Triangle Calculator

Provide 3 values including at least one side to solve the triangle — all sides, angles, area, and perimeter. Radian values like pi/3 are supported when radian mode is selected.

Modify the values and click the Calculate button to use this tool.
Angle C°
Side b
Side a
Angle A°
Side c
Angle B°

Related

Right Triangle Pythagorean Theorem Area Calculator

Rules

SSS, SAS, ASA, AAS, and SSA cases supported.

A + B + C = 180°

What is a triangle?

A triangle is a polygon with three vertices and three edges. It is the simplest polygon and the foundation of trigonometry. Triangles are classified by sides (equilateral, isosceles, scalene) or angles (acute, right, obtuse).

Triangle notation

Vertices are labeled A, B, C. Side a is opposite angle A, side b opposite B, and side c opposite C. The sum of interior angles is always 180° (π radians).

Law of sines

a/sin(A) = b/sin(B) = c/sin(C)

Used when you know two angles and one side (ASA, AAS) or two sides and a non-included angle (SSA).

Law of cosines

a² = b² + c² − 2bc·cos(A)

Used for SSS and SAS cases. Special case when A = 90° gives the Pythagorean theorem.

Area of a triangle

Area = √[s(s−a)(s−b)(s−c)]   where s = (a+b+c)/2

This is Heron's formula, used when all three sides are known.

How to use this triangle calculator

  1. Enter any 3 values — sides and/or angles.
  2. Include at least one side length.
  3. Choose degree or radian mode for angles.
  4. Click Calculate to solve all unknown sides, angles, area, and perimeter.

Frequently Asked Questions

Three values minimum, with at least one side length. Examples: three sides (SSS), two sides and included angle (SAS), two angles and one side (ASA).

When two sides and an angle opposite the shorter side are given, zero, one, or two triangles may exist. This calculator returns the primary (acute) solution when valid.

Yes. Select radian mode and enter expressions like pi/3 or pi/4.

Related Calculators

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a² + b² = c²

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