Triangle Calculator
Enter the three sides and get the area, perimeter, angles and more.
What is Triangle Calculator?
Computes area, perimeter, angles and height of any triangle from the data you have (3 sides, 2 sides + angle, base + height, etc.). Uses Heron's formula for area when only sides are known. Handy for school geometry, woodworking, construction and design.
How to use
- Pick the input type you have (3 sides, etc.).
- Fill in the matching values.
- See area, perimeter and all 3 angles.
- The triangle is drawn to scale for visual check.
Triangle types
Equilateral
All sides equal · All angles 60°
Isosceles
Two equal sides · Two equal angles
Scalene
All sides different
Right
One 90° angle · Pythagorean theorem applies
Acute
All angles less than 90°
Obtuse
One angle greater than 90°
What is Heron's formula?
Heron's formula calculates a triangle's area knowing only the three sides: A = √(s(s−a)(s−b)(s−c)), where s = (a+b+c)/2 is the semi-perimeter.
When is a triangle valid?
By the triangle inequality: the sum of any two sides must be greater than the third. If that doesn't hold, the sides can't form a triangle.