius (半径), the fundamental measurements of any circle or cylinder.
径 is built from two elements. On the left sits 彳, the double-step radical (#60), a fragment of 行 that signals walking and movement. On the right, the component 巠 relates to running straight — like warp threads pulled taut on a loom. Together they suggest moving in a direct, undeviating line. A shortcut through the forest and a diameter drawn across a circle share the same essential character.
径 has 8 strokes and sits in the secondary-school tier of the Joyo list — not taught in elementary grades. Its JLPT N1 placement reflects its role in technical and formal literary Japanese. The compounds it anchors — 直径, 半径, 口径 — appear regularly in textbooks, engineering documents, and academic writing.
Readings
On'yomi (音読み) — Chinese-derived readings
径's only on'yomi is ケイ (kei). It dominates modern usage — nearly every compound with 径 uses this reading. Mathematical, scientific, and engineering texts rely on it almost exclusively.
- 直径 (chokkei) — diameter; the straight line passing through the center of a circle from one side to the other
- 半径 (hankei) — radius; exactly half the diameter, measured from center to edge
- 口径 (koukei) — caliber, bore diameter; the interior diameter of a gun barrel, pipe, or lens
- 内径 (naikei) — inner diameter; used in plumbing, manufacturing, and mechanical engineering
- 外径 (gaikei) — outer diameter; the total width of a cylindrical object
- 孔径 (koukei) — aperture, hole diameter; used in optics and engineering to describe opening size
Kun'yomi (訓読み) — Native Japanese readings
The kun'yomi こみち (komichi) means a narrow path, lane, or small road. It carries a poetic, literary flavor — appearing in haiku, nature writing, and place names more often than in everyday speech. こみち evokes a quiet footpath winding through a traditional Japanese garden or a forested hillside. Less frequent than ケイ in modern usage, but expressive and culturally resonant.
- 径 (komichi) — narrow path, lane; a small footpath through nature or a neighborhood
- 小径 (komichi) — small path, narrow way; written with 小 for emphasis on its intimate scale
- 山径 (yamamichi) — mountain path; a trail through the mountains
Common Words & Compounds
径 appears in a small but widely used set of compounds, grouped below by topic.
Mathematical and Geometric Terms
- 直径 (chokkei) — diameter; the longest chord of a circle, passing through its center
- 半径 (hankei) — radius; exactly half the diameter, measured from center to circumference
Engineering and Technical Terms
- 内径 (naikei) — inner diameter; a critical measurement in plumbing, manufacturing, and mechanical engineering
- 外径 (gaikei) — outer diameter; the total width of a pipe, bolt, or cylindrical part
- 口径 (koukei) — caliber, bore; appears in firearms, telescopes, and fluid systems
- 管径 (kankei) — pipe diameter; used in construction, HVAC, and waterworks engineering
- 孔径 (koukei) — aperture, pore diameter; used in optics, filtration, and materials science
Path and Route Terms
- 小径 (komichi) — small path, narrow lane; a footpath through a natural setting
- 径路 (keiro) — path, route; a formal or literary variant of 経路
- 捷径 (shoukei) — shortcut; a literary word for the quickest path to a destination or goal
Example Sentences
この円の直径は10センチです。
Kono en no chokkei wa juu senchi desu.
The diameter of this circle is 10 centimeters.
地球の半径は約6,400キロメートルだ。
Chikyuu no hankei wa yaku rokusen-yonhyaku kiromeetoru da.
Earth's radius is roughly 6,400 kilometers.
森の中に細い径があった。
Mori no naka ni hosoi komichi ga atta.
A narrow path cut through the middle of the forest.
この銃の口径は9ミリです。
Kono juu no koukei wa kyuu miri desu.
This gun has a 9-millimeter bore.
古い小径を歩きながら、昔のことを思い出した。
Furui komichi wo aruki nagara, mukashi no koto wo omoidashita.
Walking the old lane, I found myself thinking of the past.
配管の内径を正確に測ってください。
Haikan no naikei wo seikaku ni hakatte kudasai.
Please measure the pipe's inner diameter accurately.
数学の授業で直径と半径の関係を学んだ。
Suugaku no jugyou de chokkei to hankei no kankei wo mananda.
In math class, we learned how diameter and radius relate to each other.
月明かりの下、細い径を二人で歩いた。
Tsukiakari no shita, hosoi komichi wo futari de aruita.
The two of us walked a narrow path under the moonlight.
この望遠鏡は口径が大きいので、暗い星もよく見える。
Kono bouenkyou wa koukei ga ookii node, kurai hoshi mo yoku mieru.
This telescope's large aperture lets it pick up even faint stars.
Memory Tip
Picture the left side 彳 as a hiker who always takes the most direct route. The right component 巠 suggests a perfectly straight line — a taut string pulled from one edge of a circle to the other through its center. That line is the diameter (直径). Cut it in half and you have the radius (半径). Send the hiker along it through the forest and it becomes a narrow path (こみち). One idea connects them all: the straightest way through. Spot 径 in any word and ask — what direct line or passage does this describe?