Triangle Perimeter Calculator

P = a + b + c. Sum of all three sides.

Inputs

Result

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How to use this calculator

  • Enter three side lengths.
  • Calc verifies triangle inequality.
  • Reads perimeter + area + type.

About this calculator

Triangle perimeter is just the sum of three sides. The triangle inequality requires each side < sum of the other two — otherwise it's not a real triangle. Heron's formula gives area from sides only: Area = √(s(s−a)(s−b)(s−c)) where s = P/2 is the semi-perimeter. Equilateral (all equal), isosceles (two equal), scalene (all different) classify by side equality.

Frequently asked

Triangle inequality?+
Each side < sum of the other two. 3-4-5: valid. 1-2-5: invalid (1+2 < 5).
How does Heron's work?+
A formula for area using only side lengths (no angles). Discovered by Heron of Alexandria, ~10-70 AD.
Equilateral special?+
All sides + angles equal (60° each). Most symmetric triangle. Perimeter = 3s, area = s²√3/4.
Right triangle?+
Where a² + b² = c² (Pythagorean). One 90° angle. Common in construction and geometry problems.
Why semi-perimeter "s"?+
Half of perimeter. Cleaner formulas use s rather than P/2 each time. Convention dating to Heron.

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