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Triangle Calculator

Solve any triangle instantly, enter three sides, or two sides and the angle between them, to get all angles, the remaining side, area, and perimeter.

Angle A

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Angle B

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Angle C

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Area

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Perimeter

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Triangle Type

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Side c

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Angle A

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Angle B

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Area

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Perimeter

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Triangle Type

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Triangle Formulas

Law of Cosines finds a missing side or angle when you know all three sides, or two sides and the included angle.

c² = a² + b² − 2ab·cos(C)

Heron’s Formula finds the area from three side lengths alone.

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

For SAS input, area is found directly: Area = ½ · a · b · sin(C). Every triangle’s interior angles always add up to 180°.

How to Use This Triangle Calculator

1

Choose 3 Sides if you know all three side lengths, or 2 Sides + Angle if you know two sides and the angle between them.

2

Enter your known values, units don’t matter, as long as all sides use the same one.

3

Results update instantly: all angles, the remaining side, area, perimeter, and triangle type.

Frequently Asked Questions

What is the triangle inequality rule?

For three lengths to form a valid triangle, the sum of any two sides must be greater than the third side. If your values don’t satisfy this, the calculator will flag them as invalid.

Does this calculator work for right triangles?

Yes, a right triangle is simply a special case where one angle equals 90°. Enter your known sides or side-angle combination as usual and the calculator will identify it.

What units does the area use?

The area is returned in square units matching whatever unit you entered for the sides (for example, if sides are in centimeters, area is in square centimeters).

How is “triangle type” determined?

Based on the calculated angles and sides, the triangle is classified as equilateral (all sides equal), isosceles (two sides equal), scalene (no sides equal), and additionally noted as right, acute, or obtuse based on its largest angle.