How to Say "Triangle" in Japanese

三角形

sankakukei[sã̠ŋka̠kɯ̟̊ke̞ː]

The Japanese word for triangle is 三角形 (sankakukei) or 三角.

Meaning: polygon (noun)

Ways to say it

“triangle” in Japanese
Japanese	Romanization (Wiktionary)	IPA
三角形	sankakukei	[sã̠ŋka̠kɯ̟̊ke̞ː] (narrow) [sã̠ŋka̠kɯ̟ke̞ː] (narrow)
三角	sankaku	[sã̠ŋka̠kɯ̟] (narrow)

The word refers to a three-sided polygon. You may see the English loanword written in katakana as トライアングル.

The term 三角形 is composed of 三角 (triangle) and 形 (shape/form).

三角形 is the precise geometric term for a triangle. In contrast, 三角 is more colloquial or is used as a prefix for other shapes.

An equilateral triangle is specifically called 正三角形.

sankakukei

In Japanese culture, the おむすび or おにぎり (rice ball) is traditionally shaped like a 三角 to mimic the shape of a mountain.
The word 角 in 三角 can also mean 'horn' or 'corner', depending on the context.
The word 三角 is used in the term 三角関係, which describes a 'love triangle' or a complicated romantic relationship between three people.

球面上では三角形の内角の和は180°よりも大きくなる。
The sum of the angles of a triangle on a spherical plane is more than 180 degrees.
This sentence shows how to use 三角 regarding the sum of angles in a triangle.
Tatoeba #2619730 by tommy_san, translation #2629820 by WestofEden
どうやったら三角のおにぎりができるの？何度やっても丸いおにぎりしかできないよ。
How do you make a triangle-shaped onigiri? No matter how many times I try I can only make round onigiri.
This sentence uses 三角 to describe the shape of food.
Tatoeba #1180859 by bunbuku, translation #1556974 by brymck
あの子が球面三角を書きました。
The child drew a spherical triangle.
This sentence uses 三角 to refer to a spherical triangle.
Tatoeba #903986 by Scott, translation #901587 by Spamster
形の一つは、さんかくです。
One of the shapes is a triangle.
この建物は、三角形の形をしています。
This building has a triangular shape.
ボーリングのピンの配置は、トライアングルの形ですね。
The arrangement of bowling pins is in the shape of a triangle, right?
1 more sentence on Tatoeba

Misaki and Kenta are doing their math homework together.

美咲 Misaki

この三角形の面積はどうやって計算するの？
Kono sankakkei no menseki wa dōyatte keisan suru no?
How do you calculate the area of this triangle?
健太 Kenta

底辺と高さを掛けて、半分にするんだよ。
Teihen to takasa o kakete, hanbun ni suru nda yo.
You multiply the base and the height, then divide by two.
美咲 Misaki

なるほど、三角形は意外と簡単だね。
Naruhodo, sankakkei wa igaito kantan da ne.
I see, triangles are surprisingly simple.
健太 Kenta

そうだね。次の問題に行こうか。
Sō da ne. Tsugi no mondai ni ikō ka.
That's true. Shall we move on to the next problem?

Sources

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