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三平方の定理計算ツール

Enter two sides of a right triangle to calculate the remaining side using the Pythagorean theorem (a²+b²=c²). Angles, area, and perimeter are shown at the same time.

Last updated: 2026/10/16 Calculator Feedback & bug reports
Side Input
Results
Calculated side
--
Angle A (opposite side a) --
Angle B (opposite side b) --
Angle C (right angle) 90°
Area --
Perimeter --
The Pythagorean theorem holds only for right triangles. Angles, area, and perimeter are approximate values calculated from the entered sides.

How to Use

1

Choose the side to solve

Under "Solve for", choose the side you want to calculate (hypotenuse c, side a, or side b).

2

Enter the two known sides

Enter the lengths of the other two sides and the selected side is calculated in real time.

3

Check angles and area

Along with the calculated side, the interior angles, area, and perimeter are shown. Use "Copy Results" to copy everything at once.

About the Pythagorean Theorem

The Pythagorean theorem states that in a right triangle, the sum of the squares of the two legs equals the square of the hypotenuse (a²+b²=c²). With this tool you only need to enter two sides; it finds the remaining side and also computes the interior angles, area, and perimeter. All processing runs in your browser and your input is never sent to any server.

Features

  • Auto-calculate the missing sideChoose which side to solve for, enter two sides, and the unknown side is calculated instantly.
  • Angles, area & perimeterNot just the side length, but the two interior angles, area, and perimeter are shown too.
  • Clear diagramA right triangle diagram with labeled sides and angles helps you see exactly what is being calculated.
  • Private & completely freeAll calculations stay in your browser. No registration, free to use as many times as you like.

Use Cases

  • Checking math homework on the Pythagorean theorem
  • Finding diagonal lengths in DIY and woodworking
  • Verifying right angles and distances in drawings
  • Reasoning about screen width/height from a diagonal size
  • Estimating straight-line distance between two points

FAQ

What is the formula for the Pythagorean theorem?
It is a²+b²=c², where a and b are the two legs (sides forming the right angle) and c is the hypotenuse (the longest side). To find the hypotenuse, c=√(a²+b²); to find a leg, a=√(c²−b²).
Can I calculate a leg, not only the hypotenuse?
Yes. Choose side a or side b under "Solve for", and the remaining leg is calculated from the hypotenuse and the known leg. In that case the hypotenuse c must be longer than the other side.
How are the angles calculated?
The interior angles are derived from the ratios of the sides using inverse trigonometric functions (arctan / arcsin). Angle C is always 90° (the right angle), and angles A and B add up to 90°.
Can I use decimals and units?
Yes. Side lengths can be decimals. If you enter a unit such as cm or m, the same unit is shown with the results (no unit conversion is performed).
Is my input sent to a server?
No. All calculations happen entirely in your browser, and the numbers you enter are never sent to or stored on any server, so you can use it with peace of mind.

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Related Guides

What Is the Pythagorean Theorem? Meaning of a²+b²=c², Finding the Hypotenuse, and Famous Triples Learn what the Pythagorean theorem is from the ground up: the relationship a²+b²=c² describes in a right triangle, how to find the hypotenuse, famous integer ratios like 3:4:5, real-world uses such as distance between two points, and the idea behind the proof.