Math

Triangle Calculator

Solve triangles with sides and angles.

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Inputs

Provide all three sides to compute area and perimeter.

Side ASide BSide C

Results

If the sides do not form a valid triangle, results are hidden.

Area

Perimeter

Type

Scalene

Angle A

36.87

Angle B

53.13

Angle C

Semiperimeter

Inradius

Circumradius

2.5

Altitudes

Side	Altitude
A	4
B	3
C	2.4

Area from base and height

Use this when you know the base length and height.

BaseHeight

Area

How this calculator works

Computes area, perimeter, angles, and triangle type from three sides using Heron's formula and the law of cosines. Validates the triangle inequality before returning results.

Quick guide

Jump straight to the section you need, then return to the calculator.

Inputs Outputs Assumptions Tips Formulas Examples Reference table Visual breakdown References

Inputs

Side A length
Side B length
Side C length
Angle unit (degrees or radians)

Outputs

Triangle area
Triangle perimeter
Triangle type (equilateral, isosceles, scalene)
All three angles

Assumptions

Sides must satisfy the triangle inequality
Units are consistent across all sides

Tips

If results show invalid, check side lengths
Keep units consistent (cm, in, or m)

Formula guide

Use these formulas to audit the output or explain it to someone else.

3 formulas

s = (a + b + c) / 2

Area = sqrt(s(s-a)(s-b)(s-c))

Perimeter = a + b + c

Usage examples

Review a ready-made scenario, copy it, then tweak inputs to match your case.

Example

2 inputs2 outputs

3-4-5 triangle

Inputs

Side A 3, Side B 4, Side C 5
All sides in the same unit

Outputs

Area equals 6
Perimeter equals 12

This classic right triangle is useful for checks.

Example

2 inputs2 outputs

Isosceles example

Inputs

Side A 5, Side B 5, Side C 6
All sides in the same unit

Outputs

Area about 12
Perimeter equals 16

Area is approximate due to square root rounding.

Triangle validity check

4 of 4 rows

Sides	Valid	Note
3, 4, 5	Yes	Satisfies inequality
2, 2, 5	No	2 + 2 is not greater than 5
5, 5, 9	No	5 + 5 is not greater than 9
5, 5, 6	Yes	Valid isosceles triangle

A + B must be greater than C for all side pairs.

Area scaling (3-4-5 family)

Highest: 9-12-15 (54)Lowest: 3-4-5 (6)

Sort

Triangle setArea

Focus point

3-4-5

Position

#1 of 3

Original order

Share of total

7.14%

Total: 84

PositiveNegative

Scaling sides by 2 multiplies area by 4.

References

Heron formula for triangle area

FAQ

Common questions

What does the Triangle Calculator do?

Computes area, perimeter, angles, and triangle type from three sides using Heron's formula and the law of cosines. Validates the triangle inequality before returning results. It is part of our math toolkit.

What inputs do I need?

Typical inputs include Side A length, Side B length, Side C length.

How are the results calculated?

We follow the formulas and assumptions outlined in the "How this calculator works" section. You will see outputs like Triangle area, Triangle perimeter, Triangle type (equilateral, isosceles, scalene).