三角形の面積計算機

概要

The area of a triangle is the space enclosed by its three sides. Depending on what you know — base and height, or all three sides — different formulas apply. This calculator computes the area instantly in your browser using the right method for your inputs.

使い方（ステップ）

1. 1

   Choose your known values

   Enter base and height, or all three side lengths.

2. 2

   Calculate

   The tool applies the matching formula automatically.

3. 3

   Read the area

   The enclosed area is shown in square units.

仕組み

With a base and height, area = ½ × base × height. When you know all three side lengths instead, the tool uses Heron's formula: compute the semi-perimeter s = (a + b + c) ÷ 2, then area = √(s(s−a)(s−b)(s−c)). Both give the same area; the tool picks the formula that matches the measurements you provide.

こんなときに使う

Finding the area of a triangular plot or surface. Solving a geometry assignment. Calculating material for a triangular shape in a project.

よくある質問